Nitrogen use is complex issue for organic vegetable growers

The longer researchers study, the more subtle and complex organic nutrient management seems to become.

In his early cover crop studies, University of California Cooperative Extension farm advisor Gene Miyao found that a legume-mix cover crop boosted processing tomato yields by several tons in the first year, but the legume cover crop was not producing huge amounts of available nitrogen.

More recently, Miyao saw significant yield increases in processing tomatoes with the application of chicken manure compost on commercial farms in Woodland and Dixon in 2011. He repeated those trials with success in 2012 on Sacramento Valley processing tomato farms.

The benefit from chicken manure compost remains a bit of a mystery, but the size of the bonus is substantial.

"This is the most dramatic increase in yields we've seen with manures. We're going to pursue this; we're pretty excited about it. The difference we're seeing with the compost or cover crops is not a plant nutrient effect. There's a difference in nutrients, but it's not great. Something is happening with the soil," Miyao said.

Miyao and other researchers discussed the complexities of organic fertility at the seminar on Organic Production—Healthy Plants sponsored by the Organic Fertilizer Association of California and the California Association of Pest Control Advisors in Woodland.

Organic nitrogen can be far more variable than widely used synthetic nitrogen in its effects on the crop.

"The difference between organic and synthetic fertilizer is with the organic some of the nitrogen can be immobilized. The higher the carbon-to-nitrogen ratio, the higher the amount of nitrogen that is fixed," said Tim Stemwedel, owner of California Organic Fertilizers Inc., Fresno.

The complexities of organic nitrogen affect when the nutrient will become available to the crop, and also when it will pose a danger of leaching as nitrate into underground water.

Organic nitrogen can still leach when microbes release nitrogen after the crop is finished, when the nitrogen is in ammonium or nitrate form, when you apply too much, or when you fail to match applications with crop demand, according to Stemwedel.

Among sources of organic nitrogen, compost has relatively low potential for leaching, according to Stemwedel. Fishmeal also has low leaching potential if it is applied as base nitrogen.

Pelleted manure or guano, however, are relatively high in ammonia or nitrate and are likely sources for eventual regulation.

"The higher the carbon in your organic fertilizer, the more you're going to be feeding the soil and not your crop," Stemwedel said.

The Organic Fertilizer Association of California certifies organic fertilizers in much the same way as the Organic Materials Research Institute certifies organic insecticides, fungicides and herbicides. The modern organic fertilizer label should include information that will help in estimating how quickly the nitrogen will become available.

"They've gone through the initial registration of organic fertilizers in a rapid manner. Your state's doing a good job for you," said Tom Quick, global sales manager for Grow More, a Los Angeles-area supplier of organic fertilizer products.

Stemwedel's prescription for minimizing organic nitrogen leaching is to use less fertilizer, avoid overtopping, combine low levels of preplant fertilizer with in-season applications and rotate with a trap crop.

Miyao believes a preplant soil test can be useful in managing organic nitrogen.

"In our studies, anything about 15 to 16 parts per million nitrates in the soil did not respond to additional applications," Miyao said.

A typical processing tomato program is to apply 180 pounds of nitrogen, have the plant take up 240 pounds, and remove 150 pounds with the harvest.

"For canning tomatoes, the plant requirement is about 240 pounds of nitrogen per acre for the season, and reaches a peak of around nine pounds a day. That peak is at flowering and early fruit set. That 60-pound difference between what you apply and what the plants take up is coming from somewhere," Miyao said.

Most of that 60 pounds is coming from residual nitrogen put into the soil by crop residue and cover crops.

"It's clear that winter-grown cover crops reduce rain runoff from the fields. Legumes also contribute to the nitrogen supply, but the organic matter may be more important," Miyao said.

Miyao first saw the effect of winter cover crops in reducing rain runoff by 40 percent in a Sacramento Valley processing tomato trial.

He tried triticale and wheat cover crops, and found that both gave processing tomato yield boosts.

There was a significant boost in tomato yields in two of his five 2006-2007 triticale trials. As a bonus, rainfall runoff was reduced up to 50 percent by the cover crops.

There are issues, however, with the seedbed condition and those issues must be managed, Miyao said.

Conventional tomato growers were, at first, not really interested in cover crops because they had difficulty getting a good seedbed after incorporating the cover crop, he said. Studies then showed it is possible to get a yield benefit even if the cover crop is terminated early, as early as the middle of February.

"That became the strategy—early termination," Miyao said. "If the triticale gets beyond 10 to 12 inches tall, it gets difficult to manage. RoundUp works in conventional systems, but there is no easy burndown in organic systems."

It is more complex and expensive for organic tomato growers to manage their nitrogen, or even to manage the residue from their indispensable cover crop.

Organic canning tomato production is more than 260,000 tons annually, according to Miyao, which is about 2 percent of the state's canning tomato crop.

The organic market is attractive because, while canning tomato yields have doubled since the 1960s, to nearly 50 tons an acre, conventional growers have faced consistently tough prices.

"The highest price for conventional process tomatoes has never crossed 4 cents a pound," Miyao said.

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