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Fluid Journal : Fall 2010
appropriate. The objective of build- maintenance fertility programs is to manage soil test levels rather than trying to predict precisely how much P and/or K would be required for optimum crop production in a given year for a particular situation. At low soil test values, build- maintenance programs are designed to increase soil test levels to a desired soil test value (e.g., the critical value) over a specified time frame, and then maintain soil test levels within a targeted maintenance range. The identified maintenance range generally lies just above the critical soil test value. No fertilizer is suggested at soil test values greater than the maintenance range. In Figure 1. Iowa State University and Kansas State University P Correlation Data general, crop yield will be about 100 percent of maximum yield, and the risk of yield loss due to insufficient fertility is minimized with this approach. The nutrient sufficiency approach generally suggests lower P and/or K application rates in the early years of adoption (if soil test values are low initially) and will eventually approach crop removal as soil tests equilibrate in the crop responsive range. Build- maintenance rates will generally be higher in the initial years (if soil test values are low initially) until soil test values are increased to the desired soils test value when crop removal application rates maintain soil test values in the desired soil test range. Some states slowly build soil tests to a soil test value near (but below) the critical value and then maintain them with crop removal maintenance rates. Other states may have a different variation with portions adapted from both the nutrient sufficiency and build-maintenance approaches. Geographic regions. Why do different institutions/people adopt different approaches to P and K fertilization? Is it because crops respond differently in different geographic regions, making one approach better than others for a given region? Not really. Actually, soil test correlation research conducted across wide geographic areas provides very similar results if the soil sampling depth is the same. For a specific P or K soil test procedure that is appropriate for a given geographic area, similar long-term average relative yields at various soil test values have been found. Figure 1 uses P correlation data from Iowa State University and Kansas State University to illustrate this point. While there are significant differences in soils and climate between these two states, the resulting crop response research data are very similar. The general interpretation of Bray P1 soil test values in the heart of the Iowa Corn Belt is the same as that on the Great Plains of Kansas. Similar conclusions result for other comparisons across the U.S. and Canada--if the same soil test extractions/procedures are used. The science is essentially the same: it is the interpretation of the science that often varies. Critical value. While the established P critical value for Iowa State and Kansas State has been set at 20 ppm Bray P1 (Figure 1), others have generally set the critical values anywhere between 15 and 25 ppm Bray P1. These differences may result from the specific mathematical model used to describe the relationship between soil test and relative crop yield and other subjective factors such as an allowance made recognizing that fields exhibit spatial and temporal variability in soil test values. And although not explicitly recognized, those developing soil-test-based fertilizer recommendations introduce their own bias concerning the best approach for interpreting response data and managing P and K fertility programs. It is not so much a difference in research data that causes differences in approaches to Fall 2010 The Fluid Journal 10
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