The Illinois Soil Nitrogen Test (ISNT) was developed for this purpose, and involves estimation of gaseous ammonia liberated by heating the soil with strong alkali in a Mason jar. Several studies have provided evidence that the ISNT is predictive of yield response by corn to nitrogen fertilization, but there have also been negative evaluations in which concern has been raised that test values represent a constant proportion of total soil nitrogen rather than a microbial fraction that would be potentially available.
A study was conducted from 2004 to 2006 at the University of Illinois to clarify the chemical nature of what the ISNT measures and its relationship to microbial growth in agricultural soils. A multifaceted approach was taken, involving recovery tests with pure organic nitrogen compounds, statistical analyses of different nitrogen fractions measured for 26 Illinois agricultural soils, and incubation studies to determine incorporation of labeled nitrogen into soil nitrogen fractions.
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