Written by: Franz Niederholzer, UC Farm Advisor, Colusa/Sutter/Yuba Counties, Allan Fulton, UC Farm Advisor, Tehama/Shasta/Glenn/Colusa Counties
It’s June – the best time to apply gypsum to the soil surface in orchards with flood or wide coverage sprinklers. Why now?
But first, what does gypsum do and not do? Adding gypsum to the soil can significantly increase the rate of irrigation water infiltration when using 1) very clean (usually canal/surface) irrigation water (EC < 0.5 dS/m); 2) when the soil surface sodium adsorption ration (SAR) is 5 to 10x that of the irrigation water EC; or 3) when calcium to magnesium ratios in the water are not at least 1:1. Adding gypsum also provides additional calcium and sulfate for nutrition, if needed. Gypsum, calcium sulfate, is a neutral salt so it affects soil pH very slowly causing it to seek neutral soil pH (7.0). It won’t break up hard pans or soil layers with distinctly different soil textures or compaction that impede water infiltration. Gypsum stabilizes the soil. It reduces dispersion of larger soil aggregates when a dry soil is irrigated. In turn this reduces the formation of soil crusts and helps maintain more soil porosity and higher water intake rates.
How much gypsum is recommended to improve irrigation water infiltration for the conditions described above?
- Injecting 500 to 1000 lbs finely ground gypsum per acre foot of water should increase irrigation water EC by 0.15-0.3 dS/m, enough to improve infiltration of very clean water or reduce the effects of sodium and magnesium.
- If not using micro-irrigation, broadcast up to one ton/acre of finely ground gypsum onto the soil surface and do not till it into the soil. It will dissolve in the water as irrigations are applied and improve the water quality. The best time to apply gypsum on the soil surface to improve irrigation water infiltration is in June, when soil infiltration rates tend to decline and as higher summer temperatures (and water use) set in. The effect on infiltration may diminish after about 12 inches or more water has been applied and infiltrated into the soil, carrying the calcium deeper into the soil so it no longer amends the water quality.
- If the benefit of the gypsum appears to wear off, a second application might be needed. One application at a higher rate of up to four tons per acre may be considered as a more practical and attractive alternative. The potential problem is that the single application at a higher rate may result in much higher concentrations of dissolved calcium than is actually needed to improve the irrigation water quality and infiltration so the calcium is quickly leached below the top few inches of surface soil after only a few irrigations. Then the calcium levels are not sufficiently elevated during later irrigations and slower infiltration rates resume. In almond growing regions 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 to improve water infiltration at the soil surface the next season. The money spent for higher rates of gypsum to manage slow infiltration could be saved for more timely use.
Bob Beede, UC Farm Advisor in Kings County, put it this way years ago – “Gyp in June”.