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Fluid Journal : Fluid Journal 2002-2004
3 Fluid Journal Spring 2002 regions. For example, in the northern Great Plains less than 20 percent of the samples are below 160 ppm, while in the northeast and southeast more than 70 percent are below 160 ppm. For North America, 43 percent of soil samples analyzed test medium or below in K. Once again, considerable variation exists among states and provinces (Figure 3). Western states and provinces generally have fewer soils in the medium or below K categories than those in the East. The higher K levels of the West reflect the less weathered status of western soils. However, in states such as California, where 44 percent of soils test medium or below in K, crop removal over several decades with limited nutrient addition has significantly reduced soil K levels. Historical trends Historical trends in soil test K levels differ markedly among major corn producing states. The trend for the eastern three states (Illinois, Indiana, and Ohio) is very similar to the P trend observed for these states. Percent medium and below in K reached a low point in 1989 and has been steadily increasing ever since. Currently, over 60 percent of samples test medium or below in K. This is in striking contrast to Iowa and Minnesota, where percent medium and below has been steadily declining in linear fashion from 1968 to 2001, suggesting a steady build-up of low and medium testing soils into the high or very high categories. In 2001, only 26 percent of the samples from these two states tested medium or lower in K, which in this case means below 120 ppm. In some respects, this observation appears in conflict with recent field observations from this region of an increase in frequency of K deficiency symptoms in corn and soybeans. However, many soils from the region test between 120 ppm and 200 ppm where K availability problems have been observed. For Minnesota and Iowa, 53 percent of samples test below 160 ppm, and 78 percent test below 200 ppm. Furthermore, ongoing studies by Iowa State University suggest that the existing soil test procedure and associated interpretation may overestimate the K-supplying capacity of some soils. Percent medium or below in Nebraska has been stable throughout the summary period at 5 to 10 percent. Percent of soils testing medium or below in K may be trending upwards slightly in the northern Great Plains (Alberta, Saskatchewan, Manitoba, South Dakota, and North Dakota) and downwards slightly in Idaho, Oregon, and Washington, although very little change in any direction has occurred in these two regions for the last several summaries. Although variable, an upward trend has occurred in California, increasing from 20 percent in 1986 to approximately 45 percent in 2001. A slow decline in soil test K levels would be expected in much of the West as crops continue to remove K that is not replaced by fertilization. K removal Potassium budgets of eastern Corn Belt states have removal-to-use ratios that run higher than P, close to 1.0 when recoverable manure K is considered. In spite of this, some of these states such as Illinois and Indiana appear to be showing trends of declining soil test levels. This should not be surprising since these budgets are calculated at a statewide level and can mask significant nutrient distribution problems. In addition, no account was made for soil erosion losses or potential loss of plant- available K to unavailable forms in the soil. When the USDA-ERS estimate of manure applied to corn and soybeans is included, K removal by these crops exceeds use in the vicinity of 1997 (Figure 4). Considering 50 to 70 percent of the soil samples from the leading corn states (Iowa, Minnesota, Illinois, Indiana, Ohio---Nebraska is excluded) are less than 160 ppm in soil K, it would seem that continued mining of soil K would be detrimental to crop yields and profitability of the region. Question It is plain to see that crop removal of P and K is increasing because yields are increasing. Do we know enough about optimum soil test levels and nutrient needs at high yield levels in the cultural systems and fields of today? Dr. Fixen is senior vice president and Dr. Murrell is north central director of the Potash and Phosphate Institute.
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