The goal of any crop producer should be to get the biggest return on fertilizer investment, not necessarily to get the highest yield.
Scott Sturgul, of the University of Wisconsin-Extension Soil and Nutrient Management Program, said the biggest return on investment comes when targeting the lowest testing fields.
Speaking at a seminar sponsored by Washington County Land and Water Conservation Department together with UW-Extension, Sturgul also reminded growers to fully utilize credits for the previous crop and to take advantage of the nutrients provided by manure.
While high profile agricultural inputs such as nitrogen, seed traits, herbicides and even some micronutrients get lots of attention these days, he said it is important not to overlook the other duo of plant nutrients contained in the fertilizer blend — phosphorus and potassium.
Elevated fertilizer prices, particularly potassium, have contributed to reductions in soil fertility. He suggested that with the lower prices available this year for potassium, it would be a good chance for farmers who have been skimping in this area to replenish the soil.
Fertilizer prices were not the only contributing factor to decreases in available phosphorus and potassium in the soil, Sturgul said. Those levels have gradually gone down in the last decade because of increased interest in soil testing, the establishment of nutrient management plans on more farms and more corn silage production.
"Corn silage removes a lot more P and K than corn for grain," he points out. "That's why soil testing is more important than ever."
Phosphorus deficiency in soil is rare, but if it is there, emerging corn will have a purple color to it.
Stephanie Egner, project technician for Washington County Land & Water Conservation, said she has seen many fields in Washington County that test low in phosphorus.
Sturgul commended her department and the farmers of Washington County for their increased interest in soil testing and monitoring nutrients. He pointed out that Washington County had 2200 soil samples 10 years ago and 11,000 samples now.
"Nutrient management is a good thing," he said. "It's a better utilization of soil resources.".
He says because of all the interest in better managing soil, farmers now need to be sure they are not mining the soil of valuable P and K.
"Potassium is the element we worry about," Sturgul said. "We remove a lot of it with our forages — alfalfa and corn silage. Further, it will leach in sandy soils."
He said it continued to build up in the soil until 1999 because farmers were not testing soil but just kept adding N, P and K at the same rate every year. Then farmers began paying closer attention to soil samples and adding only what is needed by the crop.
"Soil can replenish itself, but eventually, especially with deep root crops, the subsoil supplies will be depleted, too."
Deficiencies in corn show up when leaves on the bottom of the plant brown around the edge, an indication that the plant is pulling potassium out of the lower portion of the plant to feed the new growth. In alfalfa, the deficiency is expressed in white dots at the tips on lower leaves of the plant.
It takes a long time for phosphorus levels to be drawn down but a much shorter time for depletion in potassium, he added. It also takes much longer for phosphorus to build up than it takes for potassium.
"The optimum way to get a good return on investment in fertilizer is to apply just what the crop needs, no more, no less," he said.
"If your dollars are limited, apply fertilizer to the fields that test the lowest in the necessary nutrients. The return will be much greater than on fields that are slightly low."