A look at P and K levels in Iowa soils

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Some data suggests that Iowa farmers may be mining their soils when it comes to phosphorus (P) and potassium (K), said Joe Thelen, field representative with Midwest Labs. But while lab results over the years show ups and downs, Thelen said results from soils going through Midwest Labs have changed only slightly over the past few years.

The maps included here were produced by the Mosaic Company and are colored according to reported county use of P and K and average removal rates based on reported county average yields. Red counties are those where overall county application of P and K was below the level that normal corn and soybean production would remove. Yellow counties are in those where application and removal were about equal, while in the green counties, application of P and K was more than was needed to support the crops produced.

Speaking at the Iowa Soybean Association On-Farm Network Conference in February, Thelen said that while some Iowa farmers seem to be applying less P and K than their crops are using, the situation doesn’t require drastic measures—yet.

International Plant Nutrition Institute (IPNI) figures show that the number of soil tests for P and K nearly doubled in Iowa from 2005 to 2010. At the same time, 46 percent of the Iowa samples tested by all soil testing labs (Bray 1 test) and reported to IPNI were below 20 ppm P, the level which Iowa State University soils specialists say is optimal for corn and soybean production on most soils in the state.

Midwest Labs’ own data shows that P levels are declining in Iowa soils. The number of tests in the high or very high P ranges dropped nearly half from 2008-09 to 2011-12. At the same time, the number in the median range (15-21 ppm) stayed about the same over the four year period, but the number of samples in the low-to-very low categories increased.

Note that while there are more yellow (adequate) and green (high) counties on the K map, the number of counties in red is still significant.

Thelen said that while the Mosaic and IPNI numbers may suggest a trend in Iowa soil levels, Midwest Labs’ own Iowa soil tests over the same time period suggest fewer soils testing in the very low, low or medium ranges, and an increase of those testing high or very high.

“What our data says is that more than half the soil tests were below the critical K level,” Thelen says. “While it’s not a cause for concern, it does suggest we need to pay better attention to soil K levels.”

Thelen averaged soil pH, P1, P2, K and OM (organic matter) levels for all soil tests that were sent from an Iowa ZIP code from 2000 through 2013. Soil pH appeared to be fairly consistent over the 14-year period. P1 and P2 tests for phosphate increased through 2007, then it dropped off until 2013, when levels rose again. Average K increased slightly over the 14-year period. “It also appears that we’re slowly rebuilding organic matter levels,” he said.

For Thelen's entire talk at the 2014 On-Farm Network conference, click here.

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