MEASUREMENT OF NITRATE-SUPPLYING POWER OF SOILS BY AN INCUBATION METHOD AND CORRELATION WITH CROP YIELD RESPONSE

Abstract

The results of an incubation method for measuring the capacity of Ontario soils to accumulate nitrate were extremely variable. Placement of the soil sample between two layers of vermiculite in the incubation tube and addition of sufficient distilled water to moisten the top layer of vermiculite, the soil and a portion of the bottom layer of vermiculite resulted in satisfactory reproducibility of results. Wide fluctuations in results, due to prolonged air-dry storage before analysis or to time of sampling during the growing season, were reduced by storing all samples in the moist state at 10 °C. for 2 weeks before analysis.The correlation between the logarithm of the percentage yield decrement, which is a measure of crop response, and nitrate-supplying power as measured by the modified incubation method was highly significant for winter wheat (r = −0.437), oats (r = −0.411) and potatoes (r = −0.651). Soils having a nitrate-supplying power of 50 p.p.m. of N or more by the proposed modified method gave little or no increase in yield of wheat, oats, or potatoes when additional nitrogen was applied as fertilizer.