If you're pushing hard for high soybean yields, pay close attention to phosphorus (P) and potassium (K) fertility levels. They could be holding down yields. And don't shy away from applying manure ahead of soybeans.

Soybeans absorb significant amounts of nitrogen from manure, and benefit from the P and K, recent research shows.

That's something that soybean yield-contest winners have been contending for some time. Now some university agronomists and soil scientists are giving their blessing to that notion, too - with certain precautions.

Randy Killorn, an Iowa State University extension agronomist, looked at the efficiency of manure-nutrient use by soybeans. He used various application rates, starting with a very low rate and ending up with rates sufficient to supply almost as much nitrogen as would be needed by a corn crop.

"As expected, soybeans did not respond to the nitrogen with increased yields. However, they did respond to the P and K in the manure," Killorn reports.

"A big concern was that, by applying high rates of manure ahead of soybean production, we'd be overapplying nitrogen, which could result in excess nitrates leaching from the soil," Killorn says. "What we found was that soybeans are not only very good at scavenging for P and K in the soil, they are also very good at using up excess nitrogen."

Mike Schmitt, a University of Minnesota extension soil specialist, agrees. He says a soybean crop usually removes more nitrogen and potash than a comparable corn crop.

Both Schmitt and Killorn have looked at the environmental effect of using manure on beans. Their conclusion: If a field to be planted to soybeans needs P and K, growers need not shy away from using manure to meet those needs.

However, applying manure ahead of beans should be done with some caution, they say.

"Our recommendation is that manure be applied to stalk fields in the fall and incorporated immediately if the field is to go into soybeans," Killorn says.

One goal of his study was to apply and incorporate manure while maintaining adequate residue to protect soils from erosion.

"On many Iowa soils, incorporating manure after a soybean crop leaves too littleresidue cover, so we decided to try applying manure after corn instead," Killorn reports. "Residue cover was more than adequate in most cases, and there was no negative effect on the soybeans if the application was not made too near to planting time."

Schmitt points out that soybeans are more likely to be affected by nutrient salt and ammonia injury during germination than is corn.

"If manure applications are not made and incorporated well, there is a very real risk of crop injury," he says.

Schmitt also warns that, if manure is applied in both years of a corn-soybean rotation, phosphate levels may get too high.

"If we're going to use manure for soybeans, we need to limit application rates to avoid phosphate buildup in the soil, or we'll then be dealing with higher losses to surface water," he says.

Another concern is that manure acidifies the soil. While both corn and beans do best at a pH between 6.0 and 7.0, soybean yields are more affected if the pH drops below that range.

"Soybean yield response to soil pH is from a combination of factors," Schmitt explains. "As pH increases, the availability of aluminum and manganese decreases in the soil, which reduces the toxicity effects of these two elements to the sensitive soybean plant. Also, as pH increases up to 7.0, the availability of soil phosphorus increases.

"Near-neutral pH also favors the growth of soil bacteria, especially rhizobium sp., which are essential to the symbiotic fixation of atmospheric nitrogen in the soybean plant's roots," he says. "In addition, the higher pH increases the availability of molybdenum, a trace element that is important for nodulating bacteria."

While soybeans can benefit from livestock manure, Schmitt recommends that growers apply manure first to corn fields, based on nutrient needs and environmental risk. Then don't be afraid to use manure on soybeans if the nutrients are needed and the environmental risk is low.