By Dennis Pollock, Ag Alert
To cover crop or not to cover crop. That’s a decision based on weighing advantages and costs. It’s a particularly challenging question considering what the costs may be in the San Joaquin Valley, where rain is often sparse.
Farmers recently discussed the water dynamics of the valley as they debated the use of perennial cover croppings for grapes and almonds.
Steven Cardoza with Cardoza Ranches in Fresno County said he remains a true believer in cover cropping. Cardoza farms organic raisins in Easton. He was 12 years old when his father, Dwayne Cardoza, chairman of the Raisin Bargaining Association, first planted a cover crop.
Cardoza now praises the progressiveness of his father. He said he wants to be able to farm for decades, and he said he believes cover cropping will afford him the chance to do that. He said it makes his land more resilient and less dependent on inputs in future years.
Cardoza left the family farm for a time for college and a sales job. He later returned to farming. He took to experimenting with cover cropping, reduced tillage composting and using drones and moisture probes.
The California Department of Food and Agriculture healthy soils program is now helping Cardoza conduct research on cover cropping, including compost application as well as mulch. The CDFA program provides financial incentives to California farmers and ranchers to implement conservation management practices that sequester carbon, reduce atmospheric greenhouse gases and improve soil health.
Cardoza recently shifted from doing crops on every other row to doing them on every single row.
“We noticed how extremely effective the cover crop is at suppressing weeds,” he said. “The fact that there is a cover crop in every row means that we’re not disking every other row, dramatically minimizing our tillage.
“Cover cropping is the tool I would never take out of my toolbox,” he added.
His plantings include nitrogen fixers, he said, “my nitrogen budget for the year.” He doesn’t apply nitrogen in any other form.
There’s also daikon radish, grass and other plantings. When done harvesting in some vineyards, he can flood irrigate and plant cover crops. With drip irrigation only, he waits for the first heavy rainstorm, then tills lightly and uses a planter.
Cardoza said thick taproots “make large pores that open up the soil.”
He said he wants living plants on as much of the ground as possible and for as long as possible.
He mows the cover regularly and gets it “down to a stubble” a month before harvest, preparing the bed before laying out grapes on paper trays to dry in the sun.
“We have to get it perfectly flat like everybody else,” he said. But unlike growers who have no cover, there is not the need for frequent tillage.
Cardoza said places where cover cropping has been done for the longest periods have the highest water retention levels. He uses soil moisture probes to assess levels and confirm his suspicions that more water is retained in the root zone.
“In our area, our soils are extremely sandy,” he said. “So, water just runs through it like a coffee filter. Anything you can do to keep water higher in the soil profile and in the root zone, you can have massive benefits.”
The covers also create habitat for beneficial insects.
Cardoza showcased use of a piece of machinery for mowing and mulching. The machine disperses mulch onto the berm near the vines.
Ben King, who manages orchards from Kern to Colusa counties, grows cover crops in almond and pecan groves. His cover crops include clover, peas and grasses. He said he believes it’s most important to improve the water-holding capacity of soil.
“To the extent the soil holds that water, it’s in the bank and can be accessed,” King said.
King said he has seen no negative impact on yields. But he said one year a particularly lush cover crop did attract rabbits, which gnawed on drip lines.
“We mowed earlier that year,” he said.
Both King and Cardoza use a seed drill to plant cover crops rather than broadcast seed.
King said timing is important and “ideally you want to drill before the first rain. It’s like a field of dreams: You put the seed out there and hope the rain comes.”
“The issue with broadcasting is that a lot of times you’re just providing expensive bird food,” King said.
As to choosing covers, Cardoza said the machine he uses can help shift to plants that are denser and become a source for mulch.
For almonds, King said, he looks for some plants that act as forage for pollinators. He also has mixes of legumes and mustards as well as a soil builder mix with radish, mustard, grasses and peas.
Lauren Hale, a research soil scientist with the U.S. Department of Agriculture Agricultural Research Service in Parlier, is heading research into cover crops in partnership with a state demonstration project in California.
She said researchers are looking at cover crops to enhance soil health and for regenerative agriculture and are seeking to determine whether they are practical in places where irrigation water resources are limited.
“We wanted to see if benefits conferred from the cover crop are passed to the soil below the vines,” she said.
The vineyard was established last year. In 2019, researchers broadcast seed mixtures over a “fluffy seed bed,” she said, covering seed mixtures with a quarter inch of soil. In January 2020, plants began to emerge.
One seed mixture included five introduced species, dominated by a Merced rye. Another mixture was all native species, dominated by phacelia.
Winter of 2020 brought low annual precipitation. Microsprinklers were used to help establish the cover to generate a lush, dense cover with a high biomass, which attracted many insects and bees.
The cover was allowed to reseed and dry out. It was then mowed down, and residue was left to serve as a mulch. The grape variety is Almond King, which is harvested in autumn.
There were variations in volumetric soil moisture among species when applying the same amount of water. The amount of water applied was lowered for native species.
Vines adjacent to native cover crops proved to have wider trunk diameters than those near the introduced crop.
In the fall of 2020, researchers looked at infiltration rates, but impacts are expected eventually. They also looked at the total soil microbial biomass in different soil samples.
In native treatments, there was enhanced microbial biomass.
“Did we save water?” Hale asked. “This year and in the first season, no, we did not.” A factor in that was that precipitation received from November to March 2020 was 63% of average.
But the advantages of the cover, especially the phacelia, was that “bees just love this crop,” she said. Its biomass also boosted the carbon-nitrogen ratio. That was believed to help enhance the microbial community and the soil structure.
This year, less water was applied, just 36 gallons of water per vine compared the previous year’s 156 gallons. No more water will be applied to cover crops. Precipitation was 71% of average in the cool season months. It’s not yet known what the water savings will be.
Hale said it may be that growers elect to have a dense cover in a wet, cool season, and in a dry, cool season, they don’t. It will be a question they will likely have to ponder year by year, he said.
