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 (NH 4 + amino sugar)‐N as gaseous NH 3 The NH 3 is collected in H 3 BO 3 –indicator solution, and subsequently determined by acidimetric titration. Recovery ranged from 97 to 102% when analyses were performed after treating samples with 15 N‐labeled (NH 4 ) 2 SO 4 or glucosamine, but did not exceed 6.5% with labeled glycine and was undetectable with labeled NO 3 or NO 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 −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 NO 3 pollution of ground and surface water.