| Previous | Table of Contents | Next |
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 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° F. The process is as follows: Ammonium (NH4) to nitrite (NO2) to nitrate (NO3).
(C) 2003 Northwest Argicultural Consultants. Back to Main Page