Phosphorus trends down

Written by Steve Fairchild on .

This summer when the International Plant Nutrition Institute released its latest soil sample survey, there was a new twist. For the first time, the organization did statistical analysis on long-term soil fertility trends. Looking at data from four million soil samples collected since 2001, IPNI was able to show that crop fields in parts of the Midwest are trending down in phosphorus. Missouri and Kansas were among the states leading in soil test phosphorus decline.

A look at input trends versus crop removal over that period, shows that in some areas cropland is being mined, a practice that will eventually reduce yield or require expensive fertility catch-up programs.

The trick with talking about soil nutrients in a sweeping fashion is that there is high variability in soil fertilities levels on different types of farms in the Midwest. The nearby map, shows the difference that separates Missouri and Kansas from Arkansas and Oklahoma. The kinds of crop grown have a significant effect on how much phosphorus is removed through harvest. Animal production and cycling nutrients back to the land through manure is a factor, too.

According to Dr. Tom Bruulsema, IPNI’s Phosphorous Program director, the soil sample survey revealed that “across North America, the fraction of soils testing below critical for phosphorus decreased from about 60 percent in the 1960s to a low of 40 percent in 2005, but has increased to 44 percent over the past ten years. In key states of the Corn Belt, the depletion trend continues from the mid-1980s. The 56 percent of soils currently above critical represent two levels of legacy.”

“Legacy” phosphorus refers to stores of phosphorus in soil profiles that may exist outside of cropping areas or remain unavailable to the crop. “While it is difficult to define the precise soil test level that separates “too much” from ‘optimum’ legacy, the tools of precision agriculture should equip growers to maintain soil test levels just a little above critical,” reported Bruulsema. “Variable rate technology—applying the ‘right rate’ of phosphorus in the ‘right place’ to match soil and crop need—enables the management of legacy to desirable levels.” He added, “Most soils retain most of any phosphorus applied. The little that leaks, however, can harm the environment. Acute risks of losses accompanying application of fertilizers or manures can be controlled through ‘right time’ and ‘right place.’ Timing applications to avoid periods when risks of runoff are high, and placing them into instead of on top of the soil can make large differences on the amount of phosphorus delivered to the edge of the field. Conservation practices that control soil erosion are also important in controlling losses of particulate forms of the legacy.”

As part of the organization’s continuing effort to educate growers, IPNI has developed an interactive website ( to share data from the soil test survey. This summary shows that soil tests do change over time in response to management. Regular soil sampling can pay its way


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