Why is nitrate needed in plants

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Tellus B 59, — Sigman, D. Other bacteria live freely in soils or water and can fix nitrogen without this symbiotic relationship.

These bacteria can also create forms of nitrogen that can be used by organisms. This stage takes place in the soil. Nitrogen moves from organic materials, such as manure or plant materials to an inorganic form of nitrogen that plants can use. This becomes important in the second stage of the nitrogen cycle. Mineralization happens when microbes act on organic material, such as animal manure or decomposing plant or animal material and begin to convert it to a form of nitrogen that can be used by plants.

All plants under cultivation, except legumes plants with seed pods that split in half, such as lentils, beans, peas or peanuts get the nitrogen they require through the soil. Legumes get nitrogen through fixation that occurs in their root nodules, as described above.

The first form of nitrogen produced by the process of mineralization is ammonia, NH 3. The NH 3 in the soil then reacts with water to form ammonium, NH 4. This ammonium is held in the soils and is available for use by plants that do not get nitrogen through the symbiotic nitrogen fixing relationship described above. The third stage, nitrification, also occurs in soils. Nitrates can be used by plants and animals that consume the plants.

Some bacteria in the soil can turn ammonia into nitrites. Although nitrite is not usable by plants and animals directly, other bacteria can change nitrites into nitrates—a form that is usable by plants and animals. This reaction provides energy for the bacteria engaged in this process.

The bacteria that we are talking about are called nitrosomonas and nitrobacter. Nitrobacter turns nitrites into nitrates; nitrosomonas transform ammonia to nitrites. Both kinds of bacteria can act only in the presence of oxygen, O 2 [ 7 ]. The process of nitrification is important to plants, as it produces an extra stash of available nitrogen that can be absorbed by the plants through their root systems.

The fourth stage of the nitrogen cycle is immobilization, sometimes described as the reverse of mineralization. These two processes together control the amount of nitrogen in soils. Just like plants, microorganisms living in the soil require nitrogen as an energy source. These soil microorganisms pull nitrogen from the soil when the residues of decomposing plants do not contain enough nitrogen. Immobilization, therefore, ties up nitrogen in microorganisms.

However, immobilization is important because it helps control and balance the amount of nitrogen in the soils by tying it up, or immobilizing the nitrogen, in microorganisms.

In the fifth stage of the nitrogen cycle, nitrogen returns to the air as nitrates are converted to atmospheric nitrogen N 2 by bacteria through the process we call denitrification. A plant supplied with adequate nitrogen grows rapidly and produces large amounts of succulent, green foliage.

Providing adequate nitrogen allows an annual crop, such as corn, to grow to full maturity, rather than delaying it. A nitrogen-deficient plant is generally small and develops slowly because it lacks the nitrogen necessary to manufacture adequate structural and genetic materials.

It is usually pale green or yellowish because it lacks adequate chlorophyll. Older leaves often become necrotic and die as the plant moves nitrogen from less important older tissues to more important younger ones.

On the other hand, some plants may grow so rapidly when supplied with excessive nitrogen that they develop protoplasm faster than they can build sufficient supporting material in cell walls. Such plants are often rather weak and may be prone to mechanical injury. Development of weak straw and lodging of small grains are an example of such an effect. Nitrogen fertilizer rates are determined by the crop to be grown, yield goal and quantity of nitrogen that might be provided by the soil.

Rates needed to achieve different yields with different crops vary by region, and such decisions are usually based on local recommendations and experience. The quantity of nitrogen released from the soil organic matter.

The quantity of nitrogen released by decomposition of residues of the previous crop. Any nitrogen supplied by previous applications of organic waste. Any nitrogen carried over from previous fertilizer applications.

For example, corn following alfalfa usually requires less additional nitrogen than corn following corn, and less nitrogen fertilizer is needed to reach a given yield goal when manure is applied. As with rates, credits are usually based on local conditions. Soil testing is being suggested more often as an alternative to taking nitrogen credits. This strategy, the pre-side-dress nitrogen soil test PSNT , has received a great deal of publicity and seems to provide some indication of whether additional side-dressed nitrogen is needed or not.

Placement decisions should maximize availability of nitrogen to crops and minimize potential losses. Broadcast applications accomplish this objective. Banding does also when all crop rows are directly next to a band. For corn, banding anhydrous ammonia or urea ammonium nitrate UAN in alternate row middles is usually as effective as banding in each middle because all rows have access to the fertilizer.

Moist soil conditions are necessary for nutrient uptake. Placement below the soil surface can increase nitrogen availability under dry conditions because roots are more likely to find nitrogen in moist soil with such placement. Injecting side-dressed UAN may produce higher corn yields than surface application in years when dry weather follows side-dressing.

In years when rainfall occurs shortly after application, subsurface placement is not as critical. Subsurface placement is normally used to control nitrogen losses.

Anhydrous ammonia must be placed and sealed below the surface to eliminate direct volatilization losses of the gaseous ammonia. Nitrates are present in high levels in plant fertilisers.

Without nitrates, the amount of chlorophyll in leaves reduces. This means leaves turn a pale green or yellow colour.

This reduces the plant's ability to photosynthesise and grow properly, which reduces the farmers' crop yield.