Assessing Soil Water Status to Optimize Cover Crop Termination in a Semi-Arid Environment.

Well managed cover crops effectively improve soil health as demonstrated across the corn belt of the continental US where frequent summer rains sustain season-long crop growth.  In the Inland Pacific Northwest (IPNW) most precipitation is received during the winter months when temperatures are low, and stored as soil profile moisture.  In lower rainfall zones crop rotations include a summer fallow to increase stored soil water for the subsequent crop/growing season.  Given the infrequent and minor precipitation events that occur when conditions favor crop development, cover cropping systems remain more a curiosity than a viable soil management option.  The lack of cover crop soil water management guidelines and concerns over luxurious soil water use are inhibiting adoption of this practice.  In 2014, we reported preliminary data on the overall water removal characteristics of cover crops versus three fallow systems for five sites across a rainfall gradient in southeastern Washington.  During year two of the study (2015) we sampled soil and biomass at a three-week interval on three of the five sites to monitor changes in gravimetric soil water and cover crop biomass accumulation and assess soil water removal by evapotranspiration under a cover crop relative to evaporation under fallow.  We will present our findings and discuss the water balance trade-off of building soil quality with a cover cropping system versus conventional fallow.