Cover crops and compost help build soil fertility

The next generation of soil tests, in addition to levels of available nitrogen, potassium and phosphorous, could also include measurements of beneficial mychorizal fungi, and, as indicators of microbial life, the number of nematodes that feed on bacteria or fungi.

While gains in yield have come from two generations of managing the chemistry available to crops, the next step in long-term, sustainable food production could come from better understanding of the life of the soil that feeds the plants.

"Current practices may not be profitable under coming environmental reforms," said Gerald Davis, agronomist for Cal Organic Farms/Grimmway. "If we have limits on the amount of fertilizer you can apply, you will need a buffer of fertility in your soil."

He made his remarks as researchers and growers discussed what has already been learned about the life beneath their feet, and how it can be managed to produce good crop yields under difficult conditions, during the Organic Grower Summit in Monterey, co-sponsored by the Organic Produce Network and California Certified Organic Farmers.

When Davis began working at Cal Organic in 1993, the company had 1,200 acres. By the time Grimmway bought the operation, it had grown to 6,500 acres, and today the company has 35,000-plus acres of certified organic land.

Management practices like cover crops, compost, rotation and conservation tillage have been used for generations to build soil fertility, increase water-holding capacity and suppress crop diseases.

What is new, however, is the ability to understand through experience and science how well these practices are working.

As an example, Davis advised, "The higher your bacterial and fungal nematode feeding nematodes you have in your lab reports, the more you have in your soil for them to feed on."

Although he works with the firm's organic side, and the process of transitioning ground from the conventional, Davis said he believes many of the practices, including cover cropping, composting and minimizing soil disturbance, could help conventional growers become more resilient in the face of difficult climate and regulation.

The process of building fertility in the soil can take at least a decade and the learning process never ends because the process is far more complex than applying synthetic nutrients through the drip line.

"We have seen the benefits of improved soil fertility from 40 years of a program of cover crops and compost, but we have reached a plateau we cannot get beyond unless we reduce tillage," said Tom Willey, who grew rotations of Mediterranean vegetables in Madera County. "Unfortunately, as organic growers we are dependent on tillage because we don't have effective herbicides."

With as much as 80 percent of the soil biomass in the top eight inches, Willey said the challenge of reducing tillage to leave that living community in place remains a work in progress.

"Nutrient management in organic farming is more complicated and less predictable than in conventional farming," said Ehsan Toosi, director of research and development at True Organic products. "In organic farming you deliver packages of nutrients, not single nutrients. Carbon is usually involved in the delivery, and you deal more with soil biology than soil chemistry."

Organic fertility adds a new wrinkle to conventional fertilizer management where growers are advised to follow the four Rs —the right amount of the right material in the right place at the right time.

"In organic farming, the key R is the ratio among the nutrients," Toosi advised. "The right carbon-to-nitrogen ratio, and to phosphorous and sulfur, is critical. Carbon to nitrogen should be 20 to 30 to 1, carbon to phosphorous or sulfur around 80 to 100 to 1. If the ratio is too wide, availability is too slow, and if it is too narrow, it is too fast."

The ratio of carbon to nitrogen is paramount because the living microbes eat their helping of nitrogen first and then the crop gets what is left.

"You can use high-carbon materials, but you have to be aware of what you're going to do next," said Tim Stemwedel, founder and president of Fresno-based California Organic Fertilizers. "If you're going to plant a crop, you will have a nitrogen deficiency because the microbes eat first. Organic fertilizer should have about a 5-to-1 ratio if the nitrogen is going to be available."

The biology of the soil must be managed, and that includes supplying the microbes with water.

"Leaving ground totally dry stops the mineralization process," Stemwedel cautioned. "Carbon dioxide in soil plus water makes carbonic acid, which releases nutrients. Healthy soil releases less carbon if you manage it well; it releases more carbon if you don't."

A major tool for building the soil and both adding and capturing nutrients is growing cover crops that are incorporated before the subsequent cash crop.

"Cover crops increase soil organic matter and reduce nutrient leaching," Stemwedel said. "It lets us recover nutrients including minor ones we may not usually think about. It also serves as a bank that releases nitrogen for your crop."

But even the most successful farming operations never stop learning new things about this complex process of building a bank of nutrients by growing and incorporating into the soil.

"For years, we used vetch as our cover crop," said Tim Schultz, vice president of research and development at Lundberg Family Farms. "A couple researchers, one at Washington State and the other at Davis, said we might get more nitrogen from fava beans. We have narrowed it down to about four varieties of fava bean that give us twice the nitrogen of vetch, and it looks like the Washington State University V4 fava variety will be our choice."

Lundberg Farms has also learned that the time of incorporation makes a world of difference in the fertility cover crops contribute.

"If you disk it in early March, you don't get much nitrogen, but if you wait until early April, you get a lot of nitrogen," Schultz said. "We hope to get enough nitrogen to grow our rice without bringing in any other inputs."

Growers at the summit conceded that the process of building soil biology is complex, sometimes mysteriously perplexing, and always time consuming.

"Most of the ground we transitioned may have been productive in conventional systems but when we switched to organic it was crippled," said Davis from Cal Organics/Grimmway. "It takes a long time to build the microbial community after you transition away from the soluble, conventional fertilizers."

The long-term commitment to soil building not only provides the crop with nutrients, according to Davis, but also protects it against some diseases that are more troubling in conventionally managed ground.

"The biological activity we see in our organic soil means we see far less diseases like cavity spot in carrots," he said. "If we rebuild our soil aggregate structure, we can improve our situation. The soil aggregates are built by the biology. If we don't disturb them, they will do it again. We need to minimize disturbance to our soils, and that includes chemical intrusion as well as tillage."

In addition to the agronomic payoff, there are also public relations benefits to building soil that can sequester carbon and absorb and hold more water in dry times.

"Agriculture is portrayed as a major problem and polluter," Davis advised, "and we don't want that."

(Bob Johnson is a reporter in Davis. He may be contacted at bjohn11135@aol.com.)