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Fluid Journal : Fluid Journal 1996-1998
Winter 1997 agronomists about how to interpret soil tests. Some agronomists believe in replacement of nutrients withdrawn by crops. Others believe in maintaining a sufficiency level, or the level of nutrients required to achieve a certain yield goal. My opinion is that both these philosophies have problems. Replacement. While replacement of nutrients has merit as a basis of maintaining fertility levels, it is more important to build soil fertility to a level that will support higher populations, thus increasing yield and profits for the farmer. Recommending a nutrient replacement program can be a double-edged sword. First, a nutrient that is already adequate in the soil profile may be recommended. Second, simple replacement won't build fertility. At best, it will maintain it. Compounding this further are inherent nutrient losses in the soil (e.g., denitrification), which the grower seldom realizes. The strategy of nutrient replacment or maintaining fertility is a regressive approach to high-yield crop production. The success of U.S. agriculture has been a continual increasing of yields and reducing costs per bushel via a continual buildup of soil productivity. Sufficiency. The strategy of sufficiency should also be approached with caution. Sufficiency of each nutrient needs to be defined for each set of conditions including: hybrid, proposed population, grower management ability, climate, and yield goal. What are the sufficiency levels for NPK to raise 250 bushels of corn per acre? Answers can be extremely complex. The biggest problem with sufficiency, however, is in defining the levels of nutrients for a stratified soil. Shouldn't the sufficiency level be high enough in the 3- to 12-inch zone to support the more extensive root mass of the higher population required to produce a high, profitable yield? It is my opinion that nutrients should be at a high level throughout the soil profile to support and encourage this deep root growth. There is considerable evidence from research that concentration of nutrients in the soil solution should be higher as plant populations increase. Theory is: as plant populations increase, root mass becomes more and more restricted. Demand for nutrients by each plant rapidly depletes the soil solution. An adequate reserve must be available to replenish the extra nutrients withdrawn Figure 1. Stratification of phosphorus and potassium in a Sharpsburg soil, University of Nebraska. Table 1. Example of stratified nutrients in soils with different tillage methods, Nicollet soil, Iowa State University. Phosphorus (ppm) Depth Moldboard Chisel No-till Zone A (0-3") 29 55 62 Zone B (3-12") 25 30 17 Potassium (ppm) Zone A 130 235 240 Zone B 120 125 120 Table 2. Example of stratification on a western Iowa dryland, clay loam soil. Organic Ppm Depth Matter NO3-N P KSZn ____________________________________________________________ A (0-3") 2.8% 23 84 231 8.0 4.6 B (3-12") 1.7% 7 15 135 0.1 2.0 C (12-24") 2.2% 6 9 146 0.1 2.0
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