Soil Science-Fertilizer Reccomendations
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’).
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.
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).
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.