Good yields affecting nutrient levels Nutrient levels over the past several years may encourage farmers to test their soils, said Jim Gerwing, Extension soils specialist at South Dakota State University.
Phosphorous, nitrogen, and sulfur levels have dropped off within the past couple years, he said. Low zinc and potassium soil tests also are showing up more frequently.
"Fertilizer programs have been geared towards much lower yields. Now, after the good years, we've simply removed more nutrients than we're putting back in," said Gerwing.
The phosphorus levels are usually fairly stable. Over the last 15 to 20 years, average phosphorous levels in South Dakota soils have stayed the same. However, levels began to decline the last two years.
This is associated with extremely good crop yields across the state, he said.
According to Gerwing, one bushel of corn removes .35 pounds of phosphorous and one bushel of soybeans removes .75 pounds.
With higher than expected yields, one can easily see why the deficiencies are occurring, he said.
Reductions in soil nitrate levels can be attributed partly to high yields, but also is the result of leaching and denitrification from heavy rainfalls and waterlogged soils, said Gerwing.
He said farmers who practice no-till especially need to watch nitrate levels.
"As the issue of no-till slowly evolves in the state, we see lower nitrate levels," noted the specialist. "We start slowing the decay rate and, in turn, building up organic matter. The decay is an important source of nitrate."
Elevated yields and leaching are also causing lower sulfur levels in South Dakota fields. Again, no-till practices are having an effect.
"A major source of sulfur is decaying organic material and if you slow that down, supply slows down," he said.
Research has indicated early growth responses to sulfur, but have not always resulted in yield increases, he reported.
Sulfur deficiencies showing in the four- to seven- leaf stage of corn may indicate a need for starter sulfur the next few years, he recommended.
Zinc is another nutrient that changes slowly over time. The increased detection of zinc-deficient areas may be due to better sampling, said Gerwing.
"In the past, whole fields were averaged together and deficiencies didn't show up," he explained. "Grid and zone sampling allows deficiencies in specific areas of a field to be found."
Gerwing said zinc deficiencies are common to corn, soybeans and potatoes.
Potassium levels are beginning to suffer in the eastern edge of the state. "The general consensus is South Dakota potassium soil tests are extremely high. However, the eastern quarter of the state has some very potassium-deficient soils," he said.
Severe decreases in yields and production in eastern South Dakota on some corn hybrids have been due to a lack of potassium in the soil.
Gerwing recommends sampling at least the more eroded or hillside areas to rule out potential problems.
Sampling can lead to greater profits. "Nutrient deficiencies can reduce yield, but we don't want to put more nutrients out there than we need either," he said.
Environmental concerns are becoming more and more of an issue, especially with nitrogen and phosphorous.
"Not that it's more of a problem now, but the non-farming community is becoming less tolerant of nutrient losses," he said. "By utilizing soil sampling, farmers can show them they're not just indiscriminately throwing fertilizer out there."
Lab work can usually be done within three days, so time still remains to have soil sampled.