Northwest Agricultural Consultants: Interpreting Soil & Plant Tissue Tests

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Northwest Agricultural Consultants: Interpreting Soil and Tissue Tests


This publication is a non-technical attempt at discussion of those controllable factors which are most commonly considered when evaluating a soil or plant tissue test report for the purpose of making specific fertility recommendations and an attempt to increase the application value of our test reports by increased understanding of our reported test results.

We are not attempting to consider all factors, and a report recipient should remember that in diagnosing the needs of plants, all factors, both controlled and uncontrolled, involved with crop yield need to be considered. A proper interpretation of the assembled facts must then be made and a fertility program decided on.

Selection of the kinds and amounts of soil fertility treatments depend, broadly upon (1) the crop yield goal and its nutrient requirements, (2) the ability of a given soil to supply these nutrients, (3) the climatic factors affecting fertility response, (4) the management factor, and (5) the presence or absence of inhibitory chemicals or pathogens. The economic maximizing of factor 2 within the limits of the other factors is the goal of any fertilizer application recommendation.

Factor 2 has, historically, been the factor farmers could most readily change. Early efforts were largely the result of chance observation or trial-and-error. As the body of scientific knowledge increased, a systematic scientific approach to the problems of soil fertility evolved. An outgrowth of this is chemical soil and plant tissue testing to ascertain the fertility status of a soil or plant and predict crop response or need for the addition of fertilizer.

The technical complexities and details of the chemical test methods are of limited interest here. They are discussed only where needed for clarification or background information. We are striving solely describe the meaning of test results and facilitate conversion of the chemical soil and plant tissue test figures into recommended amount of fertility treatment.




The author wishes to acknowledge the liberal use of the published research data and the fertilizer guides of Washington State University, University of Idaho, and Oregon State University. This data along with my own experience and the experiences reported back to me by clients determined the final conclusions in making these recommendations.

I also wish to acknowledge the many helpful suggestions from the people in the fertilizer and chemical industry and the people in the food processing industry whose experiences have formed guidelines for fertilizer recommendations to growers.

General Properties of Soil

The Colloidal Complex

The primary soil factors or properties which influence crop production are organic matter, mineral composition, soil atmosphere, soil texture, and soil moisture. These factors are closely interrelated in determining the supply and availability of the chemical elements necessary for plant growth. Certain portions of the organic and much of the mineral-clay fractions exhibit a large capacity for absorbing ions within a colloidal complex.

From the Greek word "Kolla," meaning glue-like, we derive the word colloid. When used in soil descriptions it identifies those minute, plastic, sticky portions of the soil having large surface areas in comparison to diameter, high base attraction, and capacities to trade or exchange one base element for another. From the complex interactions of these factors we derive the term "colloidal complex."

So when we speak of the colloidal properties or colloidal complex of the soil, we mean the ability of the soil to hold, by absorption, various plant food elements, such as potassium, calcium, magnesium, zinc, manganese, copper, iron, sodium, etc., and to release or exchange these elements under plant growth conditions.

Soil colloids are made up principally of inorganic material, primarily clays such as montmorillonite, kaolinite, or illite, but highly decomposed organic matter or humus also has colloidal properties. In fact, for a given unit mass, the humus colloids are about 3 to 4 times more active than the clay colloids.

These minute particles of clay and humus have certain properties such as plasticity, aggregation, and the many absorption exchange reactions by ions on the surface of the colloidal particles. These properties are illustrated by the soil being readily molded when wet, by its formation of granular structure in proper moisture, its capacity to absorb many chemical ions out of solutions and conversely plaiting them back into solution from the soil by exchange processes.

It is through this process of absorption and exchange that the colloidal complex can be increased in fertility potential through soil treatments. Plants exchange hydrogen for the nutrient elements they need as their roots make contact with the colloidal surfaces or the surrounding soil solution. Water percolating through the soil exchanges hydrogen ions for other bases or cations and the exchanged cations are lost through leaching or removed by cropping. The quantity of hydrogen ions on the colloids goes up and the quantity of the other elements goes down. Treatment then becomes a problem of replacing the hydrogen ions again with the proper amounts and balance of the other plant food elements.

Soils containing much sand and little organic matter or clay have a low ion exchange capacity. They neither swell nor shrink much and are easily tilled, but they generally do not absorb large amounts of plant food, have low reserve supplies, and do not retain plant food added in large quantities.

Soils with a high clay content (especially montmorillonite types} or containing moderate to high organic matter, have high exchange capacities. They swell and shrink considerably and can present tillage problems, but they absorb larger amounts of added plant food and have greater reserve supplying capacity.

The interactions between the plant root, the soil colloid, the soil water solution, the soil air, and the parent material mineral reserves is diagrammed in Figure 1.

Salts and Soil Solutions

All soil systems contain some salts dissolved in the soil water. The soil water is a dynamic solution which contains at least a trace of every plant food element. These elements, ionized in water, are usually referred to as the active ions, and those absorbed or held on the colloidal surface but capable of exchange are called reserve ions. In most instances, the ions held either as absorbed ions on the colloid surface or as active ions in its surrounding film of water can be considered available to the plant. Unfertilized soils contain varying amounts of these salts in the soil water system. When these soils are treated with commercial fertilizer or manure the salt content of the system increases. This is a desirable situation as long as the nutrient ions remain in balance and do not exceed the toxic limits of concentration of an individual element or total salts. It is usually simpler to fertilize a salt-free soil for increased productivity than a soil which has been fertilized until some nutrient is out of balance due to an excess of one or more fertilizer salts.

Soil Weathering

Another important factor of soil fertility is associated with the weathering of soil minerals. The silt and sand separates of soils which contain weatherable minerals will, when encouraged by organic matter and clay, transfer ions from the crystals of these minerals to the surface of the colloidal complex. In cases where the sand, silt, and clay separates are composed of mainly quartz, or other extremely resistant minerals, the supply of nutrient ions on the colloid is not restored by "resting of the soil". It can be maintained only by the addition of nutrient elements contained in fertilizers, manures, and plant residues.

A diagrammatic presentation of the constant and continuing interactions going on between the plant root, the plant residue, the soil colloidal complex, the soil solution and the mineral reserves for plant nutrition is shown below;

[][1] [1]: /s/p5fig.gif

Organic Matter

The solid portion of the soil, as opposed to air space and water, is composed of either organic (carbonaceous) or inorganic material. The inorganic material comes from the chemical or mechanical decomposition of the parent rock or the wind or water deposited sand and silt material. This decomposing inorganic material supplies the phosphorus, potassium, calcium, and magnesium, plus the essential minor elements for plant growth.

The organic matter of the soil is derived from plant and animal residues, applied manure, fungi. bacteria, worms, insects, etc., in various stages of decomposition.

Organic matter is a very important factor in crop production for the following reasons:

  1. Serves as a feed for bacteria, fungi and other beneficial organisms.

  2. Aids in bringing insoluble soil minerals into solution, and therefore available to the plant.

  3. Improves the physical condition (tilthe) and aeration of the soil.

  4. Increases the water-holding capacity and the water infiltration rate of the soil.

  5. Serves as a major source of certain plant food elements such as nitrogen and sulfur.

  6. Is an aid in reducing soil erosion by wind and water.

It is recommended that growers utilize those practices which maintain and replenish soil organic matter. These practices include: 1) Growing and plowing under green manure crops. 2) Conserving and applying manure. 3) Utilizing all crop residues by returning them to the soil. 4) Controlling wind and water erosion.

The soil test for organic matter is an indication of soil productivity and from the percent organic matter the estimated nitrogen release to the crop can be calculated. The amount of available nitrogen released to the crop is about 25 to 50 Ibs. of actual nitrogen per acre per year for each percent organic matter. The actual amount released will depend upon such factors as moisture, temperatures, length of growing season, crop grown, etc.

Principle of Using Soil Test Values to Make Fertilizer Recommendations

For any given set of conditions for crop production, such as crop to be grown, soil moisture from rainfall and/or irrigation, crop variety, temperature, soil texture soil depth, cultural practices, etc. there is an optimum amount of each plant food element required to produce maximum yield. The principle of using soil test values to make fertilizer recommendations assumes that if the soil is sampled and analyzed for a given element, and the amount of that element is determined, the difference between the amount in the soil and the optimum level can be added in the form of fertilizer This principle is presented in the diagram in Figure 2.

However, several other factors must be considered to interpret soil test results such as yield goal, depth of soil sampled, distribution of the element in the soil profile, availability of the element, and method used to analyze the soil. Higher yield goals will require higher levels of nutrients than lower yield goals. The concentration of different elements vary with soil depth. For example, the concentration of phosphorus decreases with depth in the soil profile. Therefore, a 0-6 inch soil sample will usually test higher for phosphorus than a sample representing the 0-12 inch soil depth. The soil depth represented by the sample should therefore be evaluated in making fertilizer recommendations.

The total amount of a plant food element present may be many times the amount reported in a soil test. However, much of the element may be "tied up" in insoluble or unavailable forms, and therefore not usable by the plant. Soil testing procedures use extracting solutions which dissolve only that portion of the element estimated to be available to the plant.

Soil testing procedures are constantly being revised to give better estimates of available plant food nutrients.


Nitrogen, phosphorous, and sulfur, being acid forming materials or anions, must be considered as special cases. While many chemical forms of nitrogen, phosphorus, and sulfur exist in the soil, it is principally in the form of nitrate (NO3), orthaphosphate (H2PO4), and sulfate (SO4), that plants can utilize these negatively charged elements. Nitrate nitrogen with one negative charge is readily available for plant feeding, but at the same time, quite subject to leaching.

It should also be pointed out that the processes by which nitrogen, phosphorus, and sulfur are converted into the nitrate, phosphate, and sulfate forms from other chemical forms is accomplished or facilitated by the action of certain types of soil bacteria. For this reason, the amount of organic matter present in a soil to supply fool for bacteria becomes a matter of significant importance.

Nitrogen is the element that gives plants their dark green color and induces rapid succulent growth. It increases the yield of leaves, fruit, and seed crops. Sufficient nitrogen increases the protein content of the vegetative parts and seed of feed and food crops. Nitrogen in the soil feeds the beneficial soil microorganisms during their decomposition of organic matter.

Nitrogen deficiency is characterized by light or yellowish green color and dwarfed growth. More severe deficiency will cause drying up or "firing`' of the leaves, especially at the base of the plant. The firing starts at the tip of the bottom leaves and proceeds down along the midrib of the leaf.

Nitrogen does not exist in the soil in natural mineral form as do the other plant nutrients. It must come from the air. However, most plants cannot get or use nitrogen from the air in its elemental form. It must first become chemically fixed or combined with one or more elements such as hydrogen to form ammonium (NH4) or oxygen to form nitrate (NO3) before plants can use it.

The natural procedures through which nitrogen in the air is combined into a form that plants can use and is brought into position for use may be explained as follows: A very small amount of nitrogen is combined with oxygen or hydrogen during electrical storms, perhaps 5 pounds per acre per year may be applied in rainfall during electrical storms, but the main sources of natural nitrogen supply come through fixation by nitrogen fixing bacteria in the soil, and conversion of plant and animal residues.

Certain soil bacteria and other organisms fix atmospheric nitrogen as part of their life processes. Two distinct types are the symbiotic and the non-symbiotic organisms. The symbiotic bacteria are those associated with legume crops. In return for the supply of food and minerals they get from the plant, these bacteria supply the plant with part of its nitrogen needs, but generally not more than 5O to 70% of the plant needs.

The non-symbiotic bacteria live independently and without the support of higher plants. There are two different groups of non-symbiotic bacteria, the aerobic which require oxygen and the anaerobic which do not need oxygen. These bacteria can supply as much as 50 pounds per acre per year, but generally supply less than 20 pounds.

Nitrogen, returned to the soil in the form of manure and the remains of former plant and animal life, is eventually reduced by biological decomposition, oxidation or reduction, and is finally mineralized to yield nitrate nitrogen for plant use.

Test results report the organic matter content as a percent of the soil weight. Organic matter usually contains about one-twentieth or 5% nitrogen. Thus, a 3% organic matter soil (considering an acre foot depth of a soil to weigh 4,000,000 lbs.) would contain about 6,000 Ibs. nitrogen per acre foot of soil. However, only 1 to 4% of the total nitrogen in the organic faction in the soil will become available to the plant during an average growing season. Deviations from this amount will be found in poorly drained soils and very highly organic (muck) soils, etc. The type of material undergoing decomposition, the stage of decomposition, the soil temperature, and aeration will also affect these values. Ignoring the deviations and using averages, the normal soil above would supply about 90-180 Ibs. nitrate N per acre to the crop during a normal growing season. But remember, these calculations are based on a 72" depth and a soil of 4,000,000 lbs./acre foot. The organic matter in many soils does not extend down past the plowlayer in anywhere near the same concentration as exists in the plowlayer, and the sample is usually taken from just the plowlayer. In addition, most agriculturally important soils weigh less than 4,000,000 lbs./acre foot. Using a figure nearer 3,500,000 lbs. would probably give more realistic numbers for most soils. Based upon the lighter soil weight, with the sample considered as taken to 7" depth and a weight of 2,000,000 lbs., the example as given would indicate a supply of about 45-90 lbs. N/acre/season. See organic matter to nitrogen conversion factor for individual crops on nitrogen recommendation graphs.

The Process of Nitrification

The process of nitrification does not only involve the nitrogen fixing bacteria, but is also concerned with decomposition of organic matter and the conversion of certain synthetic chemical nitrogen fertilizers that may be applied to the soil. This process takes place when soil conditions such as aeration and moisture content are favorable and the soil temperature is above 45 degrees F. The process is as follows: Ammonium (NH4) to nitrite (NO2) to nitrate (NO3).

When any nitrogenous organic or fertilizer substances are added to the soil, numerous bacteria, fungi, and other soil organisms attack the materials, breaking them down into the nitrate (NO3-) form. The soil conditions that especially favor this process are good drainage, good aeration, warm soil temperature, adequate moisture, good organic matter supply, and a satisfactory pH range of 6.0 to 7.5.

Conversely in some soils a process of de-nitrification may take place where nitrate reverts to nitrite, possibly back to the ammonia form, and even to gaseous nitrogen. Such conditions may prevail in tight, soggy soils with pH range below 5.8 and low organic matter supply.

Estimating Nitrogen Needs

The most important factor in determining the nitrogen need is the crop to be grown. When the desired crop yield goal has been established, the nitrogen requirement can be computed. Your soil test report reveals the organic matter content and from this can be calculated the pounds of estimated nitrogen release for the growing season, taking into consideration the soil type and its geographical location. This estimated nitrogen release should be added to the reported test nitrogen carried over or present in the full soil root zone as nitrate and as ammonia in the surface foot. This total is then subtracted from the total nitrogen required by the crop to attain a calculated and reasonable potential yield. The difference or net is the approximate amount of nitrogen to be applied as fertilizer, but does not include losses from leaching, volatization, erosion, etc.

Other Factors to Consider

The plus or minus effects (on the total nitrogen supply) of crop residues, manure applications, legumes, nitrogen sources and forms, and at what time during the growing season the organic nitrogen will be released are other factors to be considered, and should be included in calculations of crop nitrogen need.

Plant Residues

The table below indicates some approximate amounts of nitrogen added to or subtracted from the soil by various ground covers, mulches, and crop residues worked into the soil, and serve as an estimate or guideline for inclusion in the calculation of total nitrogen needs.

MATERIAL lbs. Nitrogen released or tied up per ton (dry weight basis)
  1st Year 2nd Year
Green Alfalfa or Clover plowed down +30 lbs. N +10 lbs. N
Alfalfa, Hay Mulch +5 lbs. N  
Corn Stover -20 lbs. N  
Grass Cover -20 lbs. N  
1/2 Grass, 1/2 Legume 0  
Manure +7 lbs. N +3 lbs. N
Bean or Pea Grain Stubble +5 lbs. to 10 lbs. N  
Straw Mulch -10 lbs. to -20 lbs. N  
Weeds -20 lbs. N  

Nitrogen Carry-Over

The carry-over of nitrogen in dry years is a factor which must be considered in calculating the nitrogen needs. During periods of little or no leaching and low crop production, much of the applied nitrogen plus as much as one-half the amount of nitrogen from normal nitrification may be carried over to the next growing season. However, under conditions of high rainfall and severe leaching, no nitrogen carryover can be counted and in fact, mild to severe losses of applied nitrogen may be encountered.

Nitrogen and Irrigation

The necessity for nitrogen control, under irrigation, poses added problems as well as potentially greater benefits. Under irrigation an excess of water is generally applied to avoid the build up of soil salts. In addition many high yielding, high value, intensive production crops are sensitive to moisture stress and suffer yield reductions even though the soil is still relatively high in available moisture, or drops to low levels for only short periods of time. This leads to frequent irrigations when the additional soil holding capacity is very limited. Nitrate nitrogen goes into water solution readily and quickly and moves with the water penetration front.

Given the high solubility of nitrates, the need for slight amounts of leach water, and the need to prevent moisture stress, it becomes readily apparent that leach losses of nitrogen can be very serious if irrigation is not closely controlled. Excessive water application can completely destroy the best of fertility programs.

The method of water application and the method of nitrogen application also influence the nitrogen fertility program. In rill, border, or other methods of surface flow application, the amount of water penetrating into the soil is proportional to the distance from the turnout point as a contact time and penetration rate function. In order to bring the end of the run up to the desired moisture status the start of the run must receive an excess. This influences the nitrogen fertility level. Sprinkler systems (aside from cost) also have application pattern. An addition of nitrogen through the water may help to lessen the leach-loss problem in both methods but again there are distribution pattern problems which must be considered. Results of good water management together with good nitrogen control can be instrumental in obtaining maximum net dollar return from crops.

Nitrogen Application

The problems of when, how, and what type of nitrogen to apply should also receive some attention. The following consideration should be kept in mind:

  1. Nitrogen in the nitrate form is immediately available to plants and should be used in cases where an immediate source of available nitrogen is demanded, especially in low organic matter soils where microbial activity may be limited. Nitrate nitrogen is especially useful for late fall or early spring seedings where the soil is subject to low temperatures. A limiting factor in the use of the nitrate nitrogen form is its susceptibility to loss by leaching from the soil under certain excess moisture conditions.

  2. While some plants have the ability to utilize a small amount of ammonium by direct cation exchange in many cases the ammonium nitrogen must be converted by the soil micro-organisms to the nitrate form before utilization by plants. Ammonium nitrogen does have the advantage of being absorbed on the soil colloidal system and thus is less subject to leaching. Ammonium nitrogen can, and should, be applied ahead of the growing season, as it will be held in the soil until nitrification converts it into the more readily usable nitrate form. Late fall or winter application may be of benefit in many areas. If the winter temperatures are sufficiently cold to stop nitrification or if precipitations is low to moderate, leach losses will be minimal.

  3. Better field working conditions often exist in the fall in comparison to spring. Timing, equipment, fertilizer, and labor situations may be more favorable in the fall. Crop response field patterns to guide the soil testing are also fresher in mind. These considerations may be sufficient to offset loss considerations.

  4. The urea form of nitrogen, although readily soluble in water, is generally not subject to leaching because it is converted to ammonia and as such, held in the soil until nitrification takes place. However the urea form of nitrogen is leachable before the conversion to ammonia takes place.

  5. The release of nitrate nitrogen from organic matter does not take place until after the soil has been warmed and then reaches a peak release rate somewhat late in the season. This should be kept in mind when planning a nitrogen fertility program.

Recommedation Rates for Nitrogen

Every crop and variety of crop in any area has slightlydifferent nitrogen needs for the most economic response. Every area and each soil type within anarea will require different amounts of nitrogen per acre for best response for like crops. Farm and soil management, moisture control, weed control, insect and disease control, trash or residue to be plowed under, residual soil fertility, plus many other factors influence recommendation rates. The following example amounts reflect high yield values for unlimitiing moisture, 150+ day growing seasons, and good crop management.

Mint 100-300 lbs./acre Beans 40-150 lbs./acre
Field Corn 100-350 lbs./acre Sugar Beets 50-240 lbs./acre
Potatoes 100-400 lbs./acre Pasture 80-600 lbs./acre
Wheat 60-260 lbs./acre Barley 50-200 lbs./acre
Sweet Corn 100-300 lbs./acre Peas 30-160 lbs./acre

* Soil nitrogen supply should be definitely "tailing off" or getting low by time of crop maturity. Applications should be made with this factor in mind.

Nitrogen Charts

Irrigated Alfalfa Asparagus Beans
Sugar Beets Field corn (Silage or Grain) Sweet Corn
Hops Mint Onions
Improved Irrigated Pasture Irrigated Peas (Green or Dry Seed) Early Potatoes
Late Potatoes Dryland Winter Wheat Irrigated Wheat


Phosphorus in the soil and determination of its availability to plants is a very complex problem. It is hard to predict the effects of phosphorus fertilizers upon crops for all kinds of soils and for different growing season lengths. The satisfactory utilization of phosphorus is dependent not only upon the phosphate concentration, but upon the concentration of the other plant food elements, as well as soil temperature, moisture, pH, and the soil micro-organisms.

All soils have some phosphorus reserve in compounds of different chemical form such as phosphates of iron, aluminum, calcium etc., and though these reserves may be measured in large amounts in the soil, plants may still suffer from phosphorus deficiency. The natural release of phosphorus from these compounds may be severely limited due to certain physiological and biological conditions of the soil resulting in the continuation of the compounds in insoluble or unavailable forms of phosphorus.

Plants absorb phosphorus primarily in the form of ions of ortho or dihydrogen phosphate {H2PO4). The difficulty in supplying enough of this available form of phosphorus is that the reactions of soils tend to make water soluble phosphates into water insoluble phosphates, thus adding to the phosphorus reserves which are not as available to plants. Acid soils containing excess iron and aluminum, and basic soils containing excess calcium, cause a chemical recombination of acidic available forms of water soluble phosphates into forms less soluble.

Much of the soluble phosphorous is built into the bodies of the soil micro-organisms and subsequently becomes part of the soil humus. Therefore, supplying the phosphorus needs of plants is partly dependent upon the amount of phosphorus ions released from the phosphorus reserves by the bio-chemical processes of the soil. To supply enough phosphorus for plant needs, a reserve of phosphorus in excess of soil biological needs must be maintained, as well as proper soil conditions for maximum biological activity.

Phosphorus does not leach easily from the soil and normally is not lost unless the whole soil body is removed. However, there is evidence a minor amount of phosphorus movement occurs in our sandier soils.

Supplying Phosphorous

The addition of phosphorus to the soil may have a three fold purpose:

  1. To furnish an active form of phosphorus as a starter fertilizer for immediate stimulation of the seedling crop.

  2. To provide a continuing supply of available phosphorus for the crop during the entire growing season.

  3. To insure a good reserve supply of phosphorus in the inorganic or mineral, the organic, and the absorbed forms. (Phosphorus storage in all these forms is possible and desirable.)

The first objective is best insured by applying readily available, acidic, or soluble forms of phosphate at the time of seeding, as a band application, as a mild starter solution, or both. The second objective requires the deeper incorporation of adequate amounts of available forms to supply the crop through the growing season. The third objective can be reached in some soils (pH 6.5 or less, and with adequate organic matter level) by using the highly pulverized natural mineral forms such as rock phosphate. In other soils of more basic pH, and in some acid soils, the only material of appreciable value is acid formulations or soluble forms.

Determining Phosphorous Deficiencies

There are several chemical methods by which to determine the amount of phosphorus in soils in terms of availability. Currently we are employing the sodium bicarbonate (NaHCO3) method which is the current standard method adopted by the Soil Improvement Committee of the Pacific Northwest Universities and the fertilizer industry representatives. Other methods are under investigation, especially for acid soils.

Sodium Bicarbonate Phosphorous

The sodium bicarbonate (NaHCO3) method is based on the carbon dioxide reaction theory in which CO2 (carbon dioxide) given off by roots, soil organisms, etc., combines with H2O (water) to form H2CO3 (carbonic acid). This is weakly effective in dissolving alkaline phosphates, and there are some other chemical changes affecting calcium phosphates and other compounds which contribute to the results. The weak extraction of dilute NaHCO3 correlates rather well with crop response to acid phosphate fertilizers and the need for additional phosphorus on slightly acid (down to pH 6.5) soils and on slightly basic (7.0 + pH) to highly basic soils. In basic soils the phosphorus exists in large part as alkaline earth phosphates. Acidification of the soil or lowering of the pH tends to dissolve these and bring the phosphorus into solution. In acid soils the phosphorus generally has a significant fraction combined with iron and aluminum as phosphates. These exist when the pH is acidic. When the pH is raised toward the neutral point by the addition of an alkali element (sodium, potassium) or an alkaline earth (calcium, magnesium) these iron and aluminum phosphate compounds tend to dissociate, and the phosphorus becomes more available. This is the effect of a sodium bicarbonate test (extraction at pH 8.5) on acidic soils, and accounts for the large test values reported from acid soils.

In general phosphorus may be considered to exist as relatively insoluble compounds on both sides of a soil reaction point of approximately 6.6 pH. The solubility of the phosphorus in these compounds increases as the pH approaches this point, and this is the region of maximum phosphorus solubility and long term availability.

Strongly acidic soils have the major portion of the phosphorus as weak-acid-insoluble compounds which are soluble in basic solutions. As the natural pH of the soil rises proportionately less of these compounds are present and proportionately more of the alkaline phosphate compounds. These latter compounds are less soluble in basic solutions. As a result the critical values for crop needs as shown by the bicarbonate test will change as the soil pH changes. At a soil pH of approximately 6.2 or less a bicarbonate reading of 60-70 ppm P can be considered adequate for most crops and usually no further crop response can be detected from added phosphorus. As the soil pH rises, the critical value of the extractable phosphorus level will decrease. At the neutral point 1:20 bicarb extraction phosphorus test reading at 25-30 may be considered as adequate for most field crops with a high yield goal.

As a result of this change in critical values the bicarbonate test is difficult to interpret on acid soils and is usually replaced by the sodium acetate extraction. We do not recommend the use of the sodium bicarbonate test for naturally acidic soils with pH below 6.2.

For high value, high yield crops such as potatoes, maintenance applications of 80-100 lbs. P2O5 are recommended even for high soil test readings. However, if the soil test results indicate extremely high readings additional phosphorus applications may be detrimental because it may induce a deficiency of other elements, such as zinc.

The ranges given below indicate adequacy readings for low phosphorus demand crops to adequacy readings for most high demand crops. Some agronomists prefer to add 5-10 points to the ranges of values given for soils above pH 7.1 as an offset to expected reversion of added phosphorus during the growing season. This practice is not proven, but may have merit.

ppm P in Soil by Sodium Bicarbonate Test 1:20 Extraction Ratio
  Adequacy Readings
pH (Natural) Low P Demand Crops High P Demand Crops
6.0 33 75
6.2 31 70
6.4 28 65
6.6 22 60
6.8 16 40
7.0 12 27
7.2 11 25
7.4 12 31
7.6+ 13 35

The accompanying table (below) indicates relative ratings for different soil test reading.

Phosphorous Soil Test Rating Table1:20 Extraction Ratio - 7.4+ pH
Rating reading ppm P
Very Low 0-4
Low 5-9
Medium 10-15
High 16-40
Very High 41-60
Extremely High 61+

The next page contains phosphorous recommendation charts.

Phosphorous Recommendations


During the early years of commercial fertilizer use, nearly all fertilizer elements were in the sulfate form. Although not specifically planned for, the amount applied often exceeded the total amount of the three other elements combined. Also, sulfates were supplied to the soil by rain, snow and dust. Near industrial centers, where coal was burned for fuel, as much as 100 lbs. of sulfur per acre per year was measured, with a computed average of 15 lbs. to 30 lbs. sulfur per acre for most farms near northern industrial areas. Non-industrial areas received 3 lbs. to 5 lbs. sulfur per acre annually. Thus it was hard to visualize that sulfur deficiency would be a problem in many soils for some time. However, the use of highly concentrated fertilizers containing little or no sulfur, less use of manure, and conversion from coal to gas and electricity for fuel by industries and homes has drastically reduced the amount of sulfur supplied to soils; increased crop yields with great residue removal from the field and sometimes increased leach loss from irrigation have all combined to alter soil sulfur supplies.

An additional reason for sulfur testing in some areas is the high and increasing sulfur content of water used for irrigation. This may have an influence on soil maintenance and reclamation problems, especially as regards amendments for routine use.

The Sulfur Test Values

It should be remembered that sulfur, like phosphorus, is usually found in relatively small amounts in soils. Whereas, the phosphorus content is largely represented in the inorganic mineral form, sulfur is largely found in the organic form. As in the case of nitrogen, sulfur transformations are largely biological. They go on readily in most soils and the transformation may be indicated in a general way as follows:

Decomposition of Organic Matter
Organic sulfur, protiens and other organic combinations Decay products of which H2S and S2 are simple forms Sulfites SO3 Sulfates SO4

The last stages of sulfur oxidation are brought about largely by certain types of soil bacteria. The sulfate compounds that result become the available source for plant uptake. Because most sulfur-containing mineral materials are highly soluble and the sulfate portion subject to leaching, the best way of building sulfur reserves in soils is by adding all available organic materials and maintaining an adequate organic matter content.

Where satisfactory organic sulfur reserves cannot be maintained, certain fertilizers will have to be depended upon to supply the crop with its sulfate requirements.

Our laboratory has been analyzing soils for sulfur determinations. Approximate levels for most crops have been established with the following amounts of available sulfur offered as general guidelines:

Rating Test Result for Soluble Sulfate Sulfur-ppm S
  Irrigated Dryland
Very Low 0-3 0-2
Low 4-6 3-4
Medium 7-9 5-6
High 10-20 7-10
Very High 21-39 11-15
Extremely High 40+ 16+

Sulfur deficiencies, like other nutrient deficiencies, depend to a large extent on the crop yield. For high yield, intensive cropping, the test values need to be in the high reading range for the particular crop type; for lower yields a smaller supply of sulfur is adequate. The ratio of nitrogen to sulfur in the plant tissue may be a better indication of sulfur deficiency; but this can only be used to correct a deficiency on the current crop already growing or on next yearn crop. Obviously it cannot be used for fertilization at the previous planting time.

On the following page is a chart giving some rough guidelines for sulfur needs and amounts to add according to the soil test. You should also consider the distribution of sulfur in the profile to a depth of 3 feet. If the lower 2 feet have relatively high sulfur, the recommended amounts can be lowered, and only the seedling establishment needs consideration. If the lower depth is very low, you should consider slight increases in recommended amounts.

Sulfur deficiencies should be corrected on an approximate pound deficit to pound added basis. For ease and convenience in blending we suggest rounding a recommendation upward in units of 5 Ibs. per acre, rather than attempting application of an exact poundage per acre as shown to be needed by the test.

To convert ppm S to pounds per acre S multiply the ppm by 3.2 for sandy loam soils, by 3.5 for loamy sand soils, and by 4.0 for sandy soils.

Three considerations are involved in determining soil needs for three major cations:

  1. The Cation Exchange Capacity. Different soils hold by absorption different total amounts of potassium, calcium, magnesium, sodium, hydrogen and other cations. This total absorbed amount, called Cation Exchange Capacity, depends on the kinds and amounts of clay, silt, sand, and organic matter in the soil.

  2. The Ratio of these Elements. Change the ratios among the elements within this total to achieve an adequate blend of the three nutritionally important cations (K, Ca, Mg).

  3. The Degree of Saturation Desired. In most instances it is not necessary nor economically desirable to completely saturate the exchange complex with the changeable base elements. Economic response ceases when the crop need for nutrient base elements is satisfied or nearly satisfied. This normally occurs well before the saturation point of the exchange capacity is reached. A 75-90% saturation of the exchange capacity with a balanced ratio of exchangeable bases will usually be completely adequate for high yields of most crops.

Recommending Applications of Cations

Potassium, Calcium, and Magnesium

Potassium can be applied at approximately a pound of potassium for each pound of potassium deficiency.

Some difficulties have been experienced with single heavy applications of muriate potash (KCl), although potassium sulfate (K2SO4) applications of 1000 lbs. have been made without ill effects. To avoid zones of extreme salt concentration, not more than 400 pounds of potassium salt should generally be added at one time. Should heavy application be desirable, or necessary, it is well to split applications into two or three applications per year. If a single heavy application is to be made, it should be done well in advance of cropping and thoroughly incorporated into the soil.

Cation balance is involved along with exchange capacity. A balanced soil with 9600 lbs. of exchangeable Ca per acre (exchange capacity of 32 meq./100 grams) should contain about 800 lbs. Mg per acre and 650 lbs. K per acre. Or the milliequivalents of Ca should be 65-80% of exchangeable cations per acre, Mg 10- 15% and K 2-5%. Local soils, crops, conditions, and percent of saturation desired will influence final recommendations.

Some additional recommendation problems to consider are the reserve supplying power of the soil in question, the rate of exchange of the K in the soil reserve, and the test procedure used by the laboratory. There are differences between tests and between different soils in regards to the amount of K which will extract by the same test. These are questions which can best be answered by soil fertility specialists familiar with the area and not by a general purpose manual such as this. They are questions, however, which should be considered with regard to the best answers available.

The crop need for K should be considered, as well as yield goals and economics in view of these factors of reserve supply and applied amounts. In some instances it is not economic to maintain the soil at the ideal level of potassium. For these soils and crops a lesser saturation percentage must be decided upon as determined by the prevailing factors.

Alfalfa Clover Evergreen Shrubs
Asparagus Corn (grain) Flowers
Beans Corn (silage) Grass
Carrots Flowering Shrubs Hay (grass)
Beets, Sugar Flowers (garden) Pasture (grass)
Corn (sweet) Fruit (small) Shrubs
Field Beans Grain (small) Trees (fruit)
Golf Greens-Fairways Lentils Trees (shade
Grapes Milo Trees (nut)
Greenhouse Crops Mint Trees (timber)
Home Garden-Lawns Silage  
Hops Sorghum  
Melons Soybeans  
Small Fruits    
Vegetable Crops    

Potassium Recommendations

Soil Micronutrients

Trace Elements

There is a growing awareness of the need for knowing the trace element content of soils and plants. Although only required in small amounts by plants, their deficiency or toxicity can have just as much effect on crop production as any of the major elements.

The trace elements which are at present receiving most attention for which we are analyzing are boron (B), zinc (Zn), manganese (Mn), copper (Cu), iron (Fe), molybdenum (Mo), and in some instances chloride (Cl) and arsenic (As).

Other elements that have received some attention as affecting plant growth such as aluminum (Al), barium (Ba), chromium (Cr), cobalt (Co), fluorine (F), lead (Pb), nickel (Ni), selenium (Se), strontium (Sr), titantium (Ti), tungsten (W), and vanadium (V) may be included in the future for soil trace analysis.

The rate of availability of soil trace elements is a vitally important factor. The availability of trace elements is more susceptible than major elements to the influence of various soil factors such as pH, water availability, soil structure, and most importantly, the ratio of the other plant nutrients present in the soil. Therefore, it is necessary to determine which portions of the trace elements in the soil are available to plants and how much each plant species requires.

This measurement of availability and plant requirement is made more difficult by the small quantities of element being dealt with, usually in the parts per million range, but sometimes in parts per billion. Because of this, sampling and analysis should be done with the utmost care and precision. (See section on Soil Sampling Techniques for Trace Elements, and see section on Toxicity.)

Using Trace Element Analyses

Several considerations should be kept in mind when trace element analyses are being used.

  1. Very small amounts are required and the margin between deficiency and toxicity is quite narrow for some trace elements such as boron and molybdenum.

  2. Analysis for, or application of, minor elements should not be given serious consideration until the major elements are in balance.

  3. Toxicities or deficiencies can be induced by the following conditions:

  • When pH is rapidly changing up or down. (Large limestone applications, soil acidification).
  • Soil sterilization, both steam and chemical.
  • Irrigation water high in an element (i.e., boron) or material that accumulates by deposition or concentrates during soil moisture evaporation losses.
  • Application of fertilizer compounds which form soluble toxic substances.
  • Leached or accumulated spray material.
  • Guessing as to what is needed and applying a "shotgun" mixture.


Since there are many factors that affect the availability of micronutrients and as the levels needed by plants vary a great deal, only very general ranges of deficiencies or toxicities are given. As mentioned previously, the balance of the major elements and the pH can have a great effect on minor element utilization. To better interpret the test results the major element test should accompany the minor element analysis.

As more information becomes available from other sources, as well as the correlated data from our own soil trace element analyses, plant tissue analyses, and field response data, an attempt will be made to publish more exact ranges of soil micronutrients for various crops.

Your interest and assistance in this area will always be welcomed.

Following is a micronutrient rating schedule which can be used in evaluating soil test results for these elements. Very low or low readings would indicate a probably increased yield response to soil or foliar applications of the micronutrient. High or very high levels would indicate probably no additional yield response from application of the element. Additional applications, even at minor rates, may depress plant growth from toxicity of the applied element or by inducing a deficiency of another element. A soil analyses plus a plant tissue gives a bettor evaluation of micronutrient level than either one alone. (See section on plant tissue evaluation.)

  Sensitive Crops Tolerant Crops Acid Soils Alkaline Soils
Very Low <.20 <.30 <.05 <.05
Low .21-.50 .31-.60 .06-.10 .06-.20
Medium .51-1.00 .61-1.20 .11-.25 .21-.40
High 1.01-1.75 1.21-2.50 .26-1.00 .41-.90
Very High 1.76+ 2.51+ 1.10* 1.50
Toxic or Excessive 3.00 4.00 2.00 1.50
Very Low <.4 <.6 <.1 <3.0
Low .8 1.0 .2 5.0
Medium 1.0 3.0 .3 7.0
High 2.0 20.0 .4 20.0
Very High 3.0 30.0 .6 30.0
Excessive 3.0+? 50.0 3.0+? 50.0+?

Fertilizing With Micronutrients

Following are some representative recommendations on selected crops for rates and methods of application of microsnutrients. Recommend using lower rater or only on small observation plots if experience has not been gained in the area with a particular micronutrient.

  Rate of Element lbs. per acre Method of Application
Sugar Beets, Alfalfa 2-3 Soil Broadcast
Potatoes, Corn, Small Grain 1-2 Soil Broadcast
Deciduous Trees 1-3 Soil Broadcast
Beans Not recommended  
Corn, Beans 5-15 Soil Broadcast
Potatoes, Alfalfa, Wheat 5-10 Soil Broadcast
Beets, Peas 5-10 Soil Broadcast
Deciduous Trees 5-12 Dormant Spray
Corn, Beens 40-160 Soil Broadcast
Sugar Beets, Onions 20-80 Soil Broadcast
Corn, Small Grain 2-6 Soil Broadcast
Vegetables 1-8 Soil Broadcast
Deciduous Trees 1-2 Foliar Spray
Small Grain 1-3 Foliar Spray
Vegetables, Field Beans 1/2-1 Foliar Spray
Deciduous Trees 1/8-1/4 Foliar Spray
MOLYBDENUM Ounces per acre  
Peas, Sugar Beets 2-3 Soil Broadcast
Clover 1/2-1 Soil Broadcast
Pasture 1/4-1/2 Soil Broadcast

Graphs showing recommended rates of boron and zinc to apply to selected crops depending upon soil test values are given below.

Several micronutrient materials for each of the elements listed above with their approximate analyses are listed below.

Borax (Na2B4O7-H2O) 11.6% Boron
Solubor 20.9% Boron
Sodium Pentaborate 18% Boron
Fertilizer Borate-46 14% Boron
Fertilizer Borate-65 20% Boron
Boric Acid (H3BO3) 17% Boron
Boron-frited trace elements 2-6% Boron
Zinc Oxide (ZnO) 77.2% Zinc
Zinc Sulfate (Monohydrate) (ZnSO4-H2O) 35% Zinc, 18.6% Sulfur
Zinc Carbonate (ZnCO3) 52% Zinc
Zinc Sulfide (ZnS) 67% Zinc, 33% Sulfur
Zinc Chelates 9-14% Zinc
Manganese Oxide (CuO) 63% Manganese
Manganese Sulfate (MnSO4-3H2O) 27% Manganese, 14% Sulfur
Manganese Chelate (EDTA) 12% Manganese
Manganous Oxide (MnO) 48-61% Manganese
Manganese Chloride (MnCl2) 17% Manganese
Manganese Carbonate (MnCO3) 31% Manganese
Copper Oxide (CuO) 75% Copper
Copper Oxide (Cu2O) 89% Copper
Copper Sulfates 13-53% Copper, 12% Sulfur
Copper Ammonium Phosphate 32% Copper
Copper Chelate (Na2Cu EDTA) 13% Copper
Ferrous Sulfate (FeSO4-7H2O) 20% Iron, 11.5% Sulfur
Ferric Sulfate (Fe2(SO4)3-4H2O) 21% Iron, 14.5% Sulfur
Ferrous Oxide (FeO) 77% Iron
Ferric Oxide (Fe2O3) 69% Iron
Ferrous Ammonium Sulfate 14% Iron, 16% Sulfur
Ferrous Ammonium Sulfate 29% Iron
Iron Chelates 5-14% Iron
Sodium Molybdate (Na2MoO4-2H2O) 37-39% Molybdenum
Ammonium Molybdate ((NH4)6MoO24-4H2O) 54% Molybdenum
Molybdenum Trioxide (MoO3) 66% Molybdenum
Molybdnum Sulfide (MoS2) 60% Molybdenum
Molybdenum frited trace elements 2-3% Molybdenum
Percentage may very slightly depending upon source of raw materials and methods of manufacture.

Soil Moisture and Fertility Logging

The monitoring of soil moisture content and fertility levels at regular intervals on a crop during the growing season is a relatively new concept in crop production. This concept assumes there is an optimum level of moisture and plant nutrients in the soil and plant tissue for each stage of growth, and that the levels will change as the crop matures.

Levels of soil moisture and nitrogen and plant tissue nitrogen that have produced high yields and quality have been worked out for several crops. Examples of soil moisture, total profile nitrogen (1 foot ammonia nitrogen plus 5 feet nitrate nitrogen), and petiole nitrates for late potatoes and sugar beets sampled at 4 or 5 day intervals are given on the following two pages. For potatoes, sugar beets and most other field crops, moisture levels should be maintained between 70 and 90 percent of field capacity during the growing season. For seed crops such as grain, peas, beans, etc. the moisture level can be allowed to decline after the hard dough stage of seed development.

Optimum nitrogen level in the soil and plant tissue varies greatly with the crop and with the stage of development. For potatoes, about 250 lbs. of available nitrogen in the soil profile is adequate for germination and early development. Of this amount, at least 150 lbs. should be in the top foot. Enough nitrogen should be added during the growing season to maintain only a gradual decline during tuber development. Consumptive use of nitrogen during this development period is about 4 to 6 pounds of nitrogen per day. A carry over at harvest time of 75 to 100 pounds of nitrogen (15-20 lbs. per foot) is desirable. If the level drops below this amount, probably a deficiency and resulting yield depression occurred during final tuber development. Carry over levels much above this amount promote excess vine development, delayed maturity, and smaller tuber size which results in less yield. With these levels of soil nitrogen, the concentration of nitrate nitrogen in the petiole (petiole from fourth leaf back from tip of stem) will be about 28,000 to 32,000 ppm in the early development stage and will gradually decline to 6,000 to lOOOO ppm by the end of August. If either the soil or petiole nitrogen levels vary greatly from the proposed chart, the supplemental nitrogen additions can be increased or decreased. By evaluating both soil and petiole chemical analyses, better fertilizer application decisions can be made than by evaluating only one or the other. Observations indicate that those irrigation and fertilizer practices which produce maximum yield also give highest quality factors such as size, shape, high solids and least amount of such factors as regrowth, knobbiness and hollowheart.

For sugar beets, about 200 to 250 pounds of nitrogen in the soil profile is adequate for germination and early development. Again, the major portion of this amount should be in to the upper levels of the soil profile. This level should be maintained to be about 180 to 220 pounds on June 30. If leaching is kept to a minimum, this is adequate for the season to produce maximum yield. By August 30, the nitrogen level should be depleted to under 50 pounds In the profile (10 lbs. per foot) and by mid September nitrogen in the soil should be nil, under 4 lbs. per acre foot l ppm). Nitrogen levels above this amount will stimulate excessive top growth and depress sugar content without increasing yield. As with potatoes, those irrigation and fertilizer practices which promote maximum yield of sugar beets will also give highest quality as measured by sugar content

Optimum levels of nitrate nitrogen in the beet petioles (petiole of youngest manure leaf) at the beginning of the season, ranges between 2S,000 to 30,000 ppm. This should gradually decline to under 3,000 ppm by August 30 and under 800 ppm by mid September.

If soil or petiole nitrogen levels are much above these levels on a given date, consideration should be given to over irrigation to leach the excess nitrogen out of the root zone. If the nitrogen levels are substantially lower than these levels on a given date, a minor application of nitrogen can be made to promote a higher yield potential without depressing sugar content.

Timing and Application Methods for Soil

Fertility Materials

Fertilizer materials listed with pertinent points of application.

Broadcast Applications

  1. Liming materials or other pH correcting materials to achieve desired soil pH with proper balance of calcium and magnesium, such as dolomite limestone, calcite limestone, calcium sulfate, ferric sulfate, ferrous sulphate, flowers of sulfur, etc. apply prior to cropping and with thorough incorporation.

  2. Rock Phosphate - this material can be spread any time, even on a growing crop; use on distinctly acid soils only.

  3. Potash (muriate of potash, sulfate of potash, sulfate of potash magnesia, and in some cases, potassium nitrate) preferably well before planting to allow for diffusion and avoid salt stress, especially so with heavy application rates.

  4. Phosphates (super phosphate, triple super phosphate, phosphate in mixed fertilizers, ammonium phosphates) broadcast preferably prior to planting, or banding at planting time.

  5. Nitrogen Compounds, Dry or Liquid Forms (1) In fall materials containing ammonium or urea forms of nitrogen; ammonium sulfate, urea, etc. Ammonium nitrate fall application can be used effectively where winters are cool and rainfall is not more than about 14". (2) In spring: Same as in fall plus nitrate forms. Type of material and timing of applications should be planned to coincide with time and amount of maximum release desired. Split applications quite often give maximum return for dollars spent.

  6. Minor elements when needed to correct known sold deficiencies when added as liquid or dry material, or as foliar spray.

Row or Band Applications

Since row or band applications are almost always made at time of seeding, or early in the growing season, the primary purpose of row fertilization (that of supplying readily available plant food for stimulation of entire plant) should be kept in mind. Large applications (especially those causing high salt concentrations) may be detrimental to early plant growth. Dry fertilizer banded at seeding time should be placed 1 1/2 - 2 inches to the side and 1 - 2 inches below the seed. In some cases placing the fertilizer 1 to 2 inches directly below the seed has given the crop a earlier and better response.

Top, Side Dressing, or Split Applications

The lower the soil's exchange capacity (holding power), the greater the need for these practices; although plants may run short of nutrients during the growing season on any soil type due to many factors. Soils with very low exchange capacity, low moisture capacity, and generally low retention capacity may require repeated small applications during the growing season.

  1. Any material as needed during crop growth.

  2. Additional nitrogen.

  3. Trace element sprays.

  4. Injector solutions.

Fertilizer Treatment Problems

  1. Nitrogen

    • Higher amount of nitrogen required to achieve large yields can best be applied and utilized by split application.
    • When weather conditions are not conducive to good crop growth (cold-wet), nitrate nitrogen maybe most effective.
  2. Nitrogen-Phosphates. In situations where phosphorus uptake is low (causing poor fruit set, poor root development, etc.), ammonium phosphates or nitrogen phosphate mix solutions may be more effective.

  3. Potash. All forms of commercially available potash are readily soluble and available. The other element or elements contained in the material to be used may be the deciding factor as to the most desirable source of potash Such materials as potassium nitrate, potassium sulfate, potassium chloride, and sulfate of potash magnesia are all recommended materials (see appendix).

  4. Secondary, and Trace Elements. Use most effective method of application when troubles develop during the growing season. Foliar sprays may need to be used for maximum over-all results. Most micronutrient compounds are soluble. Check with your supplier. Response to soil application may be very limited during first year.

Fertilizer Application In Irrigation Water

Water soluble fertilizers, especially nitrogen, are effectively side-dressed through surface or sprinkler irrigation water. Timing and quantity per acre can be closely controlled for maximum response. However, the following factors must be considered.

  1. Only water soluble ancT compatible fertilizers may be used.

  2. Uniformity of application can only be as good as the uniformity of the water distribution and ire fiItratEon pattern.

  3. Corrosion of irrigation equipment may result unless fertilizer is applied as a very dilute solution {20 parts per million or less).

  4. Metering equipment, water control, and delivery equipment must be regulated h assure constant, steady delivery of a uniform solution.

Concentrated fertilizer solutions are frequently the most economic buy in fertilizers, and usually these are liquids. Application by injector rig in some instances is the only way to apply this material, and for some applications it is the best way. Conditions which uniquely favor injection include the following:

  1. Volatile material which must be placed under the surface to prevent or minimize losses.

  2. Liquid materials which are relatively insoluble in soil solution can be placed down into the root zone effectively by injection.

  3. Injection not only effectivity places the material in the root zone but can do so with a minimum of surface disturbance. This can be advantageous in both dryland and irrigated farming, for prevention of moisture loss while fertilizing, pasture renovation, side dress placement on row crops, etc., where surface application would delay crop response.

Where these conditions exist, they may be sufficiently important to warrant the application of a material by injector which would normally be applied by other methods. In the application of fertilizer for crop production there is no universal best methods or materials. This should be kept firmly in mind when applying, writing, or interpreting fertilizer recommendations. Local conditions or individual circumstances can alter any fertilizer program; amounts, methods, or kinds.

Factors Affecting Irregularities in Analyses

  1. Varied depth of samplings. For example normally a 24 inch depth soil sample will analyze substantially less for phosphate than a 12 inch sample depth.

  2. Combining unlike soil areas into one complete sample.

  3. Combining like soil areas with different past liming, fertilizer, or cropping histories into one composite sample.

  4. Combining an insufficient number of sub samples into composite from extremely varied or land-leveled fields.

  5. Attempting to use single composite sample for too large an acreage.

  6. Varying amounts of organic matter or undecomposed organic matter in sample.

  7. Soft rocks in sample

  8. Fertilizer or liming materials improperly applied or not thoroughly mixed in sol.

    • Material still on top of soil

    • Coarse materials not dissolved or not extract soluble.

    • Banded fertilizer applications not constituing a proper proportion of sample.

  9. Sheet erosion (wend or water) of material applied.

  10. Leaching of certain elements due to materials used, rates of application, or excessive water.

  11. Drought-too dry for fertilizers to dissolve and become part of soils system.

  12. Necessary soil microbes not present for proper release or conversion of fertilizers to available forms.

  13. Forced drying of soil sample at too high of temperatures.

  14. Soils that have been sampled with contaminated equipment, or dried, or processed in contaminated containers. Sample equipment hauled in a pickup with fertilizer or stored in a warehouse next to fertilizer has been a source of false high soil test results.

  15. Improper packaging of samples, allowing contaminants to become part of sample.

  16. Mixing sample identity, either field numbers or soil depths of profile samples.

Procedure for Taking Soil Samples

General Recommendations

If a soil test is to be a reliable guide for the addition of fertilizers or lime, the sample tested must represent the soil condition of the area sampled. The specific purpose of the test must be kept in mind and the completeness of the test desired. Read and follow applicable instructions carefully; the laboratory results will tell you only what is in the sample you send. It is the sample takers responsibility to take a truly representative and unbiased sample of the field area in question.

  1. Soils that differ in soil type, appearance, crop growth, or past treatment should be sampled separately, provided the area is large enough to justify fertilizing separately. A soil map can be of help in distinguishing areas and in recording location of sample.

  2. Several different tools such as an augur, a soil sampling tube, or a spade may be used in taking soil samples.3. Scrape away surface litter. if an auger or a soil sampling tube is used, obtain a small portion of soil by making a boring about 7-12 inches deep, or if plowing or tilling deeper, sample to plow depth. If a tool such as a spade is used dig a V-shaped hole to sample depth; then cut a thin slice of soil from one side of the hole.4. Avoid area or conditions that are different, such as areas Where fertilizer or liming materials have spilled, gate areas where livestock have congregated, poorly drained areas, dead furrow, tillage or fertilizer corners, or fertilizer band areas of last year's crops. It is also advisable to stay at least 50 feet from barns, roads, lanes, or fence rows.

  3. Because of soil variations, it is necessary that each sample consist of small portions of soil obtained from approximately 20 locations in the soil area, as illustrated by the diagram on the following page. After captaining these portions of scrip, mix them together for a representative sample.

  4. If recommendations are desired, fill out the "fertilizer recommendation questionnaire" as completely as possible since this information is very important in making recommendations. Be sure the sample numbers on the "analyses requested" form correspond with the numbers on the sample bag. Where soil is very varied, and especially where land leveling has been done, or erosion and deposition are severe, the field should be checked on a Soil Variability Grid Test.

Sampling for Soil Variability Grid Test

If variability proves to be great enough to be a problem in attaining uniform fertility for uniform crop response, then serious consideration should be given to mapping poor response areas as the crop grows. When the crop is off or dormant the field should be checked on an incremental or grid basis against the response map to clearly outline and determine the fertility status of poor or low areas. Then fertilization of inadequate spots should be carefully done to bring the field to a uniform fertility pattern.

When sampling for the grid, consideration should be given to carefully outlining of problem areas. This may necessitate deviating slightly from an absolutely uniform pattern. Careful grid sampling will necessitate samples from relatively small acreages (2-4 acres) and with spacing varied to check the map of known poor spots.

An example of grid sampling and mapping is shown on following page.

Recommendations for Taking Soil

Samples for Full Profile Tests

Variation in soil test results can be moderate to extreme due to nonrepresentative samples taken from a field. This variation can be due to one of the following: (1) Natural fertility variation from area to area in the fields; (2) Previous injections of fertilizer in bands; (3) Differential depletion of elements in rows verses center of rows; (4) Differential leaching of elements in irrigation furrows verses hills, etc.

Following is a sample procedure designed to average out this variation We recommend sampling at least 10 locations per field and at least 12 cores per location of the first foot. For the second foot, take 2 to 3 cores at each of the 10 locations, and for the sub soil (3, 4, 5, 6, foot) 1 core at each of the 10 locations. Each foot is kept in a separate container.

At each sample location, a line of holes should be taken perpendicular to present or previous crop rows, irrigation rills, fertilizer injection bands, etc. The line of holes should equal the spacing between rows, rills or bands. For example, if sampling is in a field previously cropped to corn with 36 inch rows, the line of 12 holes should be a hole every 3 inches for 36 inches perpendicular to the previous rows. If sampling a wheat field previously fertilized with anhydrous ammonia with 18 inch shank spacing, the line of 12 holes would be a hole every 1 1/2 inches for l8 inches perpendicular to the fertilizer injection bands.

To facilitate handling of soil after sampling the 12 cores at each sample location the soil for first foot can be mixed and reduced to l/10 volume and kept in a separate container. After sampling the 10 locations, the soil for each foot is thoroughly mixed and reduced to desired volume to send to the laboratory. For complete analyses a one-pound sample is adequate for all tests, and for a basic test only a one-half pound sample is adequate.

Sampling Soil For Micronutrient Analysis

Due to the greater possibility of contamination in soil sampling for micronutrient analysis, some special precautions are suggested:

  1. Where zinc, irons or copper analyses are desired, care should be taken not to use any galvanized, soft steel, or brass equipment (sampling equipment or containers).

  2. Be extremely careful to avoid any fertilizer dust contamination.

  3. The safest general equipment found for micronutrient sampling consists of stainless steel soil probe, clean plastic bucket for placing and mixing soil cores, and plastic bags for drying and shipping samples. Clean cloth bags or paper sacks have been reported as being satisfactory. However, we do not recommend their use, unless plastic lined.

Sampling Soil For Nitrate And Ammonia Nitrogen

  1. Rapid changes in nitrates and ammonia occur after taking a soil sample when the sample is stored moist and warm. If several days will elapse before analysis, the sample should be immediately dried at 40 degrees - 50 degrees C. (100 degrees- 110 degrees F) and then mailed. Normally, if wet samples arrive at the laboratory within 2-3 days changes in nitrogen is nil.

  2. Deeper depth sampling is required to effectively determine the total of available nitrogen. Sampling to a 6 foot depth at 1 foot intervals is recommended where possible. See "Recommendations for taking Soil Samples for Full Profile Tests".

  3. Where rill or furrow irrigation is practiced and a nitrogen test is desired, make composite samples separately for each depth. Take samples in a line at right angles to the furrow, extending one-half the distance between furrows. Take at least 12 equally spaced sample locations: for example, 3 in the furrow bottom, 6 up the shoulder, and 3 on the row crown.

  4. Where banding or injection is practiced, sample as for rill irrigation.

Special Soil Problems

Low pH (Acid Soils)

Strongly acidic soils pose a problem for many crops. There are three major facets of the problem which should be considered.

  1. Some crops, regardless of the supply of available cations, just do not do well when the soil reaction is acidic. There are plant physiological problems involved in which the plant metabolism or chemistry is unable to function properly. For those types of crops which must have a more nearly neutral pH the only answer is to increase the pH or grow a more adaptable crop.

  2. In many soils with very tow pH there is, simply, a drastic shortage of the needed cations, resulting in deficient crops and poor growth. Where this is the case, banding of cations to achieve partial saturation of the exchange complex may furnish adequate nutrients. It is a case of supplying the crop with the needed amounts of plant food.

  3. The solubility of some elements is increased in low pH soils, while others decrease. This may pose problems of toxicity and of deficiency. Manganese toxicity may be a problem at low pH, while the problem with molybdenum may be insolubility and deficiency. The most favorable range in terms of availability of plant food elements is the pH range of 6.2to 6.8. However, a pH range of 6.0 to 7.8 is satisfactory for most field crops. At this range the problems of toxicity, solubility, etc., are most nearly balanced off, for the majority of soils. Molybdenum is a very slight problem occasionally, but seed treatment, spray or soil treatment will correct the problem. The problem is most pronounced on seedling legumes. Sometimes as little as 2 oz. per acre seed treatment is sufficient. One pound ammonia molybdate per acre as spray or soil treatment is usually adequate. Be careful to guard against toxicity to livestock from ingestion of treated forage, and especially so if using sprays.

If acid sensitive crops are to be grown on soils with a pH below 6.0, a test should be run on the soil to determine the amount of lime required to bring the pH up to a satisfactory level. The lime requirement may be up to 6 tons per acre depending upon how acid the soil is, soil texture, cation exchange capacity, and present base saturation desired.

High Salt Concentrations

Excessive concentrations of various salts occur naturally in many soils as a result of the weathering of parent material high in the contributing cations or anions. Other soils are developing or have developed high salt concentrations from irrigation or ocean spray drift and continual accumulation.

One effect of high soil salt concentration is to produce water stress in the growing crop. The crop may wilt or even die from drought while the soil is apparently moist or even wet The osmotic or absorbing power of the plant for water is too slight to remove the water from the salty solution. Accumulation of salt in the plant adds to the physiological problem. The measure of the salt problem is the soluble salt reading in millimhos per centimeter (abbreviated mmhos/cm}. The following illustrates some common problem ranges and crop salt tolerances.

0-2.0 mmhos/cm Salinity effects mostly negligible.
2.1-4.0 mmhos/cm Yields sensitive crops may be restricted.
4.1-8.0 mmhos/cm Yields of most crops restricted.
8.1-16.0 mmhos/cm Only salt tolerant crops yield satisfactorily.
16.1 mmhos/cm and over Only a few very tolerant crops yield satisfactorily.

Different crops vary in their tolerance to salt concentrations in the soil. Also, most crops are more sensitive to high salt concentrations in the germination and seedling stage of growth, and more tolerant to high salt concentrations as the crop matures. Therefore, if the excess salt can be leached by irrigation 2 to 6 inches deep prior to seeding, some crops may establish themselves and tolerate a higher salt concentration in the subsoil. Following is a list of crops with low, medium, or high tolerance to salt concentrations in the root zone.

Relative Salt Tolerance of Selected Crops
Low Medium High
Beans Grape Beets
Strawberry Cantaloupe Asparagus
Celery Tomato Spinach
Alsike Clover Corn Salt Grass
Ladino Clover Potatoes Wheat Grass
Red Clover Alfalfa Barley
Radish Wheat Birdsfoot Trefoil
  Sweet Clover Cotton
  Brome Grass Rape

Associated with many salt problems is a problem of sodium. The sodium tends to displace the other cations on the exchange complex, to accumulate In the soil solution, and to interfere internally with the plant physiology. The sodium problem is more complex. Sodium may exist in soil either as free salt or as part of the exchange complex. Free sodium will leach readily or wash from the soil. Exchange absorbed sodium must be displaced by another cation, such as calcium. Gypsum (calcium sulfate) is commonly used for this purpose. Prerequisite for gypsum applications to reclaim an alkali soil is good drainage, followed by leaching with excess water by flooding or sprinkling.

Sodium content of soils should not exceed 2-3 meq/7100 g or 15% of the cation concentration depending upon the particular soil and situation.

Soils are classified by salt content as follows:

Non-Saline - Non-Alkali Soluble salt conductivity less than 4 mmhos/cm, exchangeable sodium less than 15% of exchange capacity.
Saline - Non-Alkali Soluble salt conductivity more than 4 mmhos/cm, exchangeable sodium less than 15% of exchange capacity. The soil pH value is normally below 8.5.
Non-Saline - Alkali Soluble salt conductivity more than 4 mmhos/cm, exchangeable change capacity.
Saline - Alkali Soluble salt greater than 4 mmhos/cm, exchangeable sodium greater than 15% of exchange capacity.

When requesting soluble salt and sodium the laboratory normally determines a total sodium. Be sure to designate your request as exchangeable sodium if desired for reclamation. Be sure to keep the type of analysis in mind when interpreting the test results or making reclamation recommendations.


Several elements and many commercially prepared compounds are very toxic to plants, animals , or both. The best advice is to follow manufacturers directions carefully on commercial compounds and to check to find out how much if any has previously been applied to the soil in question.

The major naturally toxic elements are arsenic, boron, manganese, selenium, and molybdenum.

Arsenic in soil is usually non-toxic to plants at 5 ppm or less, becomes toxic to sensitive seedlings at about 20 ppm, and is lethal to replants of perennial material at about 80 ppm, although established, well-rooted crops may continue to survive and not show damage.

Selenium may be required in small amounts in animal nutrition, but is very toxic in larger amounts. One part per million or more, of soluble selenium in soil can be considered to be dangerous to animals being fed the forage grown on the soil.

Boron should always be checked as part of a salinity diagnosis, especially an arid soils or any soil receiving supplemental irrigation.

Manganese is most generally toxic on soils of pH less than 5, and is not generally a serious problem on soils of near neutral (6.5), or more basic pH. Manganese becomes much less soluble as the pH increases from the strongly acid side, and this solubility decrease is often the major dividend from line application.

Molybdenum exhibits a reverse action; as pH rises the molybdenum becomes soluble. Where mild molybdenum deficiencies exist, lime applications often overcome the problem. Other acidic soils often contain sufficient molybdenum that toxic forage results when liming is done. Some alkaline soils also contain sufficient molybdenum to produce toxic forage.

When potentially toxic trace elements are being considered for application, the soil in question should be thoroughly analyzed and the side effects for future crops as well as the current or next crop should be carefully considered. It cannot be emphasized too strongly that over-application of these elements can be very difficult to rectify, but when used wisely on deficient soils they are a tremendous crop production assist.

Plant Tissue Analyses

Many times plant analysis provides useful information in diagnosing nutrient deficiencies in the plant. However, research information is limited in providing necessary references that have been correlated with field response. Plant analysis can vary greatly with tire stage of growth at sampling time, as well as the plant part sampled. It is, therefore, necessary to sample the tissue at the indicated plant maturity listed in the tables below in order for interpretation tone effective. In addition to a tissue analysis, a soil test will many times confirm the nutrient deficiency and provide useful information in diagnosing problem areas.

Some general principles should be observed when plant sampling. Do not sample diseased or mechanically damaged tissue. Do not sample tissue that has been sprayed. Your~g leaves' old- leaves and seeds are not usually representative of the nutrient status of she whole plant Tissue should be placed in a clean paper bag or envelope and mailed to the laboratory. E)o not seal tissue in a plastic bag since decomposition is hastened without air. When no samplir!g instructions are given a good rule of thumb is to sample the upper most recently matured tissue.

Tables giving some tissue analyses to use as a guide to interpret the nutrient status of several field and fruit crops are on the following two pages. These values are compiled from data reported from other areas and our own laboratory results.

If the analyses is in the low range, you could expect a growth response to the addition of that element. If the analyses is in the high range, you would normally not expect a growth response by applying additional amounts of the element. Reading in the intermediate range are results from crops with normal growth and yields. These values will be revised as more data becomes available.

Interpreting Soil & Plant Tissue Tests by Marr Waddoups, Agronomist COPYRIGHT 2019