Science From The Field

Mind Your P & K

A 2016 Pioneer study found from 20% to 73% of soil samples were below the critical soil test level for phosphorus recommended in each state, Image provided by Pioneer

Science From the Field highlights Pioneer’s industry-leading research to help growers find profitable agronomic solutions for their farming operations.

Many farmers are stumbling on a critical step while trying to climb the stairway to higher yields. Recent studies by Pioneer and the International Plant Nutrition Institute (IPNI) indicate that inadequate levels of phosphorus (P) and potassium (K) in the soil are likely limiting yields on up to 80% of farms in some states. It’s a situation brought on by farmers responding to lower commodity prices by reducing fertilizer rates, and by farmers not taking into account the increased nutrient removal of recent high yield crops.

“The results are disturbing because managing soil fertility is critical for achieving a crop’s full yield potential and maximum profitability,” says Matt Clover, agronomy manager with Pioneer. “Recent years have seen record yields in many areas and because of low commodity prices farmers have needed the additional bushels. But now there are concerns that we can’t continue producing those high yields with declining soil fertility levels.”

Clover adds that part of the problem comes from the complex way that P and K behave in the soil. “There’s confusion about how much P and K vary across a field, how much of what’s applied gets to the plant this season, how much is available later--or perhaps never--how much is lost and how much is removed in the grain. We know that farmers struggle with this because when we ask them how they hope to increase yields in the future they tell us that learning to manage P and K is second only to using improved genetics.”

LOWER LEVELS. A survey by IPNI of soil samples submitted to labs across the U.S. has sparked Pioneer’s interest in declining soil fertility levels. The 2015 results of the annual state-by-state analysis found from 31% to 83% of the samples were below the state’s critical level for phosphorus and 9% to 65% were below the critical level for potassium. Critical soil test levels for given nutrients vary with geography, but they’re based on university research that pegs the level below which a profitable yield response would be expected in the year of application.

In 2016, Pioneer agronomists and Encirca® certified services agents launched their own survey of soil fertility levels in Midwestern states. The study included 22,402 soil samples collected from 8,925 fields in 12 states. The samples were submitted to the same soil test lab to maintain consistency and the findings were strikingly similar to those of the IPNI study.

In both surveys, North Dakota and South Dakota had the most samples below the critical level for phosphorus while Indiana had the fewest. Minnesota, Iowa, Wisconsin, Michigan and North Dakota had the most samples testing below the critical level for potassium while Indiana, Kansas and Nebraska the fewest.

HIGH YIELDS REMOVE MORE. Clover and other soil fertility experts believe these soil test levels are low--and trending lower--because farmers aren’t recognizing the increased amount of P and K being removed with the grain as yields have increased.

Using a rate of one bushel of corn removes .35 pounds of P2O5 and .25 pounds of K2O and comparing USDA average yields in 1986 to those in 2016 illustrates the situation. In South Dakota, for example, average corn yields increased from 78 bushels per acre to 140 bushels. This increased the amount of P removed with the grain from 27 pounds per acre to 49 pounds and K removed from 19 pounds per acre to 35 pounds.

“That’s an 80% increase in the removal rate in that state,” says Clover. The increase in the removal rate in the other 11 states in the study ranged from 43% in Kansas, Illinois and Indiana to 58% in Minnesota and 70% in North Dakota, he says.

“These studies indicate that it’s difficult to manage P and K levels in tough economic times,” continues Clover. “Farmers know that the soil serves as a bank to store these nutrients so they want to make withdrawals when profits are tight, but in many cases increasing yield levels have already reduced any reserves. The best advice is to know your soil test levels and don’t reduce application rates in low testing soils. Also, consider applying nutrients before each crop--rather than the traditional two-year spread in corn/soybean rotations--and avoid practices that inhibit root development.”


One bushel of corn removes .35 pounds of P2O5 and .25 pounds of K2O.*

One bushel of soybeans removes .73 pounds of P2O5 and 1.2 pounds of K2O.*

USDA data from 1986 to 2016 shows the application rates of P and K have not increased at the same pace as the removal.

A survey of multi-state soil samples taken in 2015 by IPNI found from 31% to 83% of the samples were below the state’s critical level for P and 9% to 65% were below the critical level for K.

*According to IPNI


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