Effects of recycled water on soil salinity levels for cool season vegetables

Agriculture in Monterey County, California is more than a $3 billion per year industry. Over-pumping of ground water has caused sea water to intrude into wells located near the coast. In an effort to reduce ground water extraction in the northern Salinas Valley, the Monterey Regional Water Pollution Control Agency in partnership with the Monterey County Water Resources Agency began providing recycled water to 4,900 hectares of prime farmland used to grow cool season vegetables in April 1998. The dominant soil types in this region are clay loam and heavy clay soils, both of which are susceptible to sodium accumulation and water penetration problems. Recycled water, blended with well water, is used to irrigate artichokes, broccoli, Brussels sprouts, celery, cauliflower, lettuce, and strawberries. Because of grower concerns that salts, particularly Na and Cl, in the recycled water would reduce yield and quality of their crops a long term study was developed to monitor salinity levels in commercial vegetable fields. Soil salinity levels were monitored at 4 control and test sites beginning in the spring of 2000. The control sites received well water and the adjacent test sites received an approximate 2:1 blend of recycled and well water. Control and test sites were paired so that they could be compared under the same soil, crop, drainage systems and farming practices. The soil was sampled three times per year from all sites: spring (before planting), mid-summer, and late fall. Composites of 4 cores were collected from the 0 to 90-cm depth at 30-cm intervals. Soil samples were analyzed for pH, electrical conductivity (ECe), extractable cations (B, Ca, Mg, Na, and K) and extractable anions (Cl, NO3, and SO4). After 3 years of monitoring, the data showed that using recycled water for vegetable production increased ECe (saturated paste extract) of the soil profile from 2.0 to 2.9 dS/m but decreased the sodium adsorption ratio (SAR) from 2.9 to 2.6. The SAR and EC of soil samples from all sites were in a range acceptable for vegetable production.