You wouldn’t attempt to shoot a moving target while blindfolded, yet that’s exactly what some growers do when it comes to managing phosphorus (P).

Increasing yields demand more phosphorus to optimize plant functions, yet farmers often make applications based on habit, says Dale Leikam, president of the Fluid Fertilizer Foundation. A once foolproof habit of the past could end up being more of a shot in the dark, maybe a very costly one. 

“Phosphorus is probably the single most limiting nutrient in the Plains states next to nitrogen,” Leikam says.

Mark Epler of Pioneer Stock Farms, Columbus, KS, knows just how essential P is to his farm’s profitability. For more than two decades he’s monitored soil tests for P, adjusting to optimize yields while controlling fertilizer costs.

“If you get into a rut putting on the same amount every year, you could easily be wasting it,” Epler says.

Determining how much P to apply can be a quandary since levels decline at differing rates depending on soil characteristics, fertilizer-management history and removal during crop harvests, says Jason Warren, assistant professor of soil and water conservation and management at Oklahoma State University.

There are two main approaches to managing P.

  • The sufficiency approach attempts to, on the average, optimize profitability in the year of application while recommending no P application at soil-test levels above a critical soil test level. This method achieves about 95% of maximum crop yield, Leikam says.
  • The build-maintenance approach builds soil test values to the critical value over a number of years, maintains them in a range slightly above the critical value and recommends no P at soil test values above this target range, Leikam says. A build-maintenance program achieves about 100% of maximum yield, he adds.

The sufficiency approach generally minimizes P inputs in the early years while the build-maintenance approach provides for increased flexibility and crop yields. Both approaches eventually suggest crop removal rates of P in the long run, Leikam says.

“If we build soil-test P, it will hold itself there for a while and gradually be depleted if we stop fertilizing,” Warren adds.

He cautions growers to reassess levels the year after skipping application since the rate of decline is uncertain.

Regardless of the approach, eliminating soil-test variability is critical for determining the optimal P application, Leikam says. Taking an ample number of soil subsamples from equal depths and using GPS to standardize test locations builds consistency and provides reliable field histories sooner.

Implementing zero-P check strips also helps growers obtain more accurate data. The strips help predict when yield might respond to P decline, Warren says.

Phosphorus monitoring and management has not only helped Epler save on fertilizer costs, but also has helped improve soil health.

“It’s been a way to keep a thumb on the pulse of what’s going on chemically in the soil,” he says.