Managing nitrogen fertilization in vegetables for yield, pest management, and the environment

In the 1970s, the Ouija Board was the rage for a new generation of young people. They hesitantly moved their trembling fingers to slowly maneuver the planchette across the magical playing board to mystically spell out answers to puzzling questions.

Determining the best fertility management for vegetable crops is far from a game of guessing, and is best determined by scientific research to maximize crop yield and quality, control costs, reduce pest pressure, and avoid environmental scrutiny.

Tim Hartz, University of California, Davis Extension vegetable specialist, outlined the latest fertility management data on vegetables to pest control advisers attending the 2007 California Association of Pest Control Advisers annual conference and Agri-Expo held in late October in Reno, NV.

Fertilizer and yields

Does excessive fertilization reduce vegetable yields? With leafy crops like celery, cauliflower, and lettuce, clearly an upper threshold exists where salt damage eventually causes problems. “However in the real world, we really don’t see that much,” Hartz said.

In fruiting crops like processing tomatoes, it’s absolutely possible for higher N rates to sacrifice fruit set for additional vegetative growth. A broad range of fertilizer practices will cause about the same yield even though vine growth may vary. The concept that excessive fertilizer results in lower yields is somewhat rare.

Yet excessive fertilizer can lead to problems, for example, when a 15 percent to 20 percent cracking rate in the profitable carrot cut and peel market results in culls. Anything can exaggerate cracking in sensitive cultivars.

“In carrot trials conducted a few years back studying N fertility and the cracking phenomenon, higher N use than required for maximum growth enhanced cracking and in some the cracking doubled,” Hartz said. “That’s surprising since carrots efficiently utilize N.”

Do higher N rates affect the post-harvest life of vegetables? Arguments for years have centered on both sides of the equation — high N enhances post-harvest problems or high N allows “the product to have some legs” and extends life in the post-harvest realm.

In UC Davis research on the post-harvest storage of broccoli, the results indicated reduced post-harvest life with higher N. There was no evidence that additional N created useful benefits.

In lettuce, UC Davis conducted 11 commercial field trials a few years back to determine the most effective N application rates.

“Some growers used the seasonal average of 257 pounds. In other tests, season N use was reduced by 100 pounds, then harvested, and the product was stored for two weeks at 40 degrees Fahrenheit intentionally trying to abuse the product to induce post-harvest problems.”

The end results showed essentially no difference between the N amounts. Additional fertility to increase carrying capacity has no foundation, Hartz said.

Fertilizer and pests

UC Davis conducted tests in San Joaquin Valley cotton fields with N and aphids, examining the difference between side-dress N amounts and aphid colonization.

“You really have no N impact until the 150-pound rate is surpassed which in this case was enough to maximize link yield and there was suddenly a large jump in aphid numbers,” Hartz said.

Tests with the whitefly had similar dynamics. The higher N rate, especially beyond the amount needed for good growth, showed enhanced reproduction of the second insect on higher N plants. A lengthy list of evidence supports this conclusion.

Does high N application impact disease pressure? The answer is a mixed bag. Clear evidence suggests plants suffering from the lack of N are more susceptible to a whole range of issues. On the other side, like with corky root in lettuce, good data suggests that excessive N fertility enhances disease pressure. For most diseases, there is a broad middle ground where N management is not likely to make a large difference, Hartz said.

Fertilizer and the environment

When Congress passed the Clean Water Act in the 1970s, the ensuing regulatory chase collared industries and municipalities with exposed pipes dumping pollutants into rivers. In the last decade, enforcement has changed course to non-point pollution sources.

In California, that activity is mostly agriculture and enforcement is mostly targeting irrigated horticultural production agriculture, Hartz said.

The federal drinking water standard is 10 parts per million N03-N (nitrates-nitrogen). Where is nutrient enforcement headed? Phosphorus levels are lurking in the background.

“Once monitoring programs determine repeatedly that orthophosphate content in runoff water is higher than any other place in the country, this will draw scrutiny,” Hartz said. “The issue is predominantly one of stimulation of algae growth because algae growth in the stimulation phase, bloom, and die back induces unfortunate things. As we go forward, we not only have the drinking water standard but the environmental standard for algae control.”

The severity of the problem depends on farm location. Farmers in Kern County, Calif. have nothing to sweat about since runoff is not an issue. The Salinas and Santa Maria is a whole different story, Hartz said.

Nutrient best management practices

Hartz offered the PCAs best management practices (BMPs) for efficient and effective nutrient management:

1– Use a realistic template for N requirements in crops.

In a 2004-2005 survey of commercial lettuce fields, results showed high N rates are unnecessary. The mean of 11 N fertilization trials in head and romaine lettuce indicated plants fail to take up excess fertilizer.

A reasonable template to follow is the industry’s current median rate — 170 pounds N per acre per season in lettuce and 185 pounds for tomatoes. Higher rates are seldom justified.

2- Base phosphorus (P) fertilization on the P soil test.

Is phosphorus application always necessary? In elevated, background soils with plenty of existing phosphorus, extra phosphorus fails to enter the plant. Extra P only enriches the soil base creating environmental problems.

3- Adjust N template by in-season monitoring.

Tissue sampling as a BMP may be useful in identifying yield-limiting deficiency. If the N is above a deficiency level, tissue nutrient concentration is unreliable for estimating current soil N supply and is therefore of little value as a fertilizer BMP.

4– Manage irrigation effectively.

Converting to drip irrigation saves nitrogen, but only because losses are limited by leaching.

5- Grow winter cover crops to ‘trap’ residual N and reduce runoff and erosion.

Fall-planted vegetation can reduce the flood of potential effects from a two-inch rainstorm. One environmental win-win is mustard crops that reduce runoff volume and sediment loss.

Some proclaim that mustard crops contain compounds that break down and release products that act as a bio-fumigant, similar to the effects from the product Vapam. However, tests conducted to determine this potential impact on soil-borne pests failed to find a single circumstance where that was true. There was no replacement for chemical fumigation.

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