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Soil Studies Aimed To Help Organic Growers on Fertilizer

Organic growers face a challenge to delivering precise levels of fertilizer to their cool season vegetable crops because levels of nitrogen made available from the soil and from the organic fertilizers are complex and difficult to produce.

Having an answer may be increasingly important. That’s because the Central Coast Regional Water Quality Control Board has adopted rules that will allow growers to apply only limited amounts of nitrogen over and above what is removed with the crop at harvest.

While the regulations most seriously impact conventional growers who rely almost entirely on fertilizers that make nitrogen readily available, organic growers would also would be affected, said Richard Smith: University of California Cooperative Extension vegetable crops farm advisor.

“Large organic growers could be seriously impacted by the new water quality fertilizer standard, but possibly less so than conventional growers because the materials they use are not completely available” Smith said. “Only a portion of the nitrogen in organic fertilizers becomes mineralized to nitrate. The rest becomes slowly available over time.”

During the past four years, Smith and other researchers who have studied soil fertility observed on farms that contribute to the Central Coast’s estimated $766 million cool season organic vegetable production. The crops include lettuces, Cole crops, spinach, strawberries, kale and arugula.

In an article published in the October issue of California Agriculture, Smith and fellow researchers summarize findings from the studies at Monterey County organic cool season vegetable operations that typically farm 300 - 1000 acres. Organic fields were paired with neighboring conventional fields with similar crop and soil mixes.

The goal of the fertilizer management is to make enough nitrogen available to the crop without having nitrate nitrogen leach below the root zone.

“It’s very difficult to precisely apply nitrogen in organic systems” Smith said “improving nitrogen fertilizer applications will revolve around growers experience and skills, soil nitrate testing, and understanding the crop growth characteristics.”

In their article “Fine-Tuning Fertilizer Applications in Organic Cool Season Leafy Green Crops Can Increase Soil Quality and Yields” Smith and UCCE researcher Michael Cahn, Daniel Geisseler and Tim Hartz noted that conventional growers routinely measure soil nitrate nitrogen levels and adjust fertilizer application rates.

“In organic vegetable production, however,” they wrote, “soil nitrate tests are rarely used to guide fertilizer applications due to the uncertainty of how much and when nitrogen is mineralized from soil organic matter an organic fertilizers.”

Yet they concluded the soil testing for residual soil nitrate nitrogen can be useful in organic vegetable production. They said a quick soil test conducted in the field by dipping a color coded to strip in a tube of calcium nitrate mixed soil from the root zone can estimate nitrate levels and let growers know in a few minutes if there’s enough nitrogen available to skip or reduce a site sidedress application.

“However, to estimate figure availability of residual soil nitrogen in soil,” they wrote, “nitrate tests appear to be most useful for heavier soils. The tests are less useful for heavier soils, where nitrate nitrogen can be leached by irrigation water before it is utilized by the crop”.

They said more studies are needed using nitrate tests to fine-tune fertilizer nitrogen applications in organic cool season vegetables.

Organic fertilizer makes nitrogen available more slowly than conventional fertilizer, and the researchers provided calculations for how slow the release is. The researchers said nitrogen that mineralized from dry fertilizers ranged from about 15% to 61% and liquid fertilizers range from 52% to 69% of the initial nitrogen content of the fertilizer.

A solution to nitrate leaching in organic systems would be years of cover crops and compost that build a large pool of organic nitrogen that becomes available and gradually to feed the crops.

Smith and U.S. Dept of agriculture researcher Eric Brennan produced studies the water quality control board used to relax rules for nitrogen that comes from cover crops and compost. The economics of production in Salinas Valley, however, can make it difficult or impossible for growers serving mainstream markets to take ground out of production long enough to implement this system.

“Organic vegetable growers on the Central Coast face the same production pressures as conventional growers; high land rents, scheduling pressures and food safety concerns,” the researchers wrote in the California Agriculture article. These issues create barriers to using, crops and compost which generally were not used in the organic forms we evaluated.”

Fertilizer calculations are trickier for organic production because of crops use more organic nitrogen that becomes available over the growing season and because nitrogen in organic fertilizers is available more slowly than organic than conventional fertilizers.

But the researchers concluded the quick test can still be useful in organic vegetable production.

“Soil tests for residual soil nitrate nitrogen are most accurate when done immediately prior to a fertilizer application” they wrote. “However the time lag in the release of nitrate nitrogen from organic fertilizer as a level of uncertainty to the use of nitrate nitrogen tests, especially if residual soil nitrogen is lost to leaching before the crop can utilize it.”

In organic systems there’s more organic nitrogen in the soil that will become available gradually over the growing season.

“In the 20 fields we evaluated, we observed that the net amount of nitrogen supplied by organic fertilizers applied to cool season vegetables was generally less than crop uptake” the researchers wrote. “This was because the organic fertilizers released only a portion of the nitrogen they contain during the crops cycle.”

They said it is likely that modest amounts of residual soil nitrogen and nitrogen in irrigation water “made up the difference necessary for acceptable yields.”

By bob Johnson

Reproduced here by permission of the California Farm Bureau

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