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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.
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.
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