Impacts of gypsum and winter cover crops on soil Physical properties and crop productivity when irrigated with saline water

Reuse of saline subsurface drainage water far irrigation has been identified as a potential option for managing drainage volumes and sustaining crop productivity in California's San Joaquin Valley. Soil surface structural instability, crusting and poor stand establishment may, however, be constraints in drainage reuse systems. The objective of this field study was to evaluate the effectiveness of winter cover crop incorporation and gypsum applications relative to conventional fallows for improving soil physical properties, stand establishment and crop productivity in a cropping system relying on the cyclic reuse of saline drainage for irrigation. Barley (Hordeum vulgare), Lana woolypod vetch (Vicia dasycarpa) and barley/vetch winter cover crop treatments were compared with gypsum amended and unamended winter fallows in a rotation of tomato-tomato-cotton as summer crops. Tomato seedling emergence was improved by 34% following incorporation of vetch in year 1 prior to saline irrigation application, but was unaffected by amendment treatment in year 2. Following two summer seasons in which saline drainage water was used for about 70% of the irrigation requirements, surface-applied gypsum significantly reduced soil crust strength an average of 14%, increased soil aggregate stability an average of 46%, and maintained cotton stand establishment relative to nonsaline irrigation. Emergence rates and final stand densities of cotton seedlings following incorporation of each of the cover crops, however, were significantly lower. The mechanism most likely responsible for reducing stand establishment was the formation of stubble-reinforced surface crusts that resulted in interconnected slabs that impeded timely emergence of seedlings. This type of physical impedance can create secondary effects such as increased disease which could also have played a role in reducing emergence. Yields of tomatoes irrigated with saline water were maintained relative to nonsaline irrigation in year 1, but were decreased by 33% in year 2. No reductions in cotton lint yield occurred as a result of saline irrigation in year 3. Soil electrical conductivity (ECe) increased from about 2-6 dS m(-1) during the course of this 3-year study despite leaching by winter rains. Cyclic reuse of saline subsurface drainage water may conserve good quality water and provide a means of sustaining crop productivity over short terms. Soil surface salt and boron accumulation, however, may be major constraints to this cropping strategy and will limit productivity if appropriate irrigation and crop management practices are not followed. (C) 2000 Elsevier Science B.V. All rights reserved.