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Fluid Journal : Fluid Journal 2002-2004
1 Fluid Journal Spring 2002 Figure 1. Percent of soils testing medium or below in P in 2001. In the past year, the Potash & Phosphate Institute completed a comprehensive evaluation of P and K levels in North American soils, as well as the rate at which these nutrients are removed from the soil by U.S. crops. Data in its soil test summary were provided by 34 public and 31 private laboratories and consisted of 2.5 million soil samples collected for the 2001 crop year. The purpose of this article is to highlight some of the more important findings. Current P status The median soil test P level for North America is 28 ppm Bray P1 equivalent, indicating that 50 percent of soil samples are less than this amount. Soil test P levels show a skewed frequency distribution with a broad peak running from 5 to 30 ppm and accounting for over 50 percent of the samples. Another 24 percent test greater than 50 ppm. The lowest median P levels are found in the Northern Great Plains while the highest occur in the Northeast. Regional cumulative frequency distributions show a marked trend of increasing P levels west to east across North America. Nearly 80 percent test below 20 ppm in the Northern Great Plains, while only about 10 percent test below 20 ppm in the Northeast. Across all of North America, 47 percent of the samples are medium or below in P. (The traditional protocol for the summary is to use the laboratory's definition of medium; therefore, it is not numerically consistent across North America.) As expected, considerable variation exists among states and provinces (Figure 1). The northern Great Plains has the highest frequency of medium or below P tests, with values in the 60 to 90 percent range, while a few states in the Northeast are below 20 percent. Drs. P.E. Fixen and T.S. Murrell P and K Reserves Growing Thinner Do we know enough about optimum soil test levels and nutrient needs at high yield levels is the question being asked. Historical trends Soil test levels in the Great Plains have been quite stable since the summaries were initiated: 70 to 80 percent medium or below in the north (Alberta, Saskatchewan, Manitoba, South Dakota, and North Dakota) and 50 to 65 percent in the south (Kansas, Nebraska, and Oklahoma). These data indicate that P nutrition remains an important yield -limiting factor in much of the Great Plains. In 1975, slightly over 50 percent of the samples in the summary were medium or below in the six leading Corn Belt states (Iowa, Illinois, Indiana, Minnesota, Nebraska, and Ohio). As farmers built soil test levels, percent medium or below declined to approximately 40 percent in 1989. Since then, the trend has reversed and these six states are again approaching 50 percent medium or below, similar to the 1975 level. In contrast to the stable or currently declining soil test P trends in the Great Plains and Corn Belt states are the currently increasing trends in states with large manure supplies relative to crop P demand (North Carolina, Michigan, Wisconsin, Pennsylvania, and Arkansas). Of the five, Arkansas has experienced the most dramatic increase in P test levels with percent medium or below dropping from nearly 90 percent in 1975 to 40 percent today. Over that quarter century, poultry and hog numbers nearly doubled, plus commercial phosphate use increased approximately 40 percent as well. However, 40 percent of Arkansas samples still test medium or below and would be expected to give first-year response to P. P removal Historical trends show P removal in the U.S. has been less than inputs over a period extending from 1960 to 2000. In fact, in the late 1960s and early 1970s,
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