A three year trial using gypsum to improve infiltration in border irrigated almonds in Kern County has shown a significant yield benefit to hard-shell almonds over the past two years.

So what's the big news here?

San Joaquin Valley farmers have been using gypsum to improve soil structure and infiltration for more than 50 years, so there must be an obvious benefit or they would stop using it! And there's the rub — the obvious benefit that was seen for one particular field is often not seen in another field, however, growers know that the free calcium is a good thing.

Dissolved gypsum supplies free calcium to counteract too much sodium in the soil or irrigation water, or increase salinity in the very pure snow-melt runoff used to irrigate the east side of the valley. This prevents dispersion and sealing of the soil surface.

The old standard rule of thumb (especially for a lot of Kern County potato ground) was 3 to 5 tons per acre of manure and 1 to 2 tons per acre pit gypsum applied in the fall and disked into the soil.

Even though most almond growers avoid any kind of tillage in the orchard (maintaining a smooth floor for efficient pickup of nuts during harvest) many growers will broadcast gypsum and compost every two to three years in the fall and winter and let the rain incorporate the material into the soil. This is helpful for refilling the root zone during the dormant season, but the material is long gone when you hit the peak of the season in June and July.

The soil surface reseals and you can't get enough water into the ground to meet the demands of the crop. Nut load on the trees make it impossible to drive a spreader through the orchard and you're stuck. You may see leaves drop and even reduced nut size.

Irrigation helps

Drip and micro irrigation systems help counter this problem with more frequent irrigations. These systems are also ideal for using gypsum “solutionizer” machines that inject dissolved gypsum directly into the irrigation water so the free calcium can now be applied during the summer when crop water demand is at its peak. But these machines, the automated silo storage and high quality gypsum required for this application make it much more expensive than a winter broadcast run of the “pit” gypsum typically used for flood almonds.

In an attempt to test the benefit of a cheap mid-season alternative, we set up a trial in a 22-year-old flood block planted to Missions and Buttes with 1,260-foot checks. Using a 75 percent purity gypsum costing about one-third the price of the high grade solution gypsum we put 250 pounds total (350 pounds per acre) in two piles on either side of the alfalfa valve to simply let the water dissolve the gypsum and carry it down field. This was done one time per season, mid to the end of July, in 2001, 2002 and 2003. Flow rates ranged from 55 to 95 gallons per minute with a set duration of 12 hours. Irrigations were done with low salinity Friant-Kern Canal water.

The concentration of calcium in the water was, of course, very high in the first 100 feet of advance down the check (20 meq/l) and decreased to 3 meq/l at 900 feet and 6 hours later. Even with the higher calcium concentrations at the start of the irrigation and head of the field compared to six hours and 900 feet later, the uniformity of infiltration down the check was better than 95 percent.

Infiltration evaluations at the time of gypsum application showed no difference between the untreated and the gypsum water run checks until 2003, when there was a 15 percent increased infiltration for the gypsum. Average neutron probe measurements of soil moisture to 6 feet also showed little difference until 2003 where average peak season available moisture for the gypsum treatment ran 36 percent compared to 25 percent for the untreated checks.

Major increases

The first yields (Butte trees only) were taken in 2002 where the gypsum treatment significantly increased yield by 185 pounds per acre. This difference increased to a 335 pounds per acre advantage in 2003. Average nut size for the gypsum treatment over the two years was improved by 4.1 percent while the average yield increase was 9.5 percent; 3,000 pounds per acre compared to 2,740 pounds per acre. No significant differences in leaf tissue calcium levels were found. A third and final harvest will be taken for 2004.

The good news is the total cost of this application technique was about $4 per acre for the material and about $2 per acre for the application. The results for this grower have been dramatically better than dormant season broadcasting of gypsum.

For more information on this topic, contact me at the UCCE , Kern County office at (661) 868-6218.

There will be a field meeting on May 20 near Shafter, Calif., discussing this trial as well as the use of Watermark blocks to monitor soil moisture. Call 661-868-6200 for meeting location and times.