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Fluid Journal : Fluid Journal 2002-2004
2 Fluid Journal Spring 2002 Figure 2. Average P use on corn and soybeans relative to crop removal in the U.S. Figure 3. Percent of soils testing medium or below in K in 2001. Figure 4. Average K use on corn and soybeans relative to crop removal in the U.S. P removal by 22 commonly grown crops was only 60 percent of P inputs (fertilizer + recoverable manure). This resulted in build-up of soil test P in many regions of the U.S., especially the Corn Belt. Since 1970, the removal-to- use ratio has trended consistently higher and is now over 0.90 for the U.S. as a whole. If only manure applied to corn, soybeans, wheat, and cotton is considered in the estimate of P inputs instead of total recoverable manure, P removal in the U.S. exceeded use by approximately 20 percent in the year 2000. Since much of the U.S. corn crop is grown is rotation with soybeans, average use of P on corn and soybeans is a meaningful expression and is graphed in Figure 2. The USDA-ERS estimate of manure P applied to corn and soybeans is included. The analysis indicates that P removal has been exceeding use since the late 1980s, with the deficit growing each year. In 2000, the average deficit was over 10 lbs/A of P2O5. This is corroborated by the independent estimate of soil test trends discussed earlier that indicates soil test P levels in the Corn Belt started declining shortly after corn/soybean P budgets became negative. Current K status The median ammonium acetate equivalent K level for North America is 155 ppm. Over 50 percent of the soils test below 160 ppm K, and over a third test between 120 and 200 ppm. This distribution is compelling evidence of the need for proper and regular soil testing to carefully monitor soil K status. Many soils test near or below what most calibration research indicates is a critical level for crop response. This is especially true in the North central region where nearly 70 percent fall between 80 and 200 ppm. This frequency distribution may also help explain why we often observe interactions between K and other growth factors such as variety or hybrid, tillage, and weather. As with P, clear regional differences exist. In contrast to P, the highest median K levels occur in the Great Plains and western states while the lowest levels occur in the eastern
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