What is the Illinois Soil N Test (ISNT)? The test was developed several years ago at the University of Illinois by researchers in the Department of Natural Resources and Environmental Sciences. It is a laboratory procedure designed to measure N liberated from soil heated for 5 hours with dilute alkali solution (sodium hydroxide). The test does not measure nitrate, but does measure exchangeable ammonium and a fraction of soil organic N. The theory behind the test is to extract a specific component of organic N from soil collected before planting and N application that can reflect the amount of N mineralized in a growing season to plant-available N forms (converted to inorganic ammonium and nitrate). The concept of estimating mineralized soil organic N is not new, as many biological and chemical methods have been proposed for the past 50 years. At best some of those tests predict available soil N in closed systems, such as in pots under greenhouse conditions, and none have been proved useful in predicting plant-available N in open systems such as field conditions. Unfortunately, it is extremely difficult to predict mineralization that might occur in the field with a lab test. This is the reason there has not been a reliable test developed or recommended to date.