Soil organic matter and wheat productivity in the semiarid argentine pampas

Crop productivity under dryland conditions is largely limited by soil water availability. Soil organic matter (SOM) contents have been found to be a r eliable index of crop productivity in semiarid regions because if positivel y affects soil water-holding capacity, Our objectives were to explain diffe rences in wheat (Triticum aestivum L.) yields in response to SOM levels and related properties and to quantify the contribution of a unit increment of SOM content to soil productivity during 1991, 1992, and 1994 on a total of 134 production fields in the semiarid Argentine Pampas, Wheat yields were related to both soil water retention and total organic C (TOC) contents in the top layers (0-20 cm) in years with low moisture availability (1992 [r = 0.51, P < 0.01] and 1994 [r = 0.59, P < 0.01]), and mere related to both f etal N and available P contents in a year without water deficit stress (199 1 [r = 0.58, P < 0.01]), Wheat yields over all years were linearly related to TOC (r = 0.68, P < 0.01) when these contents were <175 g kg(-1). Depende nce of wheal yields on soil water retention and on TOC contents under water deficit was related to. the positive effect of these soil components on pl ant-available water. In the absence of water deficit (1991), nutrient avail ability was the limiting factor, Losses of 1 Mg SOM ha(-1) were associated with a decrease in wheat yield of approximately 40 kg ha(-1) These results demonstrate the importance of using cultural practices that minimize losses of soil organic C in the semiarid Argentine Pampas.