Evaluation of some chemical methods of soil nitrogen available based on 15Nitrogen technique

Abstract

The large increases in nitrogen (N) fertilizer usage in recent years have led to a recognition of the need for reliable and quick methods of measuring the N‐supplying capacities of soil. Despite this need, there is no well accepted method for testing soils for N availability that has been generally considered to be satisfactory. This study was conducted to determine the N status of the soils of the Bafra Plain and to evaluate some of the more promising methods of determining the index of soil N availability by using a ¹⁵N technique as a standard. Twelve representative soil samples were collected having a broad range of chemical and physical characteristics. In determining the available N contents of these soils, one aerobic incubation method and 12 different chemical methods were compared. In order to evaluate the availability of indigenous soil N to plants and also to investigate the N status of these soils, a greenhouse experiment was conducted with corn as the test plant. Nitrogen was applied as urea at the rate of 0, 50, 100, and 200 ppm with ¹⁵N enriched (2% a.e) urea used at the 50 ppm rate. After eight weeks of growth, plants were harvested. The increased amounts of N were not affected linearly by the dry matter, N content, and N uptake by the corn plants for most of the soils. The most suitable N rate was found to be 50 ppm N under the experimental conditions. In order to select the most suitable chemical methods, the A_N‐value (soil N availability to plant as the applied fertilizer N) was taken as the standard method. Highly significant correlation coefficients were obtained between the A_N‐value and each of the following methods: alkaline hydrolysis (r=0.900 and r=0.938, P<0.001), alkaline permanganate (r= 0.873, r=0.864 and r=0.826, P<0.001), and total N (r=0.850, P<0.001), respectively. These methods appeared worthy of consideration as a routine test for assessing the N availability for these soils.