SALINAS, Calif. - Something as simple as cover cropping can
dramatically improve vegetable yields, new organic research is showing.

The research, now in its seventh year, is conducted at the U.S. Department of Agriculture’s Salinas, Calif., research station by research horticulturist Eric Brennan.

Brennan has found through several season of research that for organic growers, adding an inexpensive cover crop can boost yields to more than 800 cartons per acre — levels commercial conventional growers see in their fields — and suppress weeds that are not only expensive to remove but deprive the soil of needed nutrients.

The study is the longest running organic trial in the state, Brennan said.

“It provides a real comprehensive data set that gets at everything from profitability to weeds to soil quality,” Brennan said.

Standing in a field a few miles outside Salinas, Brennan looks over a patchwork of plots studied in the experiment that are covered with different cover crops and seed levels.

Come spring, the plots will grow lettuce and broccoli, but during the dormant winter months they are covered with tall legume and rye cover crop mix, rye, and mustard cover crops.

Brennan said there are eight different treatments for the various plots in the experiment: Six plots are cover cropped, three at recommended rates, and three at three times the recommended rate.

Two more plots are left bare, except for one winter season, and treated with fertilizer. Those fields left bare sometimes failed to produce measurable yields, Brennan said, or produced less than 100 cartons per acre.

“They basically represent the three different cover crops that are commonly grown during the winter,” Brennan said.

What the results have shown so far is more weeds tend to grow in plots with the recommended seeding rates, but those with higher seeding rates, especially of the legume-rye mixture, tend to see greater weed suppression and higher yields, Brennan said.

Standard seeding rates for mustard and rye cover crops may turn out to be just as effective as the higher seeding rates, Brennan said, but he won’t know for sure until the end of the project next year.

The 25 acres where the experiment is conducted are certified organic, Brennan said, and undergo the same certification inspections as any other organic grower.

The vegetables produced in the farm are sold commercially, Brennan said, through Salinas wholesalers and other companies.

Even though the study is looking at organic growing systems, Brennan said the preliminary results are applicable to commercial conventional farming.

Throughout the Salinas Valley many fields are not covered during the winter months, and Brennan said this is usually due to the additional cost and time it takes to mow cover crops, disc them into the fields, and let the plant material decompose long enough to provide nutrients in the soil.

But the benefits are potentially great because Brennan’s experiment also shows fertilizer use is much lower for cover cropped fields, reducing nitrogen runoff into ground water and the Salinas River.

“We need to see a lot more cover cropping by conventional growers to improve the sustainability of these systems,” Brennan said.

“Without doing that we’re fooling ourselves that we’re farming sustainably.”