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Soil Science-Fertilizer Reccomendations

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Fertilizers are substances that supply plant nutrients or amend soil fertility

They are the most effective means of increasing crop production and of improving the quality of food and fodder. Fertilizers are used in order to supplement the natural nutrient supply in the soil, especially to correct the (yield-limiting) minimum factor. Fertilizers are soil amendments applied to promote plant growth; the main nutrients present in fertilizer are nitrogen, phosphorus, and potassium (the ‘macronutrients’) and other nutrients (‘micronutrients’) are added in smaller amounts. Fertilizers are usually directly applied to soil, and also sprayed on leaves (‘foliar feeding’).

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Soil testing measures the relative nutrient status of soils and serves as a foundation for profitable and environmentally responsible crop mineral nutrient application. Laboratory analysis quality control influences soil test value accuracy, but the quality of the soil sample may have an even greater influence on accuracy. Sample collection is extremely important in the accuracy and repeatability of a soil test. Sample handling following field collection also is important. An unrepresentative soil sample may be misleading and may result in over- or underapplication of fertilizer

Therefore, collecting and handling soil samples properly is very important.Many growers and landowners wish to track the trend of soil tests for several years. Therefore, obtaining soil samples for K analysis at the same time each year is important. Soil EC (electrical conductivity) is a measure for soluble salt content, and its value usually varies during the sampling season in response to rainfall and evapotranspiration. Soil samples for nitrate-N analysis may be taken immediately after any grain harvest. Twenty years ago, NDSU recommendations included a sampling date adjustment for sampling fields before Sept. However, close inspection of data generated in North Dakota by sampling the same site from spring through fall, and then again the following spring, found that sites sampled after small-grain harvest sometimes increased in nitrate-N, sometimes decreased in nitrate-N and sometimes stayed the same. Therefore, the sampling date adjustment is not predictive and no sampling date adjustment has been included in NDSU fertilizer N recommendations since about 2005. Sampling behind the combine is more important. Early sampling results in the ability to sample more fields due to more favorable sampling conditions (a better soil core) because fields are not tilled prior to sampling. Early sampling allows the soil test results to be used that year in fall N applications. Sampling before tillage also results in a much better 0- to 6-inch core for P, K and other nutrients and soil factors, rather than allowing tillage depth guesses to confound the depth.

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Many times we think of organic matter as the plant and animal residues we incorporate into the soil. We see a pile of leaves, manure, or plant parts and think, "Wow! I'm adding a lot of organic matter to the soil." This stuff is actually organic material, not organic matter. What's the difference between organic material and organic matter? Organic material is anything that was alive and is now in or on the soil. For it to become organic matter, it must be decomposed into humus. Humus is organic material that has been converted by microorganisms to a resistant state of decomposition. Organic material is unstable in the soil, changing form and mass readily as it decomposes. As much as 90 percent of it disappears quickly because of decomposition. Organic matter is stable in the soil. It has been decomposed until it is resistant to further decomposition. Usually, only about 5 percent of it mineralizes yearly (Barber, S. A., 1984). That rate increases if temperature, oxygen, and moisture conditions become favorable for decomposition, which often occurs with excessive tillage. It is the stable organic matter that is analyzed in the soil test

An acre of soil measured to a depth of 6 inches weighs approximately 2,000,000 pounds, which means that 1 percent organic matter in the soil would weigh about 20,000 pounds per acre. Remember that it takes at least 10 pounds of organic material to decompose to 1 pound of organic matter, so it takes at least 200,000 pounds (100 tons) of organic material applied or returned to the soil to add 1 percent stable organic matter under favorable conditions. In soils that formed under prairie vegetation, organic-matter levels are generally comparatively high because organic material was supplied from both the top growth and the roots. We don't usually think of roots as supplying organic material, but a study in the Upper Great Plains showed that a mixed prairie had an above-ground (shoot) yield of 1.4 tons of organic material per acre, while the root yield was about 4 tons per acre. The plants were producing roots that were more than twice the weight of the shoots (Tisdale, S. L., 1975).

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As shown above, the accuracy of a fertilizer recommendation depends on how well the soil sample on which the recommendation was based represents the area on which the recommendation will be used. The physical and chemical characteristics of soil in an area can vary considerably from place to place because of natural factors and the management to which the area has been subjected.

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Barber, S. A. Soil Nutrient Bioavailability: A Mechanistic Approach. New York: Wiley, 1984.

Brady, N. C. The Nature and Properties of Soils. New York: Macmillan Publishing Co., 1974.

Plaster, E. J. Soil Science and Management. 3rd ed. Albany: Delmar Publishers, 1996.

Tisdale, S. L. and W. L. Nelson. Soil Fertility and Fertilizers. 3rd ed. New York: Macmillan, 1975.

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