Chapter 10. Nitrogen

Summary

Nitrogen is an essential element of all proteins; it affects the growth of a plant and the quantity and quality of produce. The most obvious manifestation of an adequate supply is a luxuriance of leaf color and growth.

It is, however, the one nutrient most likely to be deficient.

Nitrogen is subject to losses in a greater variey of ways than any other nutrient: volatilization of ammonia, leaching and denitrification of nitrates.

Nitrogen fixation by soil organisms is the only significant way to maintain the soil supply in a sustainable system.

Table 18. Comparison Of Nitrogen Fertilizers compares nitrogen fertilizers; table 19. Application Of Nitrogen Fertilizers lists typical application rates.

Nitrogen In The Plant

Nitrogen represents Life. It is an ingredient of proteins and distinguishes them from carbohydrates. Carbohydrates are passive, storing energy or providing physical structure, but proteins control the movement of energy and materials and the growth of the plant. Sugars, starches and cellulose are carbohydrates; chlorophyll, enzymes, and hormones are proteins.

Inasmuch as proteins influence food quality as well as quantity, nitrogen has a predominant role among the soil nutrients. Nevertheless, to the casual observer the obvious effect of nitrogen is on leaf growth and color. Nitrogen fertilizer produces a luxurious growth of lush green leaves, essential for capturing the sun's energy and converting it into sugars. Nitrogen is necessary for the production of sugars and, subsequently, of sweet, ripe fruit.

If nitrogen is low, growth is stunted, and all plant functions are disturbed. Nitrogen is mobile and, and when in short supply it will drift from older leaves to younger ones. Deprived of nitrogen, the older leaves will often turn light green, yellow, or in some cases pink.

A stunted plant with such discolored leaves is a good sign of a protein deficiency in the plant, and it may indicate a nitrogen deficiency in the soil, but it also may mean that the soil is too cold or too wet or too dry, or that the plant is under attack by an insect or disease.

All soil life requires nitrogen in substantial amounts, and because supplies are usually limited, competition is vigorous. Perhaps for this reason, plants evolved to render the metabolism of nitrogen first in priority among all other processes.

This priority may mean survival under natural conditions, but it can be disastrous if the nitrogen supply is unusually high. In the presence of excess nitrogen, a plant's response is to divert energy, carbohydrates, water and minerals in order to metabolize it.

Consequently everything is thrown out of balance:

If sunlight is insufficient to provide enough energy for nitrogen metabolism, the plant accumulates nitrates and free amino acids, the latter of which may attract insects.

Stimulation of plants in a winter greenhouse, by heating the soil and fertilizing, is especially hazardous in combination with the low light conditions, because of the possible accumulation of nitrates. Vegetables harvested from a greenhouse in the afternoon of a sunny day contain fewer nitrates than those picked after a cloudy day [10].

Nitrogen In The Soil

Nitrogen Fixation

Unlike other soil nutrients, nitrogen does not originate from the soil but from the air. Some nitrogen accumulates when rainfall absorbs nitrates in the atmosphere. Some nitrogen is fixed by soil organisms associated with legumes, such as clover, alfalfa, peas, beans and a few trees (locust and acacia, for example). Some is fixed by organisms associated with non-legumes such as alder, various olive bushes (Autumn olive, Russian olive), bayberry and New Jersey tea. And some is fixed by free-living organisms (such as blue-green algae) not associated with plants.

So far as is known, the primary source of nitrogen is associated with legumes, with production in the range of 50-200 lbs per acre per year. The amount contributed by rainfall and fixation by free-living organisms rarely exceeds about 10 lbs per acre per year; an outstanding exception is the blue-green algae which inhabit flooded fields and can supply all the nitrogen needed for growing rice.

Fixation by organisms allied with non-legumes is unknown but is likely to be less than the amount associated with legumes. Such plants are pioneers, surviving in acidic soils with low nutient availability.

Not much is known about optimizing the nitrogen-fixing capability of trees and nonlegumes. Indigenous species seem to do well by themselves. Annual and perennial legumes, however, are more demanding, and an awareness of the following points should help in assuring successful results:

1. Nitrogen fixation by legumes takes place as a result of the attachment of specific bacteria (rhizobia) to the plant roots. The bacteria penetrate the roots and form small nodules on the root surface. The carbohydrates extracted from the roots by the bacteria supply enough energy for the bacteria to utilize their ability to convert nitrogen from the atmosphere into nitrates.

2. The soil that supports a legume should have a sufficient quantity of all minerals other than nitrogen. In particular, rhizobia require phosphorus, iron, molybdenum and cobalt.

Secondly, the rhizobia obtain their carbohydrates from the plant, so the plant must be healthy and vigorous, well supplied with minerals, in order to provide the bacteria with a supply of carbohydrates in addition to its own needs.

In most cases a pH above 6 is necessary for the maximum availability of minerals and therefore for growing agricultural legumes. An exception is lupines, most varieties of which are best adapted to acid conditions. Also, in sandy soils of the Atlantic coast and southeast, where organic matter is low, the pH should not be much above 6.0, otherwise trace elements may be deficient.

3. Legume seed should be treated with an appropriate bacterial inoculant (unless the same legume has been grown succussfully in the soil within the past few years), in order to assure rapid nodule formation and fixation capability. An inoculant is not absolutely necessary; the plant will attract the necessary bacteria spontaneously, but the delay in doing so may be unacceptable, and the particular rhizobia present may be an inferior variety.

Inoculant sold for alfalfa and clover is also suitable for sweetclover; garden inoculant is satisfactory for field peas and vetch; but soybeans, lupines and cowpeas require specific inoculants.

There is a benefit to inoculating clayey soils after planting, rather than inoculating the seed, because it results in a better distribution of the nodules along the root system [6].

4. Nitrogen is only fixed as the plant requires it. If the plant receives enough nitrogen, its production of carbohydrates is diverted to manufacturing proteins, and the supply to the root nodules is cut off. If nitrogen is low, carbohydrate production increases, and more becomes available to the root nodules. This feedback mechanism gives legumes an extra competitive edge, because the production of carbohydrates requires energy, which is better utilized for other purposes if the plant has no need for additional nitrogen.

So alfalfa does not respond to applications of nitrogen; it simply fixes less. Clover often does respond to manure, but any response is due to minerals in the manure, particularly potassium. Also, early spring peas may respond to nitrogen if the soil is too cold for nitrogen-fixation to be effective.

5. Legumes can contribute nitrogen to the soil before the plant is tilled under, because various portions of the roots die during the year and are sloughed off along with their nodules. The nodules decay rapidly and release nitrogen. Legume roots may die when stressed, for example by a local exhaustion of nutrients or a drought. Grasses growing with the legumes will utilize nitrogen released from these decaying roots and nodules.

Consequently, one of the best ways to maximize nitrogen fixation is to grow a non-legume as a companion crop. The non-legume sops up excess nitrogen in the soil and forces the legume to continue fixing nitrogen.

As an anecdotal example, I once saw a butternut tree planted alongside a nitrogen-fixing autumn olive shrub, which towered over neighboring butternuts planted at the same time.

During a drought, nitrogen fixation ceases, but soon after, new roots develop fresh nodules, and fixation resumes.

6. Annual legumes (such as peas, beans and soybeans), which are harvested for the pods, will not contribute nitrogen to the soil. This is because all of the fixed nitrogen is in the seed. They grow well in soil poor in nitrogen but leave little if any behind. In fact, some beans are very inefficient and require fertilizer nitrogen for optimum yield. Possibly some nitrogen is added to the soil from sloughed nodules, but most annuals make a great demand on soil nitrogen; so any net improvement owing to sloughing off of dead roots is small. To improve the nitrogen status of the soil with annual legumes, it is necessary to either:

Perennial forages, such as alfalfa, clover and trefoil, do not divert all of the nitrogen to the seed but retain a considerable amount for continued growth. Even if cut for hay, perennial legumes will add nitrogen to the soil, through the sloughing off of dead roots and nodules. Perennials, however, are slow to start fixing nitrogen and should be left to grow for at least a year before turning under.

In summary, the best way to maximize the fixation of nitrogen is to minimize available soil nitrogen, grow vigorous, healthy legumes, and keep the legumes in the vegetative stage. Where feasible, harvested seed from annuals should be used as animal feed and the manure recycled.

Immobilization Of Nitrogen

The fact that carbonaceous residues added to soil will cause the immobilization of nitrogen during decay was discussed earlier in chapter 2. Essentials of Soil Fertility , but four conclusions are worth noting here:

  1. if the residues are succulent or easily decomposed, nitrogen immobilization is only temporary
  2. adding nitrogen will not help speed up decay, because soil organisms can usually find the nitrogen they need locally. Adding organic nitrogen, however, should lead to a higher accumulation of humus (because more carbon is used for growth)
  3. nitrogen losses from denitrification are inevitable.
  4. Nitrogen fertilizer should be added before planting a crop rather than while turning residues into the soil.

Nitrogen Losses

Usually, the more one tries to force nitrogen into the soil, the greater are the chances of losses. If the soil is overfertilized, it may find a way to get rid of the nitrogen almost as fast as the farmer puts it on. If the nitrogen is spread in ammonium form, the soil may either cause it to be volatilized or to be rapidly nitrified (converted to nitrate form) and soon afterward lost as a gas by denitrification. If the nitrogen is initially in nitrate form, it may be denitrified or leach into the groundwater.

Loss of Ammonia

Volatilization of ammonia has already been discussed in relation to manure handling (chapter 6. Unprocessed Residues - Manure Losses ). It can occur in the soil after heavy applications of manure and when urea or ammonia fertilizer is used if the soil pH is high. Recent research, however, has shown that losses can be reduced by adding calcium or potassium salts to the soil1.

Nitrate Leaching

Leaching of nitrogen occurs in climates with moderate to high rainfall. Whenever excess water percolates through the soil, it carries with it any dissolved nitrogen. The principal nitrogenous constituents of soil water are nitrate salts and soluble organic substances. Ammonium salts rarely leach, because the soil has mechanisms for absorbing excesses2.

The leaching of dissolved organic materials carries away not only nitrogen but also sulfur and trace elements. To minimize leaching, the soil pH should be maintained near neutral. This maximizes biological activity, which aids in the stabilization of soluble organic substances. Also the calcium in the lime is a good binding agent and reduces the instability and solubility of organic residues.

On the other hand, some instability of organic matter is desireable, because unstable organic substances are easily attacked and will release nitrogen, phosphorus and sulfur, which then become available to plants. Unstable organic matter is also responsible for the soil's ability to keep trace elements in an available form. So the pH should not be too high; ideally it should be near neutral, and organic residues should be continually added to the soil.

Denitrification

The concept of denitrification is new to many people, but it can account for substantial losses of available nitrogen. Denitrification is likely to occur in the presence of nitrates and organic matter whenever free oxygen is low3. In the absence of air, many soil organisms can extract oxygen from nitrates, using organic matter for carbohydrates; the nitrates are converted to gaseous nitrogen or nitrous oxide. Even the best-drained soil may have anaerobic pockets at some time.

Denitrification is half of a nitrogen buffering action in the soil, the other half being nitrogen fixation by free-living organisms. Both function best in an anaerobic atmosphere and require organic matter for carbohydrates. However, if nitrates are low, nitrogen will be fixed, but if high, it will be denitrified.

Too much organic matter can encourage denitrification, because an excess produces enough biological activity to use up all of the available oxygen. The critical amount of organic matter depends upon the coarseness of the organic residues and the texture of the soil as they affect oxygen supply; an open sandy soil can handle a greater amount of compact residues than a clay soil.

Denitrification is a hazard not only in the soil but also in the hot composting process, where biological activity can be very high indeed, especially during the initial stage. Perhaps because of the diversity of organisms that can denitrify nitrates, denitrification continues even at the high range of temperatures reached by a pile.

To some extent, denitrification is influenced by the nature as well as the quantity of the organic matter. Stabilized humus is more resistant to attack than fresh residues, and it has less energy to offer. Denitrification is more likely under acid than under neutral conditions, perhaps because organic matter is not as well stabilized in an acid soil.

Therefore, to minimize the likelihood of denitrification, the soil structure should assure good aeration, and the pH should be near neutral. Organic matter is important in maintaining good structure, but unstabilized organic residues should be added slowly enough so that the soil has the opportunity to stabilize them and to maintain sufficient aeration. Organic residues should not be tilled under too deeply, otherwise they will exhaust the limited oxygen supply; usually it is best to keep residues within the top 2-3 inches of the soil.

Most important in minimizing denitrification is the necessity of keeping the nitrate level down. If soluble fertilizers are applied, they are best spread frequently in small amounts. One should never mix soluble fertilizer with fresh carbonaceous residues; the biological activity stimulated by the residues will exhaust the oxygen supply, and most if not all the fertilizer will be lost.

these remarks and experience regarding the spreading of manure. On the one hand, the need to minimize denitrification loss leads to the following conclusion: Since fresh manure containing the urine has a high content of unstable nitrogen, light applications are more conservative of nitrogen than heavy applications. Farmers with a limited supply of manure, however, have found that manure is better utilized as measured by average crop yield, by concentrating the manure in a restricted area.

Resolution of this conflict is possible: perhaps the miscellaneous value of the manure more than compensates for nitrogen loss. Or perhaps the manure has already lost much of its volatile nitrogen before spreading, and further loss is minimal. But I don't know and leave the question to the reader.)

Summary of Losses

In determining fertilizer use, some account has to be taken of these possible losses. There is no simple rule, because nitrogen losses depend upon the particular situation. Heavy applications of soluble or otherwise unstable forms of nitrogen could result in high losses. Poorly aerated soils will probably produce greater losses than well-aerated soils. Leaching losses are more likely with soils having a coarse texture, a low organic content, low biological activity or a lack of growing crops.

Other things being equal, losses probably depend upon the C/N ratio of the fertilizer. Where the C/N ratio is low, nitrogen is readily released with less chance of being used profitably; but where it is high, nitrogen is released more slowly and excesses are unlikely. According to table 18. Comparison Of Nitrogen Fertilizers , nitrogen losses from blood meal should be greater than losses from animal manure or from alfalfa pellets, while soybean or cottonseed meal should result in intermediate losses. Nitrogen losses from fresh hay should be less than losses from manure.

Nitrogen Fertilizers

Comparisons

Table 18. Comparison Of Nitrogen Fertilizers is a comparison of various nitrogen fertilizers. The first three columns list the nitrogen content, the energy index and the C/N ratio. The next four columns show the value in dollars of individual components and the total value. The last column indicates typical prices for some fertilizers4.

All of the organic fertilizers except the last three were discussed in chapter 8. Other Organic Fertilizers - Organic Byproducts . Of these three, Nitro-10 and Fertrell Super N are typical commercial organic fertilizers. Nitro-10 is made from untanned animal hides, and Fertrell Super N is a blend of organic and inorganic materials, the exact composition of which depends upon the materials availabile to the manufacturer. Fertrell specifications, however, indicate that the organic content may be up to 50%. Urea is a synthetic fertilizer. It is chemically the same as the urea in animal urine, and the nitrogen has the same high availability and low stability.

Table 18. Comparison Of Nitrogen Fertilizers includes four inorganic fertilizers. Sodium nitrate mined from Chile is a natural product and is available in two forms, one with potassium nitrate and one without. Both also contain significant amounts of boron (0.2 - 0.4 lbs/ton) and iodine. The latter is probably not important for plants but is essential for human and animal nutrition. Ammonium nitrate and ammonium sulfate are typical synthetic fertilizers.

With the energy content of organic fertilizers included in the determination of their value, table 18. Comparison Of Nitrogen Fertilizers shows that animal manure is the best buy wherever it can be obtained locally, since its cost is usually less than its value. To the purchase price must be added the cost of hauling and spreading it. In 1979 the cost of spreading manure in Maine was about $1.50 per ton, and even at several times that, it is a good investment.

Hay is reasonably priced anywhere that it is grown, especially if it can be bought as mulch hay at a lower cost. Nonleguminous hay, however, is not a good source of immediately available nitrogen, because the C/N ratio is too high, so its greatest value is as a mulch or in compost, where the nitrogen has a more long-term benefit. If alfalfa or clover or any leguminous hay can be obtained at a good price and turned under, much of its nitrogen will be available quickly.

Commercial organic nitrogen fertilizers (excluding urea) are not fairly priced, especially blood meal selling at more than ten times its value. The two most equitable commercial organic products in Maine appear to be soybean meal and alfalfa pellets, both available at feed stores in 50- or 100-lb bags. In Maine, the price of either is about twice the value. It would be worth the time required to compare costs of different organic fertilizers before purchasing sizeable quantities.

Other Considerations

With commercial organic fertilizers selling at an unjustifiable premium, why should anyone choose them over synthetic or inorganic fertilizers? There is probably no justification for choosing blood meal except for small gardens; it is expensive, and its nitrogen is available much too quickly to be used efficiently. In its action it is closer to soluble fertilizers than to organic residues.

On the other hand, alfalfa pellets have a good C/N ratio and are well balanced, with numerous other nutrients. They are easy on the soil and are the best buy among organic fertilizers. One might choose them for the same reason that a person will take out a whole life insurance policy or join a Christmas club. These return less on an investment than alternatives which may be equally safe, but the forced savings reduces the possibility that surplus funds will be squandered. Similarly, the use of commercial organic fertilizers insures that some organic residues will be added, but at a premium. For a garden or if the fertilizer is used only as a supplementary dressing, the premium may be small.

An added value of a product like alfalfa pellets is its trace elements.

The seed meals (cottonseed and soybean) fall somewhere between the extremes of blood meal and alfalfa pellets.

Naturally occurring Chilean nitrate bridges the gap between organic residues and synthetic fertilizers. It has no organic value, and the nitrogen is soluble and subject to leaching losses and denitrification. Chilean nitrate does, however, contain trace elements, and it has a liming value equivalent to about 1/2 pound of lime for each pound of Chilean nitrate, according to the approximate calculation in appendix C. Acid and Basic Nature of Fertilizers ; it is a suitable fertilizer for acid soils.

Chilean nitrate is a good choice when making a transition to the use of organic residues and cover crops, and it is probably the best choice for market gardeners who cannot obtain enough organic residues or afford the cost of commercial organic fertilizers. Owing to its sodium content, however, Chilean nitrate is not good on alkaline soils.

This brings us to fertilizers containing ammonia or ammonium (and include urea). Like urea, liquid ammonia is subject to volatilization in alkaline soils, but it is also caustic and destructive of soil life. Ammonium salts are not caustic, but they tend to acidify the soil5.

The application of these fertilizers has to be timed carefully and placed properly to avoid burning leaves and roots. Moreover they require that the soil be treated with nitrate inhibitors; these retard biological activity in order to minimize the loss of nitrogen by denitrification.

In addition, ammonium salts tend to inhibit the release of non-exchangeable potassium, which is an important source of nutrients in some soils6.

Finally, they are also the most concentrated of nitrogen fertilizers and impossible to spread in small quantities. They exist with no regard for adverse effects on the soil or nutrient imbalances. Probably nowhere is the conflict between the mass production of food and the maintenance of soil life and activity - and, most likely, the quality of the harvest - more obvious than in the use of these fertilizers.

Determining Fertilizer Application Rates

In any soil of reasonable fertility, the only value of a nitrogen fertilizer is to supplement whatever amount that biological activity releaes from the organic matter in its need for energy. In most cases the annual release falls within a range of 1 - 4% of the total nitrogen, depending on the climate and the degree to which the organic matter is subject to attack. Furthermore the success of a crop depends not only on how much nitrogen is released but when it is available.

Soil tests at best can only predict an average result of these variables. The simplest test evaluates soluble nitrates, but this depends on when the soil is tested. A test for organic matter is a valuable supplement, but that states nothing about its composition.

Most likely the best procedure is to start with an estimate and keep records. They should include information on the plant varieties, fertilizer rates and timing, the weather, and the results. They should be taken every year to see the effects of weather conditions.

There are several possibilities for an estimate:

The best time to apply is at the beginning of the season, when biological activity is sluggish.

Fertilizer Rates and Nitrogen Availability

Table 18. Comparison Of Nitrogen Fertilizers lists the amount of fertilizer required to add either 10 lbs or 30 lbs of nitrogen per acre. Twenty lbs are often added as a side dressing, and any amount from 10 to 100 lbs has been used as a starter fertilizer. In addition, table 18. Comparison Of Nitrogen Fertilizers shows the amount of phosphorus and potassium which are also added when organic fertilizers are spread.

Nitrogen from soluble fertilizers is available immediately. Most commercial organic products are about 85% as effective as soluble fertilizers during the year of application, probably because their C/N ratio is so low; the nitrogen is not released as quickly as it is from soluble fertilizers, but it is quick enough to be effective in the first year. Leather meal is an exception, however, with a nitrogen availability of about 15 - 20% in the first year of application.

Nitrogen release from the above-ground portions of leguminous cover crops is also rapid. Release from the decaying roots is difficult to predict, but probably about half becomes available during the first year. Release from the tops or roots of green nonleguminous crops may be slow during the first few weeks after incorporation, but it should pick up afterward.

Nitrogen is released from animal manure at varying rates during the first year, from about 50% for cow manure to about 90% for poultry manure. About half the nitrogen from compost is released the first year.

Nitrogen release from all organic substances, however, depends upon biological activity; if the environment is unfavorable, the nitrogen will remain unavailable. Adequate moisture, aeration and a warm soil are necessary. A cold spring or a dry summer will inhibit availability, and wet conditions may promote denitrification. Consequently one always has to be aware of the weather when planning for supplemental topdressings.


1 For further information, see a series of articles, the latest of which is [38].    [return to text]

2 One of the mechanisms is cation exchange (see chapter 14. Calcium And Soil Ph for a discussion), which keeps ammonium from leaching but maintains it in an available state. Another is the trapping of ammonium ions by crystal minerals in the soil. Such trapped ammonium is tightly bound and generally unavailable, but some is released at a slow rate. Potassium can be trapped similarly, as discussed in chapter 12. Potassium .    [return to text]

3 Recent evidence, however, is that denitrification can sometimes occur even in the presence of free oxygen    [return to text]

4 Prices are those found in Maine in 1984. They represent the lowest prices from the following sources: the Agway distribution depot in Pittsfield, Maine; Organic Growers Supply (associated with the Maine Organic Farmers and Gardeners Association); and the mail order price list from the Necessary Trading Company, New Castle, Virginia.    [return to text]

5 For example, see appendix C. Acid and Basic Nature of Fertilizers - Ammonium Sulfate for a calculation of the acidifying tendency of ammonium sulfate    [return to text]

6 Chapter 12. Potassium - Potassium In The Soil     [return to text]

© 2013 Robert Parnes

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