Function of N in plants
Nitrogen is a structural component of several essential plant parts and compounds, including
- chlorophyll
- nucleic acids (DNA, RNA) in each cell
- all proteins
As a result of these functions, corrections of N shortages result in large gains in vegetative growth, much higher protein levels, and much higher yields of grain, fruit, and vegetative plant organs. While these gains are normally desirable, excess amounts of N, either in absolute terms or sometimes in the ratio of N to other elements, can have a negative impact on some aspects of various yield components.
Nitrogen in the Soil
Nitrogen has a complicated cycle. Some features unique to N are that N does not accumulate in the soil to any significant degree. Nitrogen is highly subject to losses from the soil, yet there is a large reservoir of N in the atmosphere. Of course, most plants (non-legumes) are not able to directly utilize atmospheric N.
Plants take up N in several forms. These include
- nitrate-N (NO3-N)
- ammonium-N (NH4-N),
- urea (CO[NH2]2).
Other forms of N must be transformed to one of these forms before plants can utilize the N.
The N in organic residues exists primarily in large organic molecules which cannot be taken into plant roots. These N sources must be decomposed and transformed into one of the previously mentioned available forms of N. Therefore, a significant portion of “organic” N is not immediately available to plants.
Factors Affecting Availability
The principle factors affecting N availability to a crop are
- The amount N applied
- Excess soil moisture
- Residual soil N from previous crops or conditions
- Organic N available from applied manure, municipal sludge, or other residues.
Nitrogen loss after application has been a major concern for years. Nearly all significant pathways of N loss are the result of excess moisture. As it happens, the most loss-prone form of N is nitrate-N (NO3-N).
Interactions of N with Other Elements
NH4 and P Perhaps the best documented interaction of N with other nutrients is the relationship between ammonium-N and P uptake. Early research showed that when NH4-N was closely associated with P in a fertilizer source, the plants took up more P. When NO3-N was used, or when the NH4-N was separated from the P source, the P uptake was reduced.
Other than the previously mentioned NH4-P interaction, N does not have other similar close interactions with other nutrients. However, N shortages can dramatically reduce the uptake of most other elements. This appears to be the simple result of a plants loss of vigor, and perhaps lower demand for other nutrients with an N shortage. From our many years of doing plant analysis interpretations, it appears that an N shortage most dramatically reduces the uptake of Mg and Cu. However, our experience supports the previous statement that all other nutrients are typically affected as well.
Balances and Ratios
The primary concern of growers in this area should be to avoid excess N in relation to other nutrients. Many times we hear that “high” N rates are detrimental to crop quality (not to mention the environment). In many of these cases, the supposedly high N rate would not have been detrimental if it was balanced with proportionately strong amounts of other nutrients, especially K. Plants having high N uptake without proportional amounts of at least a few other nutrients can be more subject to disease infection and greater physical damage from insects or environmental factors. Crops that receive high rates of N fertilizer, which also have a shortage of one or more other nutrients, are not likely to properly utilize all of the applied N. This can lead to excess N in the soil or water, or excess nitrates in forage or other crops.
Plant Deficiency Symptoms
Typical N deficiency symptoms are a general chlorosis of the older leaves on a plant along with slower growth and generally smaller plants. Some plants have more specific visual symptoms. Corn, for example will have V-shaped chlorotic tissue extending from the tips of the older leaves toward the stalk, in addition to the other mentioned symptoms.
Legumes can also suffer from N deficiency, even though they are capable of making mineral N in the associated N-fixing nodules. There can be several reasons for N shortages in legumes.
Some legumes like edible beans simply don’t produce enough N in healthy nodules
Legumes that are not inoculated, and are planted on land that does not contain a resident population of the correct rhizobia will not produce the nodules required for N production.
A shortage of other nutrients or other soil condition may prevent adequate N production. For example, soybeans suffering from a K shortage are likely to suffer from N deficiency as well, because the nodules will be deprived of the sugars needed for healthy rhizobia and adequate N production.
How Nitrogen Fertilizer Affects Soil Acidity
When the nitrification process converts the ammonium ion to nitrate, hydrogen ions are released, shown by the following reaction.
| 2NH4 + | + | 3O2 |
 |
2NO3 | + | 8H+ |
| Ammonium | Oxygen | Nitrifying Bacteria | Nitrate | Hydrogen |
This is a source of soil acidity (H+), so N fertilizers containing or forming ammonium-N increase soil acidity unless the plant absorbs the ammonium ion directly.
Also, nitrate is a major factor associated with leaching of such bases as calcium (Ca), magnesium (Mg), and potassium (K) from the soil. The nitrate and bases move out together. As these bases are removed and replaced by hydrogen, soils become more acid. Nitrogen fertilizers containing such strong acid-forming anions as sulfate increase acidity more than other carriers without acidifying anions.
When the mineralization process decomposes soil organic matter, the first N product is ammonium. From that point on, the same nitrification as shown above happens, creating acids.
Nitrogen carriers such as sodium nitrate and calcium nitrate leave the associated cation (Na+ or Ca+) in the soil. This makes the soil less acid.