Seeking to improve the efficiency of nitrogen fertilizers for grain crops led to conducting this study on developing methods using 15N in 2015–2017 at the Kazakh Research Institute of Agriculture, Almalybak Village, Almaty Region, Southeastern Kazakhstan. Nitrogen use efficiency using the stable isotope 15N in microfield experiments revealed that the assimilation of nitrogen fertilizers by grain crops largely depends on the norms, timing, fertilization method, and varietal parameters of the concerned crop. Based on morphophysiological methods of monitoring plant development conditions, the nitrogen fertilizers’ role based on the development has been affirmative, with the optimal timing of their application also determined. Results show that grain crops use nitrogen productively with partial application at the beginning of tillering and tubing, respectively, and stages III and V of organogenesis. With the use of nitrogen fertilizers, the significant varietal differences were evident. The help of an isotope label established the accurate nitrogen utilization coefficients of fertilizers based on the options ranging from 14.7% to 32.2%. Using the isotope method provides an opportunity for further development of practicing the most efficient techniques of applying fertilizers, which is an imperative method for determining the effectiveness of nitrogen fertilizers.

Keywords: grain crops, microfield experience, stable nitrogen isotope, light chestnut soil, morphophysiology, utilization factor

Key findings: The effect of nitrogen fertilizers on the plant height of grain crops planted on light chestnut soils of Southeast Kazakhstan was distinguishable by a low level of natural fertility. The help of morphophysiological observation methods determined the reasonable period needing nitrogen through nitrogen fertilizers by grain crops nitrogen fertilization. With the help of a stable isotope ¹⁵N, the coefficients of nitrogen use from fertilizers (11.7%–31.7%) attained establishment, which vary based upon the time and methods of application and the varietal characteristics of the grain crops.