Soil and Rainfall Factors Influencing Yields of a Dryland Cropping System in Colorado.

The semiarid U.S. Great Plains experiences a large variation in crop yields due to variability in rainfall, soil, and other factors. We analyzed (24-yr) yields from a no-till rotation of winter wheat (Triticum aestivum L.)--corn (Zea mays L.) or sorghum [Sorghum bicolor (L.) Moench]--fallow at three sites, each with three soil types along a catena in Colorado. We investigated: (i) effects of soil organic carbon (SOC), length of slope (SL), and other soil properties on yields; and (ii) degree in which variability in annual yields are explained with water variables of rainfall during the fallow, vegetative and reproductive stages, and soil water at planting. Wheat and corn/sorghum yields were strongly related to soil properties (R² = 0.84; 0.97, respectively). Wheat yield regression had only SOC as a significant variable, whereas corn/sorghum had SOC and SL. Water factors explained 37 to 73% of annual variability in wheat yields at the site by soil level, 19 to 63% when pooled over soils within sites, and 35 to 40% pooled over all. For corn, the corresponding percentages were 16 to 64%, 26 to 57%, and 40 to 45%, respectively. Fallow rain made the highest contribution to R² in wheat whereas reproductive rain did for corn/sorghum. We conclude SOC is a stand-in for long-term effects of water availability on production, but other natural factors greatly influence the annual yield variability. Combining mean yields determined from soil properties with CV of pooled annual yields, we can estimate mean and standard deviation of yields. [ABSTRACT FROM AUTHOR]