Soil scientist Richard Mulvaney and several colleagues at the University of Illinois believe they have developed a reliable way to determine the optimum nitrogen fertilizer application rate for crops.
Mulvaney describes the Illinois Soil Nitrogen Test (ISNT) as a simple concept. “It uses two fundamental principles: the fertilizer only supplements what the soil can supply, and poor soils need more nitrogen than good soils, not the other way around,” he told attendees at January’s National No-Tillage Conference in St. Louis.
The ISNT estimates how much nitrogen the soil can supply, primarily by measuring an organic fraction of the soil that Mulvaney calls an alkali-labile organic nitrogen.
“This fraction includes substantial amounts of amino sugars,” Mulvaney says, “and we also will detect ammonium if it’s present, but normally it’s not, unless it’s recently been fertilized. And the test does not recover nitrate.”
During early development of the ISNT in 2001, Mulvaney and his colleagues used soil samples collected back in the early 1990s from on-farm applied nitrogen response trials. The samples came from 12 sites where the crop did not respond to fertilizer and 13 sites where it did.
“The excitement began to build when we found that the non-responsive group tested higher in our test than the responsive group,” he recalls. “It was possible to completely distinguish the two types of sites, assuming a critical range of between 225 and 235 parts per million. That had never been done with any other soil nitrogen test.”