The Illinois soil nitrogen test (ISNT) was initially developed as a means to identify fields where corn would not respond to fertilizer nitrogen (N) addition (Khan et al. 2001). The ISNT is a simplified version of a diffusion technique that determines different forms of N in soil hydrolysates (Mulvaney and Khan, 2001). Using stored samples, Mulvaney et al. (2001) found that N fertilizer response in corn was related to amino-sugar N (ASN); whereby as ASN increased, corn N fertilizer response decreased to zero and remained non-responsive above a threshold ASN value.
The ISNT was shown to be strongly correlated to ASN (Khan et al., 2001). Other favorable characteristics of the ISNT that could aid in the adoption of the test are that soil samples could be taken from 0-15 cm at the same time as routine soil sampling (Khan et al., 2001). Also samples could be taken in the spring prior to planting corn or the fall prior to the corn crop (Barker et al., 2006a; Hoeft et al., 2001).
More recently, Mulvaney et al. (2005) reported on data from 102 N response studies conducted in 1990-1992 and 2001-2003. In this dataset, 33 sites-years were non-responsive while 69 were responsive. The ISNT correctly predicted 31 of the non-responsive sites; meaning that two sites were predicted as being responsive but were not.
The ISNT correctly predicted 50 of the responsive sites; meaning that 19 responsive sites were predicted as being non-responsive. Incorrectly classifying responsive sites as non-responsive could have a large negative economic impact to farmers as yield losses from under fertilization would have occurred. Mulvaney et al. (2005) hypothesized why these 19 failures occurred; however, they did not provide any experimental data to substantiate the hypotheses. Fully understanding situations where the test works well and where it does not is essential to providing growers with criteria for successful use of the test.
Field research in New York the past 8 years has shown a new soil N test, the Illinois Soil Nitrogen Test (ISNT), to be the best option for determining soil N supply potential for New York corn growers. The ISNT is a laboratory test that estimates the amount of readily mineralizable soil organic N. The test has been 83% accurate in our trials predicting if soil-N supply alone could provide adequate N for a corn crop in New York.
A major factor in the Cornell N equation is soil N supply, yet soil N supply is very difficult to predict accurately. The soil N-supply values used for the Cornell N equation for corn are estimates (book values) developed for more than 600 New York soil types. Book values are based on studies of N uptake by continuous corn grown without additional N. For New York soils, soil N supply can range from 50 to 140 lbs N/acre, with 60-70 lbs N/acre typical for many common agricultural ...