WATER QUALITY FOR IRRIGATION OF ALFALFA HAY

Bob Sheesley
University of California, Farm Advisor, Emeritus
S & W Alfalfa Plant Breeder

How much salt is in your irrigation water and your soil? All water used for irrigation contains some dissolved salts. An acre-foot of water (the amount of water covering one acre one foot deep), weighs approximately 2,720,000 pounds, therefore 1ppm, of salt in an acre foot of water weighs 2.72 pounds. This means that one acre-foot of water containing only 735ppm (EC=1.15mS/cm) carries ONE TON OF SALT. Many growers apply 3.5 – 4 feet of irrigation water per year to produce a crop of alfalfa hay — or they apply 3.5 – 4 tons of salt on every acre every year if that water has an EC (Electrical Conductivity) of only 1.15mS/cm. If your irrigation water has an EC of 5mS/cm (like some well waters on the west side of the San Joaquin Valley) your 4 acre-feet of irrigation water applies 171/3 tons of salt per acre every year.

THAT’S A LOT OF SALT!

One of the hazards of irrigated agriculture is the possible accumulation of soluble salts in the root zone. Some plants can tolerate more salts than others, but all plants have a maximum tolerance. Where ample water and deep drainage is used to remove excess salt from the root zone, the salt level in the saturation extract is about 1.5 times that of the irrigation water. Where water is used more sparingly, there may be 3 times as much salt accumulated in the plant root zone. Leaching and drainage are critical over time!

Data developed by the U.S.D.A. Salinity Laboratory, Riverside, California show approximately what a grower of alfalfa hay can expect when using increasing amounts (EC) of salt in irrigation waters. This data was developed using alfalfa varieties NOT developed for salt tolerance, but are very useful in general. The data gives the required percentage of additional water required for leaching salts (LR%) from the root zone in order to not increase the soil salt content, and salt (EC) content expected to reduce hay yields by: 0%, 10%, 25%, 50% and the point of no growth at all.

 

Crop 0% 10% 25% 50% NO GROWTH
Alfalfa Hay ECw 1.3 2.2 3.6 5.9 10.3
LR% 4% 7% 12% 19%

 

S & W alfalfa varieties that are developed for salt tolerance are subjected to soils within the root zone with water having salinity levels of up to EC 15. This would be the equivalent of irrigating with water having an EC value of 10.

Alfalfa varieties SW 9215, SW 9720 and SW 8421S are currently marketed as salt tolerant varieties, and are also very productive in soils without salt problems.

U.S.D.A. and University of California researchers tested SW 9720 in well drained sand tanks at the salinity laboratory in Riverside in 2001 using irrigation nutrient solutions with EC 15 and EC 25. With a 275 day growing period, SW 9720 yielded 11.5 tons/acre with EC 15 solution and 6 tons/acre with EC 25 solution. Selective breeding of alfalfa varieties for salt tolerance does work! SW 9215 and SW 8421S have at least as much salt tolerance as SW 9720.