Answer: Here is some information on nitrates, water, and vegetable crop quality.
Nitrogen takes many forms as it works its way through the natural nitrogen cycle. Atmospheric nitrogen is very abundant, and is constantly in transformation. The atmosphere over one acre of land has approximately 35,000 tons of atmospheric nitrogen. This atmospheric nitrogen is fixed by rhizobial bacterial living in symbiotic association in leguminous plant roots. Upon decomposition, this nitrogen is released into the soil and becomes available for plant uptake. In addition, a 7-inch plow layer of soil contains on average about 1000 pounds of nitrogen. This nitrogen is released from soil organic matter at the rate of about 1 to 4 percent annually. Nitrate is the most common form of nitrogen in the soil environment. It is taken up by plant roots and metabolized into amino acids and proteins which are then used by the plant for growth and reproduction. The ammonium ion is the other form of nitrogen that occurs in soils and available for plant update, especially in association with organic fertilizers.
Synthetic nitrogen fertilizer is a nitrate form of nitrogen. It does not need to be converted by bacteria into a plant-available form, and is very soluble in water. This is the source of most nitrate pollution in groundwater, but livestock manure can contribute to this as well.
Conventionally raised vegetable crops may have nitrogen applications in excess of 400 pounds per acre during the growing season. Organic vegetables producers rely on leguminous cover crops and green manures, supplemental organic fertilizers such as plant meals and animal meals, and yearly manure and compost applications to maintain nitrogen fertility. In fact, fall-seeded leguminous cover crops can contribute 50 to 150 pounds of nitrogen per acre to the following year's crop, depending on the species and soil conditions.
Irrigating with Water High in Nitrates
Irrigating with water that is contaminated with nitrates is a practice that has been studied by soil scientists. Studies with water having nitrate concentrations of 0.1 ppm to 30 ppm have been conducted by the University of Nebraska (Watts, 2001). The EPA has designated a limit of 10 ppm in water used for human consumption.
Assuming a level of 10 ppm of nitrates in irrigation water (which is equal to 10 mg per Kg), this amounts to 20 pounds per acre of nitrogen in the field. Not all of this nitrate will be absorbed by plant roots, because the soil already has a residual amount of nitrogen in it. Watts (2001) has found that with 55 pounds per acre of residual nitrogen and the use of a starter fertilizer only, only about 50 percent of the water nitrates were utilized by the crop. The rest was leached below the rooting zone.
Nitrates and Human Health
There are health risks related to nitrates in human food. When nitrates are ingested, they are converted to nitrites. This molecule can replace oxygen on red blood cells, resulting in a condition called methemoglobinemia, or "blue baby syndrome." It is for this reason that nitrate levels in drinking water are set at 10 ppm, and at 200 ppm for baby foods.
Nitrates occur naturally in vegetables, with leafy vegetables having higher concentrations. For example, radishes and lettuce can contain up to 1.9 percent nitrate-nitrogen, which is more than most vegetables (Brown, 1993). This equates to 8.4 ppm nitrates, well under the 200 ppm limit set by the European Commission for nitrates in baby food (EIS, 2004).
Given this information it seems that irrigating with 10 ppm high-nitrate water will result in food that is safe for human consumption. However, this would be subject to plant tissue tests (which can be conducted through your local Cooperative Extension Service) and adjusted fertilizer applications based on the presence of nitrates in the water.
More detailed references on nitrates in vegetables are also available from ATTRA upon request, via our Ask a Sustainable Ag Expert service.
Brown, J.R. 1993. Nitrate in Soils and Plants. University of Missouri Extension.
European Information Service. 2004. Commission Sets Maximum Nitrate Levels for Baby Food. European Report.
Food and Fertilizer Technology Center. 2002. Nitrates in Vegetables.
Watts, D. 2001. Irrigating corn with high nitrate water. University of Nebraska Extension.
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