A simple soil test for detecting sites that are nonresponsive to nitrogen fertilization. (Division S-4--Soil Fertility & Plant Nutrition)

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 [degrees] C on a hot plate to liberate (N[H.sub.4] + amino sugar)-N as gaseous N[H.sub.3]. The N[H.sub.3] is collected in [H.sub.3]B[O.sub.3]--indicator solution, and subsequently determined by acidimetric titration. Recovery ranged from 97 to 102% when analyses were performed after treating samples with [sup.15]N-labeled [(N[H.sub.4]).sub.2]S[O.sub.4] or glucosamine, but did not exceed 6.5% with labeled glycine and was undetectable with labeled N[O.sub.3] or N[O.sub.2]. 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.sup.-1]) than for responsive (72-223 mg N [kg.sup.-1]) soils. The soil test described has important economic implications for production agriculture, and also should be of value for controlling N[O.sub.3] pollution of ground and surface water.