YIELD OF SOYBEANS AND OIL QUALITY IN RELATION TO SOIL TEMPERATURE AND MOISTURE IN A FIELD ENVIRONMENT

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Abstract
Under comparable field conditions of air temperature and solar radiation, yield of soybeans increased 43.4% when the day-degrees units of the soil were raised from a seasonal value of 859 to 1822 (> 5 C) and yield decreased 82.4% when the day-degrees for the same period were lowered to 408 for the 20-cm depth. These heat values from July 10 to September 21 correspond to mean daily soil temperatures of 11.2, 17.7, and 31.2 C. The change in yield represents a reduction on the cold soils of 208 kg/ha (3.1 bu/acre) per 1 degree C below the seasonal temperature, and an increase on the warm plots of 54 kg/ha per 1 degree C (unfertilized) above the seasonal temperature. In general, yield was related linearly to the reciprocal of the temperature and of the day-degrees. Oil concentration varied little among the three soil temperatures, although the iodine number decreased and the percentage protein increased with higher soil temperature. Coinciding with the high yields on the warm soils was a high concentration of P in the foliage, e.g., 0.15% P at 11.2 C and 0.42% at 31.9 C. During early growth, concentration of K in the plant material increased and that of Mn and Cu was reduced with high soil temperature. This resulted in a greater removal of P and K from the warm soil than from the cool soil, and little difference in removal of Mn and Cu between the low and high temperatures.

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