Organic growers target nitrogen balance for crops

Organic vegetable and berry growers face a great challenge knowing how much nitrogen will be available to their crop and when.

While conventional growers apply synthetic fertilizers and know with some certainty how much nutrients will be available to the crop, organic growers rely on a variety of ways to build soil with nutrients that become available slowly over a period of months and even years.

"A lot of the nitrogen from cover crops, crop residue and compost is being built into the soil, and that is where we get this tremendous ongoing release of nitrogen in organic systems," said Margaret Lloyd, University of California Cooperative Extension small farms advisor in Woodland. "With cover crops, residue and compost, we build this robust nitrogen source in our soil."

Hitting the target in applying nitrogen fertilizer is important for economic and environmental reasons but is challenging in organic systems because most of the nitrogen becomes available gradually, she said.

"It's very difficult to precisely apply nitrogen in organic systems," said Richard Smith, UCCE farm advisor in Salinas. "Improving nitrogen fertilizer applications will revolve around growers' experience and skills, soil-nitrate testing and understanding the crop growth characteristics."

Lloyd and Smith made their remarks as UCCE advisors and specialists presented their advice on matching available nutrients with crop needs in presentations during the virtual Organic Fertility Series.

Presentations by the seven experts on the role of cover crops, compost and crop residue in building fertile organic soil are available through the Monterey County Cooperative Extension website.

"We may think of amendments in terms of their contribution to nitrogen, but it's really many sources: the soil, cover crops, poultry manure, compost, granular fertilizer, the irrigation water," Lloyd said. "All of the nitrogen contributions from many places are building the nitrogen program in an organic system."

Micro-organisms in the soil make nitrogen available to the crop through a complex process that depends on many factors, including the weather.

"When you add crop residue or organic amendments, the nitrogen needs to be converted to ammonium or nitrate by micro-organisms," said Daniel Geisseler, UCCE specialist in nutrient management. "The main factors affecting how fast this happens are soil temperature, soil moisture and the quality of the organic source."

This process moves more quickly when the soil is warm and moist. A lower ratio of carbon to nitrogen also speeds this mineralization process, which means nitrogen becomes available fastest in most growing areas in the summer.

"In the summer, the soils are warm, and they are moist from irrigation," Geisseler said. "In the winter and spring, the temperature is limiting. In the fall, moisture is limiting if the field is fallow."

Nitrogen mineralizes slower from cover crops and residue when the percentage of nitrogen is low.

"Broccoli, lettuce and legumes are higher in nitrogen, and tomatoes, melons and young cereal cover crops are lower in nitrogen," Geisseler said.

That difference impacts what cover crop varieties to plant—and how to manage cover crops at termination and crop residue after harvest.

"Legume and mustard cover crops tend to have a higher percentage of tissue nitrogen," said Louise Jackson, a UC Davis plant sciences professor emeritus who did pioneering studies on organic nitrogen management. "If you have a legume cover crop, you should avoid a lag between incorporation and planting the cash crop in order to minimize the loss of nitrogen."

Another important nutrient and water management tool is to encourage the growth in the soil of beneficial mycorrhizal fungi, which attach to the roots and mine the soil for nutrients and water.

"Lots of information shows that continuous cropping with less tillage and less fallow time is conducive to the symbiosis between mycorrhizae and crop roots," Jackson said.

Matching the nitrogen available in the soil with the amount needed by the crop begins with an understanding of the amount taken up by various crops over the course of the season.

Spinach takes up 90 to 130 pounds an acre of nitrogen, full term lettuces from 120 to 160 pounds, and broccoli between 250 and 350 pounds, according to studies conducted by UCCE specialist Tim Hartz.

Irrigation management is another crucial piece of the puzzle of making nitrogen available in the root zone when the crop needs it.

"You can do a lot of nitrogen budgeting, but if you're not paying attention to water management, that budgeting for the season is not going to be that accurate," said Michael Cahn, UCCE irrigation farm advisor.

Spinach, for example, takes up 6 pounds a day of nitrogen in the final two weeks before harvest. That nitrogen must be available in the narrow band of soil that is reached by the shallow roots.

"In spinach, we found that most of the nitrate is coming from a narrow band between 4 and 12 inches deep, which makes irrigation management critical to keep the nitrate in this zone," Smith said.

Strawberries also have a unique cycle, as a study showed the crop at the organic field in Moss Landing took up a little more than 90 pounds of nitrogen over the course of the season. Uptake was just a pound a week in the weeks after planting but before fruiting, and then increased to 3 pounds a week while the plants were producing berries.

"You have to match nitrogen supply with the nitrogen demand of the plants," said Joji Muramoto, UCCE specialist in organic production. "These figures were taken from four years of study at an organic strawberry field in Moss Landing averaging 37,800 pounds of berries."

There can also be significant differences in how much nitrogen the plants take up from the soil, as a deep-rooted crop such as broccoli may scavenge the soil for more than more than 150 pounds.

From 2016 to 2019, UC researchers evaluated fertilizer use on 28 blocks at large-scale Salinas Valley organic vegetable operations and found that growers applied on average 205% as much nitrogen as was taken up by the crop, but the fertilizer still supplied only 85% of the nitrogen taken up by the crop.

Much of the nitrogen in the fertilizer only became available over a long period of time, but soil microbes were making other sources of nitrogen available to the crop to take up the slack.

Soil quick tests can take some of the guesswork out of fertilizer application decisions in both conventional and organic systems.

When soil tests show 20 parts per million nitrates, that translates into 70 to 75 pounds of crop available nitrogen an acre, which should be enough to feed a vegetable crop for a week or two.

Use of the soil quick test to decide when an application is needed is trickier in organic systems, however, because unlike conventional fertilizers, organic fertilizers applied after the soil nitrate supply is exhausted release their nitrogen gradually over time.

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