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Home  > Question of the Week

Question of the Week



Permalink Can vegetables take up dangerous levels of nitrate from water?

D.R.
Wisconsin

Answer: Here is some information on nitrates, water, and vegetable crop quality.

Nitrogen Basics

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.

References

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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Permalink How can I control mesquite without jeopardizing future organic production?

M.C.
Texas

Answer: Mechanical methods of mesquite control have varying levels of success. Even chemical control is not completely successful without repeated applications and constant attention to new growth. Having mesquite on your land can be exasperating, but if kept in check can actually become a beneficial contribution to a farm.

Mowing is discouraged as a control method, because this activity trains the tree to become a bush. Repeated mowings often encourage crown development, and in general make future control more difficult.

Grubbing is effective if you are able to remove the crown. This is an enlarged portion of the root just below the soil line. It is the principle carbohydrate storage structure and the source of future sproutings. However, even if the crown is removed, the new growth will often begin from buds on the roots below the crown. Even though, crown removal remains the best way to effectively reduce re-sprouting of mesquite.

If you are able, you can conduct a burn after you have grubbed up the crowns. Grubbing exposes sensitive plant tissue and builds up fuel necessary for an intensely hot burn. If the fire is not hot enough, it will not kill the plant but merely defoliate it and encourage it to re-sprout.

Mature, single-stemmed mesquite trees can increase the aesthetics of a landscape if they are not too numerous. They are also much more difficult to remove. For this reason it is advisable to concentrate on new seedlings and young bushes and trees. If the infestation is small, use a grubber or root plow to remove these plants individually. If the infestation is wide-spread, encompassing small and medium-size trees and bushes, consider root plowing, chaining, or roller chopping followed by a controlled burn to remove most of the biomass. It will most likely be necessary to grub out re-sprouts for several years after to obtain optimal control.

Make sure you contact your local USDA-NRCS office or county Extension agent if you plan on conducting a burn. They will be able to advise you on the legality and practicality of this activity, and may be able to offer assistance.

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Permalink What can you tell me about intercropping commercial field crops?

C.R.
Saskatchewan

Answer: Intensive cropping systems, rotations, and crop diversification including intercropping have been shown to be successful in some areas of the Northern Plains. Intensive cropping utilizes soil water more efficiently than fallow, reduces erosion, rebuilds soil organic matter through residue and root die-back, increases soil microbiological activity and diversity, increases nutrient availability through nitrogen carry-over, and subsequently increases yield and quality while allowing for reduced inputs.

A few common intercrop systems have proven effective in the northern plains. As with any new cropping system, its best to (1) make sure you have an adequate market outlet, (2) familiarize yourself with new planting and harvesting techniques associated with different grains, and (3) plant at the appropriate time into a well-prepared seedbed. For some fall-seeded crops, a no-till seeding into short cereal stubble can be very effective, especially with small-seeded species like canola. This helps to prevent wind erosion and desiccation of the germinating seed.

The following two scenarios have proved to be effective in the northern plains.

Lentils and Flax

The intercropping of flax and lentils is adapted to North Dakota, but is susceptible to weed pressure. Weeds are typically a problem with lentils and flax, especially early in the growing season since the young crop isn’t as competitive with many annual weeds like mustards. Post-emergence harrowing can be effective, but is site-specific in studies in ND and Washington State, and is often harmful to young plants. Results of intercropping lentils and flax in Saskatchewan are variable and are generally not recommended by the Saskatchewan Pulse Growers. In order to prevent weed problems in this cropping system, use this intercrop in rotation with cereals and forages if your soil and moisture allow. Crop rotations and high crop diversity both temporally and spatially is often a reliable management tool in preventing weeds and other crop pests. The ATTRA publication Farmscaping to Enhance Biological Control includes information on diversifying landscapes to mitigate problems associated with crop pests and disease.

Peas and Canola (Rapeseed)

Fall-planted winter peas fix nitrogen, reduce soil erosion, and help to limit soil moisture loss through soil cover and effective water cycling. They can be harvested for seed, combined with winter wheat for forage, grown for green manure to help build soil organic matter, or combined with winter cereals or canola as a multiple crop.

Canola is a crop that might be appropriate to your area. The paper by McGill University echoes the conclusions of Grant et al. (2002) in stating that winter peas, which fix atmospheric nitrogen in large amounts, can supply nitrogen to the canola crop or to the following year’s cereal grain. Canola can be planted in the fall and is known to help reduce disease problems in cereal and legume rotations, so it can have a good place in a wheat-legume rotation.

Miller et al. (2003) have shown that pulse crops, notably peas and lentils, can have a positive impact on subsequent wheat yield in the northern plains, with more consistent results on clay soil than loamy soil. This has been attributed to increased water availability for wheat following pulses, and to an increase in nitrogen availability for wheat following pulses. The bottom line, it can be argued, is that intercropping of pulses and oilseeds can (1) increase water use efficiency for subsequent crops, (2) increase nitrogen carry-over for subsequent crops, (3) increase biodiversity and associated ecological services, such as reduction in pest and disease problems, and (4) diversify cropping enterprises and thereby spreading out economic risk.

References:

Carr, P., G. Martin, and B. Melchior. 1994. Alternative Crops and Diversified Cropping Systems in Southwestern North Dakota. Dickinson, ND: North Dakota State University, Dickinson Research Extension Center.

Grant, C.A., G.A. Peterson, and C.A. Campbell. 2002. Nutrient Considerations for Diversified Cropping Systems in the Northern Great Plains, in Agron. J. 94:186-198.

Ecological Agriculture Projects. 1991. Intercropping canola and peas. McGill University.

Miller, P., Y. Gan, B. McConkey, and C. McDonald. 2003. Pulse Crops for the Northern Great Plains: II. Cropping Sequence Effects on Cereal, Oilseed, and Pulse Crops, in Agron. J. 95:980-986.

Saskatchewan Pulse Growers. Chapter 7: Lentils, in Pulse Production Manual 2000. (PDF)

Stephen Machado, Brian Tuck, and Christopher Humphreys. 2005. Alternative Rotation Crops: Peas (Pisum sativum), in Dryland Agricultural Research Annual Report. Oregon State University.

ATTRA Publications:

Farmscaping to Enhance Biological Control
Intercropping Principles and Production Practices

Additional Resource:

Sustainable Agriculture Network. Diversifying Cropping Systems. USDA: Sustainable Agriculture Research and Education Program.

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Permalink What is used to treat mastitis in organic production?

J.M.
California

Answer: Transitioning from conventional to organic production brings about many challenges. Organic health care is a hurdle that forces many producers to learn to adapt. Organic Valley cooperative provides its member producers with a packet of information that discusses many organic health care techniques and how they are used, including mastitis treatments.

There is also a short explanation of mastitis care and prevention in the Hoegger Supply Company's catalog. Another article, "Treating Mastitis without Antibiotics," has some useful information about prevention and alternative treatments of mastitis. You can also ask current organic producers and organic certifiers what they recommend for organic treatments.

Resources:

Mother Nature's Natural Animal Health Tools An Introduction for Organic Producers (PDF). Organic Valley.

Mastitis. Hoegger Supply Company Catalog. p. 74

Duval, J. 1995. Treating Mastitis Without Antibiotics.

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