With lower fertilizer prices, remember that maintaining optimum P and K levels are critical in sustaining productive soils. The past few years of near-record corn yields may have pulled more of these crucial nutrients than you've been replacing. That could spell trouble.

“The past two or three years we saw little fertilizer applied, and a lot of soil fertility removed because of good yields,” says Fabian Fernandez, assistant professor of soil fertility and plant nutrition at the University of Illinois. “The soil's fertility can be at maintenance levels, but can quickly dip below the critical level.”

Maintenance levels for P and K are set to provide adequate fertility for a crop throughout the growing season and vary depending on the soil. The critical level is the point below which a yield response to fertilizer is expected. Any additional stress on the crop, and yields suffer.

For instance, Fernandez says high-phosphorus soils in Illinois are at the critical level when soil samples indicate P levels at 30 lbs./acre. On those same soils, maintenance levels would be in the 40-60-lb./acre range.

But recent bumper yields may be giving some producers a false confidence that fertility levels are adequate. And without soil tests, experts caution that not knowing where your P and K levels are in the soils could be a risky proposition.

“Replenishing soil fertility levels makes sense, especially as soil test levels get below critical levels,” notes Tony Vyn, professor of agronomy at Purdue University. “We have had two pretty good years. With the yields we are harvesting, we are really taking a lot of fertility out of the soil.”

On average, corn removes anywhere from 0.37 to 0.43 lb. of P₂O₅ and 0.27 to 0.28 lb. of K₂O/bu. of grain produced. Soybeans remove anywhere from 0.80 to 0.85 lb. of P₂O₅ and 1.3 to 1.4 lbs. of K₂O/bu., according to the Illinois Agronomy Handbook and the Tri-State Fertilizer Recommendations. “It's easy to see how bumper yields can take a lot of critical nutrients out of the soil,” Fernandez says.

THE ACTUAL P in P₂O₅ is only 43% and the actual K in K₂O is only 83% on an equivalent weight basis.

As a general rule, it takes an application of 9 lbs. of P₂O₅/acre to increase the phosphorus level by 1 lb./acre. To build potassium levels by 1 lb./acre, it takes an application of 4 lbs. of K₂O.

It's quite possible to go several years on reduced P and K application without any noticeable affect on yield “in the first year or two after the fertilizer materials are not applied,” Vyn says. “But the depletion may become more evident as levels fall to less than critical or approach critical, and the crop is under stress from other factors. Fertility depletion is less likely to surface when all other growing conditions are optimum. But when the crop is under multiple stresses, low fertility levels can have a greater impact.”

It all comes back to proper soil testing and ensuring that fertility levels are at recommended levels to match soil conditions as well as yield potential.

A recent Illinois study collected soil samples from 600 locations in 52 counties across the state. The results showed that 19% of those fields tested sampled below the critical level for P, while 22% of the fields were at maintenance levels and 59% above maintenance levels. “For many of the fields sampled, P levels are not a concern to producing good yields,” Fernandez says.

However, that same survey showed that 45% of the samples were below critical levels for K, 24% at maintenance levels and only 31% above maintenance levels. This indicates that in many fields in Illinois, substantial amounts of K applications are needed to reach adequate fertility.

The survey showed that while some growers may be able to still forego fertilizer application and not impact yield, others need to take a hard look at their fertility levels, and adequately build up soil fertility levels to ensure they aren't shortchanging their yield potential.