A simple soil test for detecting sites that are nonresponsive to Nitrogen fertilization

Recent work indicates that accumulation of amino sugar N in soil reduces the yield response of corn (Zea mays L.) to N fertilization, and that nonresponsive sites are detectable by determination of amino sugar N in soil hydrolysates. Unfortunately, the hydrolysis process is too complicated and time-consuming for use in routine soil testing. A much simpler technique was developed to estimate amino sugar N without the need for acid hydrolysis. In this test, 1 g of air-dried soil is treated with 10 mL of 2 M NaOH in a 473-mL (1-pint) wide-mouth Mason jar, and the sample is heated for 5 h at 48 to 50°C on a hot plate to liberate (NH4 + amino sugar)-N as gaseous NH₃. The NH₃ is collected in H₃BO₃-indicator solution, and subsequently determined by acidimetric titration. Recovery ranged from 97 to 102% when analyses were performed after treating samples with ¹⁵N-labeled (NH₄)₂O₄ or glucosamine, but did not exceed 6.5% with labeled glycine and was undetectable with labeled NO₃ or NO₂. Comparative studies using 12 nonresponsive and 13 responsive soils showed a very high correlation between soil-test N and hydrolyzable amino sugar N (r = 0.90^***). Test values were significantly higher (P < 0.001) for nonresponsive (237-435 mg N kg^-1) than for responsive (72-223 mg N kg^-1) soils. The soil test described has important economic implications for production agriculture, and also should be of value for controlling NO₃ pollution of ground and surface water.