Straw Mulch Effects on Soil Temperatures and Sorghum Germination and Growth

The possibility that lower than normal temperatures in residue-covered soils may affect crops has caused concern regarding the use of conservation tillage systems in some regions. To evaluate this possibility in the Southern G::eat Plains, soil temperatures were measured at a 10- cnl depth in field plots on which 0 (check), 1, 2, 4, m 12 metric tons/ha of wheat (Triticum aestivum L.) straw were applied on the surface. The soil was Pullman clay loam, a fine, mixed, thermic Torrertic Paleustoll (order Mollisol). Objectives were to determine the effects of mulch-induced temperatures on grain sorghum [Sorghum bicolor (L.) Moench] planting dates, germination, emergence, and growth, and to obtain information concerning effects of mulch rates on soil temperatures at various seasons and under specified climatic conditions. Increasing mulch rates delayed the time that soil reached favorable temperatures for sorghum germination and growth. However, temperatures were near optimum before normal planting dates and, therefore, mulches did not affect sorghum germination and only slightly delated emergence. Since temperatures with high mulch rates rarely, if ever, reached the optimum for sorghum growth, plants on these plots grew slower early in the season than t]hose on plots with low mulch rates. Later in the season, when soil water limited growth, plants on high mulch 1ate plots grew more than on other plots because of higher water contents. Increased mulch rates decreased average soil temperature means, maximums, minimun~s, and standard deviations during all seasons of fallow front wheat harvest to sorghum planting. However, when temperature of bare soil approached or fell below 0 C, the temperature effect was reversed. The change in temperature for each metric tons/ha of mulch as compared with bare soil was greatest for the 1-metric ton/ha rate and generally decreased as mulch rates increased.