You’re putting ample water onto your almond orchard, but is it getting to where it’s needed?
This was the question posed in a recent internet posting by David Doll, University of California pomology farm advisor in Merced County.
Chemical sealing of the surface soil is among challenges that can occur, creating a crust which reduces the water movement into the soil when water is applied faster than the rate of infiltration, Doll said.
“Puddling occurs, leading to an increase in evaporation, as well as saturated soil conditions,” he said. “This impacts water use efficiency and tree health.”
There’s no single culprit for low infiltration, Doll explained. It can result from compaction, loss of minerals in the upper layer, poor water quality, or other factors including chemicals in the soil.
Doll said a season of irrigation can require between 36 and 52 inches of applied water per acre. A problem is that the water is often applied to a limited area of an orchard, defined as the wetting pattern.
For micro sprinklers, that’s around 30-60 percent of the orchard area. For drip, it’s around 20 percent. This means the wetted area may receive two to five times more water than the targeted season’s application per acre.
“In other words, if four acre feet per acre were applied using a drip system, this wets around 20 percent of the soil in the orchard floor,” Doll said.
“This is a tremendous amount of water that must pass through in order to infiltrate the soil, and it can leach away beneficial elements, which leads to chemical sealing and infiltration problems.”
Finding the case of low infiltration rates is key, and Doll said taking an analysis of the water and soil is a good place to start.
Unlike soil sampling conducted in the fall, soil sampling of the top two to three inches of soil should occur to identify the chemical imbalance, he said. Analyzing the soil and water will provide an idea of the salt load, sodium absorption rate, and the pH level plus other elements.
“This will help identify what should be applied to open the soil back up,” Doll said.
The cheapest way of adjusting management of the water is to adjust irrigation frequency, the water source, or the irrigation system.
A soil analysis of crusted surface soil can reveal an imbalance of cations. Gypsum can help remedy this, applied to the soil during dormancy or in June or suspended in irrigation water weekly or monthly, the farm advisor said.
If these types of applications aren’t possible, Doll said consider using calcium containing fertilizers including calcium nitrate, calcium ammonium nitrate, or calcium thiosulfate.
“It might be best to time the use of these in May when water applications increase and tree nitrogen demand is high,” Doll said, adding that if a crust has already formed then tillage may be needed to break it up.
Sodium increases particle dispersion and decreases aggregate formation, thereby reducing water infiltration. Generally, Doll said, this happens when the sodium calcium ratio is greater than three parts sodium to one part calcium. This effect may be amplified by alkaline irrigation water, which may precipitate or “tie up” available calcium.
He said this can often be identified by looking at the SAR (sodium absorption rate) and adjusted SAR value on a water analysis. Calcium containing amendments or water acidification can help.
Extreme soil pH on either end of the scale can also lead to soil sealing, Doll said. Adjusting pH levels to close to seven will help deal with alkaline soils that may be high in pH. He said water pH should not be dropped below five.
In low pH soils, the options are more limited, and lime or other neutralizing amendments must be added.
Soil compaction can also reduce infiltration as the weight of machinery disrupts the soil’s pore space, Doll said, adding that tilling can help but prevention is the key.
“Avoid driving in the orchard when the soil is wet and use lighter weight equipment when possible,” he said. “Remember it’s not about total weight, but the weight per square inch of wheel contacts.”
Doll said surfactants can help increase infiltration and cover crops improve infiltration as well. Biochar, a form of wood charcoal, and regular compost applications have also shown to increase infiltration rates, yet the longevity and feasibility is unknown.
The farm advisor said managing infiltration issues should occur regularly, and he warned against waiting until the issue arises.
More information can be found in a chapter in the UC ANR Production manual online.