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Reviewed October 1993

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Contents

• The Nitrogen Cycle
• Nitrate in soils
• Movement of nitrate in soils
• Using soil testing and plant analysis
• Nitrate in plants
• What causes the nitrate problem encountered in forages?
• What happens to nitrate after harvesting forages?
• Suggestions for soil fertility in forage production to minimize nitrate accumulation and maintain high forage yield
• Nitrates and vegetables
• Conversion factors

Related publications

• G9131, Sampling Plant Tissue and Soil for Analysis

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Nitrate in Soils and Plants

J.R. Brown
School of Natural Resources

Marshall Christy and George S. Smith
Department of Agronomy

Nitrogen is essential for growth and reproduction of all plant and animal life. It is a basic constituent of proteins. The form of nitrogen within plants when consumed by animals has important effects on growth and reproduction.

Several different groups of nitrogen-containing compounds may be found in plants. The amount of each form depends on plant species, maturity and environmental conditions during growth.

These nitrogen compounds may be broadly classed as either protein or non-protein compounds. Under normal growing conditions plants use nitrogen to form plant proteins. When normal growth is altered, protein formation may be slowed and the nitrogen remains in the plant as non-protein nitrogen.

Nitrate, nitrite, amides, free amino acids and small peptides make up most of the non-protein nitrogen fraction. Nitrate is of special concern in animal production and in human foods because of its potential toxicity when excessive amounts are ingested.

The Nitrogen Cycle

The origin of all nitrogen is the atmosphere, which contains about 79 percent nitrogen by volume. The atmosphere over each acre of the earth's surface consists of about 35,000 tons of elemental nitrogen. The supply is inexhaustible through natural processes.

Soil micro-organisms, free-living and those associated with legumes, fix atmospheric nitrogen. One product of the fixation process is the amino (NH₂) form of nitrogen. Decomposition of plant residues and animal waste by soil microorganisms results in the formation of the ammonium form (NH₄^-). Specific soil microorganisms oxidize the ammonium form to nitrate-nitrogen.

Ammonia and/or nitrate found in nitrogen fertilizers are produced by chemical fixation of atmospheric nitrogen (N₂). Precipitation adds about five to 10 pounds of nitrogen to soils annually. This nitrogen is in the nitrate form due to the action of lightning in presence of oxygen and nitrogen.

The "Nitrogen Cycle" includes various changes from elemental atmospheric nitrogen to inorganic, to organic, and back to inorganic forms. Details can be found in many soils and biology textbooks.

The complex reactions involved in intake and outgo of nitrogen in the soil-plant system are largely microbiological and chemical. Aeration of soil by cultivation can speed up the formation of nitrates. Nitrogenous crop and animal residues and manures in organic form are converted to the ammonium form by decomposition and mineralization.

Complex nitrification processes result in formation of nitrate-nitrogen, which is used by microorganisms and higher plants. It is subject to leaching and may be recycled to the atmosphere by denitrification.

Nitrate in soils

Nitrate is a natural material in soils. Adequate supply of nitrate is necessary for good plant growth. Probably more than 90 percent of the nitrogen absorbed by plants is in the nitrate form.

Chemical fertilizer nitrogen is often in the ammonium nitrogen (NH₄⁺) form and is rapidly converted to nitrate (NO₃^-) in the soil. The amount of crop growth is essentially the same whether nitrogen fertilizer is applied as ammonia (NH₃), ammonium or nitrate (NO₃^-). Chemical fertilizers may be composed of ammonium nitrate, ammonium phosphates, ammonium sulfate, various nitrate salts, urea and other organic forms of nitrogen.

Soil organic matter contains about 5 percent N. For each 1 percent organic matter, the 7-inch plow layer of an acre (about 2,000,000 pounds of soil) contains about 1,000 pounds of N. Microorganisms must change organic nitrogen to ammonium or nitrate before plants can use it. Usual release of available N from soil organic matter is 1 to 4 percent annually, depending on soil texture and weather conditions.

Animal manure is an excellent source of nitrogen and can contribute significantly to soil improvement. Animal manure contains about 10 pounds of N per ton, poultry manure about 20 pounds; and legume residues 20 to 80 pounds. About half of this organic nitrogen may be converted to nitrate-nitrogen and become available for plant use the year it is added to the soil. However, it is low in phosphorus content.

Excessive manure applications can result in toxic levels of nitrate in forage crops the same as excessive use of chemical nitrogen fertilizer. Adding phosphate fertilizer to manure can reduce the nitrate content in the crop produced.

Effluent from animal waste treatment facilities may lose about 50 percent of its nitrogen to the atmosphere as it is applied to soils. However, applications of large quantities of effluent or solid waste can add excessive amounts of nitrogen to the soil. Applying large amounts per acre repeatedly to the same area may add more nitrogen to the soil system than can be used.

Using feed additives in livestock feeding may contribute significant concentrations of certain elements such as copper, zinc, arsenic or others to the solid animal waste collected in lagoons or similar facilities. Such wastes continuously applied to soils may eventually result in soil levels toxic to plants and possibly to animals that consume the feed crop.

Treated urban organic wastes may contain small percentages of nitrogen and other essential plant nutrients. Such wastes may have usefulness in soil-crop systems. Urban secondary or tertiary treated sewage effluent may be a potential source of irrigation water. Chemical analysis should be made before applying it to cropland to determine the form and concentration of nitrogen and other elements that might be toxic to plants and animals.

Manures, effluents and solid wastes vary greatly in nutrient element content. An analysis helps to effectively use the materials for crop production when large amounts are involved.

Avoid use of waste materials potentially detrimental to soils, crops and animals

Movement of nitrate in soils

Nitrate-nitrogen is soluble in water and moves with soil moisture. Some may be lost by leaching in sandy soils. In heavier soils, leaching is slower and most of the nitrogen is recovered by plants. Significant quantities of nitrogen rarely leach out of the root zone in medium- and fine-textured soils when reasonable management practices are followed. Annual additions of N to the soil through rain and snow about equal the amount leached.

Nitrogen in the ammonium form (NH₄⁺) is strongly held by the negative charges of clay and soil organic matter colloids until converted to the nitrate form by bacteria.

Using soil testing and plant analysis

Soil testing is practical and useful for pinpointing soil deficiencies and fertility imbalances for crop production. Suggested soil treatments are tailored to the fertility of the soil, the cropping system and yield goals.

Plant analysis offers a method of determining if essential nutrients are getting into plants in amounts needed. This analysis is especially useful in detecting macro- and micro-nutrients and determining if they are present in amounts satisfactory for proper plant nutrition.

Nitrate in plants

Under normal growing conditions with sufficient light as a source of energy, enzyme systems in green plants rapidly reduce nitrate-N (NO₃^-) to intermediate compounds that are subsequently converted into amino-nitrogen.

Organic acids arise from carbohydrate metabolism in combination with the amino-nitrogen to yield amino acids in the plants. The amino acids are building blocks for proteins. This total process is dependent on sunlight.

Nitrate reduction occurs both in aerial portions and roots of plants. The relative importance of these two sites of nitrate conversion is considered most important.

Nitrate is not found in significant amounts in mature grain and seldom in the accompanying vegetative part of the plant under normal fertility and growth conditions. Good grain yields require conversion of nitrate to seed protein.

Young plants in the vegetative stage generally contain more nitrate than more mature plants of the same species. This is especially true of young pasture plants that have been liberally manured or fertilized with nitrogen.

Sixteen elements are known as essential nutrients for plant growth. Nitrogen is only one. The soil serves as storehouse and supplier whether the essential nutrients are native or applied as fertilizers. A deficient supply of one or more essential element creates an imbalance in plant uptake and may cause abnormal growth. Excess nitrate within the plant may result from too little of some other plant nutrient rather than an excess of nitrogen.

Phosphorus, potassium and sulfur have major roles in production of proteins, thereby decreasing nitrate within the plant. Calcium, magnesium and soil pH are closely involved in plant nutrition and crop performance as measured by yield and quality. Tables 1, 2 and 3 illustrate the importance of providing optimum mineral nutrition for plants in connection with nitrogen fertilization.

Table 1
Nitrate-N in corn plants with phosphate applied to low phosphate soil. (Department of Agronomy)

Table 2
Average yields and nitrate-nitrogen of spring growth only of 11 surface fertilized demonstration fields of established grasses in Gasconade County.

Table 3
Effects of limited essential plant nutrients. (Department of Agronomy)

¹Basic acre treatment -- 200 pounds N; 86 pounds P; 186 pounds K; 3,000 pounds Ca.

Nitrate accumulation in a tissue or organ of a plant is the result of the rate of uptake and translocation to other plant parts, and the rate of assimilation into proteins. In mature plants, nitrate accumulates primarily in stems and stalks, with greatest concentration in the basal areas.

Different species of plants accumulate different amounts of nitrate with identical nitrogen treatments as illustrated in Table 4. Sudan harvested at the earliest stage had not absorbed the fertilized nitrogen. A higher level of nitrate in Sudan with more maturity indicates either a lower enzyme capacity to reduce nitrate, better root systems or a less than adequate supply of water. Alfalfa harvested late in the season, following drought and high temperature, may contain an abnormal quantity of nitrate.

Table 4
Nitrate-nitrogen of various forages harvested at different stages of maturity, fertilized with 100 pounds N per acre. (Department of Agronomy)

Yields may be low unless a small amount of nitrate is present in corn and sorghum stalks at silage time. Forage sorghums may contain more nitrate than corn due to the distributing of nitrate within the plant. As corn and sorghums mature and grain develops, nitrate decreases in the stalks and leaves. Residual nitrate will generally decrease or disappear during ensiling if the crop is not damaged and is ensiled at proper stage.

Nitrate content of corn and sorghum silage may also be caused by weeds in the silage. Certain weeds accumulate nitrate when shaded or partially killed by herbicides. Raising the cutter bar to exclude such weeds as well as the basal corn and sorghum stalks, which may have a high nitrate content, may be beneficial. Herbicides destroy nitrate accumulating weeds, making pastures and forage crops safer.

What causes the nitrate problem encountered in forages?

All forage plant parts contain some non-protein nitrogen that can be used or excreted by animals. Concern arises with forages containing more nitrate than animals can effectively tolerate. Disruption of normal plant growth increases the probability for nitrate accumulation in leaves, stems and stalks. Factors favoring accumulation of nitrate in plants include:

• Drought (plant injury and concentration of nitrate in soil solution).
• High temperatures (rapid transpiration of water, concentrating nitrate not used).
• Shading and cloudiness (reduced protein synthesis).
• Deficiency of plant food nutrients (phosphorus, potassium, calcium).
• Excessive soil nitrogen (manures, legume, residues, effluents and solid wastes, and nitrogen fertilizer).
• Plant damage from insects and certain weed control chemicals.
• Immaturity at harvest.

What happens to nitrate after harvesting forages?

In hay, enzymes continue to reduce nitrate for a short time during curing. Little further change occurs after the moisture becomes low enough to allow storage. Microorganisms reduce the nitrate if incomplete drying or rewetting occurs.

In silage, anaerobic fermentation causes some nitrate reduction after ensiling. Nitrogen oxides may be observ ed as colored gases escaping from "fuming silos." However, toxic, colorless gases may also be present. Since nitrogen oxides are heavier than air, accumulations may occur in the silo, silo chutes, around the base and in areas with little ventilation. Juices draining from silos may contain a high concentration of nitrate.

Delay in harvesting of an immature crop for silage may result in a decreased nitrate content if further growth and grain development occurs.

Suggestions for soil fertility in forage production to minimize nitrate accumulation and maintain high forage yield

• Balance fertility -- provide adequate minerals using soil tests to guide soil treatments with plant analysis to monitor uptake in plants.
• Use phosphate and potash in addition to nitrogen when topdressing pastures and hay crops
• Limit the use of manure for Sudan and silage crops. Using chemical nitrogen fertilizers permits more control over the amount of nitrogen available to the crop. Phosphate fertilizer improves the effectiveness of manure. Chemical analysis of effluents and solid wastes is suggested before application.
• Delay harvesting of immature crops following drought if conditions might favor further growth and grain development.
• Consider amounts of nitrogen from all sources available to a crop including soil organic matter, crop residues, legumes, animal manure and chemical nitrogen fertilization.

Nitrates and vegetables

Nitrate content of plants is determined by their inherited metabolic pattern (genetics) and the available nitrate of the soil. Applying fertilizer in amounts beyond the ability of the vegetable crop to use them may result in an accumulation of nitrate.

Leafy green vegetables and some root crops naturally contain nitrates. There are wide variations between species. If an excess of nitrogen is present in the soil, the nitrate content may be too high, particularly if some other essential nutrient is not adequate.

Vegetables produced on high organic soils, and even where no fertilizer nitrogen is applied, frequently have a higher nitrate content than the same species grown in Missouri with soil nitrogen treatment. Nitrate-nitrogen values are given in Table 5 to demonstrate the range for a given vegetable.

Tomatoes with vigorous foliage are usually low in nitrate content. However, where the plant is defoliated by disease, weather or other factors, the nitrate from the soil may move directly to the fruit and accumulate.

Table 5
Nitrate content of Missouri-grown vegetables and vegetables purchased in food markets (NO₃-N, percent dry weight).

¹Grown in experimental studies with variable nitrogen (N) soil treatments from 0 to 400 pounds per acre.

Conversion factors

Table 6
Conversion factors.

ppm divided by 10,000 = percent
percent multiplied by 10,000 = ppm

G9804, reviewed October 1993