Balancing the Soil Fertility Equation: Five Ways to Reduce Nitrogen Leaching

This article was written by Jane Sooby.

Widespread leakage of nitrogen from agricultural production has become a huge problem worldwide. Recent news articles have reported that numerous small towns in California’s central valley, the country’s most productive agricultural region, must use bottled water because their municipal water supplies are contaminated with dangerous levels of nitrates. A report issued by UC Davis in early 2012 documented that in California’s Tulare Lake Basin and Salinas Valley, “roughly 254,000 people are currently at risk for nitrate contamination of their drinking water.” The “dead zone” in the Gulf of Mexico grows larger each year, depleted of oxygen by uncontrolled algal growth.

Conventional agriculture’s reckless use of chemical fertilizers is the main culprit, followed closely by large-scale concentrated livestock production. Globally, human activity has disturbed natural nutrient cycling in a variety of ways. Since discovery of the Haber-Bosch process to industrially produce nitrogen in the 1940s, scientists estimate that the Earth is saturated with twice the amount of nitrogen than can be processed through natural ecosystems. In nature, nitrogen, phosphorus, and carbon cycling are linked, and scant availability of nitrogen puts the brakes on over-production. Humans have released the brakes, resulting in a broad array of negative consequences: from an altered atmosphere to degraded soil and water quality, and reduced plant and animal biodiversity.

The good news is that the practices organic farmers use to deliver nutrients to their crops are precisely the practices that help to “re-couple” the nitrogen, phosphorus, and carbon cycles that have been disrupted by industrial agriculture. Long-term studies in Midwestern field crop rotations show that organic systems have about 50% less nitrate leaching than conventionally managed systems. Why is this? Using practices, such as cover cropping, that build soil organic matter helps to slow the release of nutrients so that they may be utilized by plants or soil organisms before being leached out of the root zone by irrigation or rain water. Still, nitrates can leach from organic or conventional farms if nutrients are not managed carefully.

Planting a winter cover crop is the number one most effective practice any farmer can use to reduce nitrate leaching. In intensive vegetable and field crop production, high levels of nitrogen remain in residues and the soil after harvest. If the land is kept fallow, winter rains will wash that nitrogen directly into groundwater. A study conducted in California coastal vegetable production showed that a winter cover crop of Merced rye reduced nitrate leaching by 70%. One experienced organic farmer uses an oat scavenger crop to absorb excess nitrogen in the winter. In Iowa, winter rye is commonly used. Mustards, grasses, legumes, or mixtures of these are typical winter covers, but the species of the cover crop isn’t as important as the crop’s ability to grow well in that location.

One of the largest challenges in organic production is matching nutrient availability to crop need. A basic tool to help growers make their fertility management decisions is soil testing. While most soil tests are aimed at informing conventional growers about levels of soluble nutrients in their soils, an organic farmer can gain valuable information from knowing the soil organic matter content, pH, and micronutrient availability. “Thinking of organic matter as fertilizer” can be a useful approach: University of California research shows that soils containing between 1-2% organic matter can, through microbial activity, release 30-60 lb. N/acre to a 60-day summer crop.

Using soil tests to develop a fertility strategy for each crop can help growers stay on top of the nutrient status of their soils, make more efficient use of applied fertilizers, and reduce the chance of over-applying nutrients.

Consistent use of cover crops and compost can build organic matter that provides background levels of soil nutrients including nitrogen. Short season crops such as lettuce, beets, and radishes may have all their nutrient needs met by residual nitrogen from a legume cover crop or a pre-plant fertilizer application. For other crops, these levels are not adequate to support full-season crop growth and nutrients must be supplemented throughout the growing season. The total amount and timing of fertilizer application will depend on the crop, soil test results, soil temperature, and other factors. Longer season, high nitrogen-requiring crops, such as peppers or tomatoes, require split applications of fertilizer divided between pre-plant application and a mid-season sidedress or foliar feed.

Planting early season crops into cool soils before soil microbes become active can pose a challenge. Because microbial activity is responsible for liberating soil nutrients, such crops might have a hard time acquiring adequate nitrogen and phosphorus from the soil alone and may need supplemental pre-plant applications of nutrients.

Managing irrigation is also an important factor in preventing nutrient loss. The perfect amount of nitrate can be present at planting but one big irrigation can wash it all away. Another factor to keep in mind is the nitrate level of the irrigation water used on the crop, which can be a significant part of the nutrient equation.

Nitrogen management is a moving target on an organic farm because it is difficult to know precisely how much nitrogen will be released or “mineralized” from the organic matter over a given cropping cycle. Richard Smith, vegetable crop advisor for Monterey County Extension, recommends that growers “Experiment. The situation is constantly changing so you have to constantly tweak what you do. Try one-half your fertilizer rate on a strip and see how it goes. Or increase the rate on a strip and see what happens. Don’t get too comfortable with ‘400 pounds of feathermeal per acre.’”

On California’s Central Coast, policies are being implemented that require growers to conduct groundwater monitoring and reporting for nitrates. As more public attention is paid to the problem, farmers nationwide are likely to see increased regulation of ground and surface discharges of nitrate in the future. Because of the higher organic matter in their soils, their use of slow-release fertility inputs, and their willingness to grow cover crops, organic farmers are in a good position to meet these regulations.

5 ways to reduce nitrate leaching from the farm:

1. Plant winter cover or scavenger crop

2. Build soil organic matter using practices such as cover cropping and compost applications

3. Conduct annual soil tests and use organic matter, pH, and micronutrient levels to plan your fertilization strategy

4. Use split applications of nitrogen fertilizers on long-season crops; rely on mineralized nitrogen for short-season crops

5. Manage irrigation carefully



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