If you’re looking for a way to reduce runoff from flood or wide-coverage sprinkler irrigation, you might consider applying gypsum to the soil surface in your orchards this month.

For one thing June is a great time to do this, says Franz Niederholzer, University of California Extension Farm Advisor for Colusa, Sutter and Yuba Counties. For another, it helps improve water infiltration, which adds to the amount of plant available water in the soil. Plus, it can also give trees a nutritional boost in the form of more calcium and sulfate, if needed.

Adding gypsum to the soil can significantly increase the rate of irrigation water infiltration under three conditions, Niederholzer reports:

  1. When using very clean (usually canal/surface) irrigation water (EC < 0.5 dS/m);
  2. When the soil surface sodium adsorption ration (SAR) is 5 to 10 greater than that of the irrigation water EC; or . . .
  3. When calcium to magnesium ratios in the water are less than 1:1.

Because gypsum – calcium sulfate – is a neutral salt it affects soil pH very slowly, causing it to seek neutral soil pH (7.0) over time, Niederholzer explains. It won’t break up hard pans or soil layers with distinctly different soil textures or reduce compaction that impedes water infiltration.

Instead, gypsum stabilizes the soil, reducing dispersion of larger soil aggregates when a dry soil is irrigated. In turn, this reduces the formation of soil crusts, opening up the soil and allowing more water to soak in rather than run off.

When using drip or micro-sprinkler irrigation Niederholzer and Allan Fulton, UCCE water resources advisor for Tehama, Glenn, and Colusa Counties, recommend injecting 500 to 1,000 pounds of finely ground gypsum per acre foot of water. This should increase irrigation water EC by 0.15-0.3 dS/m. That would be enough to improve infiltration of very clean water or reduce the effects of sodium and magnesium. Many growers using micro-irrigation do this on a regular basis through the season, they add.

So, why is June an ideal time for growers using flood or wide-coverage sprinklers to apply gypsum on the soil surface for improving irrigation water infiltration? Because that’s when soil infiltration rates tend to decline, temperatures are rising and trees start using more water.

Under these conditions, Niederholzer and Fulton advise broadcasting up to one ton per acre of finely ground gypsum onto the soil surface. Also, don’t till it into the soil. It will dissolve in the water as irrigations are applied, improving water quality.

Gypsum is of little benefit when irrigating with groundwater, these advisors note, since that source usually has enough dissolved salts to more than equal the amount gypsum would provide to clean surface water.

“The effectiveness of gypsum in improving water infiltration may diminish after about 12 inches or more water has been applied,” Niederholzer says. “As water infiltrates into the soil, it carries the calcium deeper into the soil, where it no longer improves water quality.”

If the benefit of the gypsum appears to wear off, a second application could be needed. Instead of applying gypsum twice, it might seem more practical to make just one application, but at a higher rate – up to four tons per acre. But, he notes, that might create some problems.

“The higher rate may result in much higher concentrations of dissolved calcium than is actually needed to improve the irrigation water quality and infiltration,” Niederholzer says. “Then calcium would be quickly leached below the top few inches of surface soil after only a few irrigations. Then the calcium levels would not be high enough during later irrigations and slower infiltration rates would resume.”

In areas that receive more winter rainfall, higher rates of gypsum will leach deeper into the soil profile during the winter and will not be as effective for improving water infiltration at the soil surface in no-till conditions the next season. “In that case, the money spent for higher rates of gypsum to manage slow infiltration could be saved for more timely use,” Niederholzer says.