Changes in Thermal Resources of the Growing Season and Yield of Spring Wheat in the Middle Volga Region

This article presents the results of long-term studies (1982–2018) assessing changes in thermal resources during the growing season and their impact on spring wheat yield. The work was carried out in the northern part of the Middle Volga region on gray forest soils. The soil of the experimental plot is gray forest medium loam, the humus content (according to Tyurin) is 2.8–3.2%; the content of mobile phosphorus and potassium (according to Kirsanov) is 105–184 and 79–149 mg/100 g of soil, respectively; and pHsalt is 5.6–5.8. During the observation period, the highest temperature increase was noted in July (2.4°C) and May (2.2°C), and the lowest temperature increase was noted in June (1.0°C). The sum of effective temperatures over 10°C increased by 500°C during the study period. The yield of spring wheat grown without fertilization varied greatly over the years, ranging from 0.99 to 2.97 t/ha. The highest value of this indicator was observed with an average monthly air temperature of about 13°C in May, 17°C in June, 18°C in July, and a sum of active temperatures of 1800°C. The studied thermal resources can be arranged in the following descending order according to the significance of their influence on the yield of spring wheat: average air temperature in June > average air temperature in July > sum of effective temperatures above 10°C > average air temperature in May. In three out of four cases, a second-degree polynomial equation was better suited to describe the dependence of the yield of spring wheat grain on thermal resources, and, in one case, an exponential equation was better suited.