Fertilizers
Nitrogen Fertilizers
Fertilizers common to crop production usually contain nitrogen in one or more of the following forms: nitrate, ammonia, ammonium, or urea (CO(NH2)2). Each form has specific properties that determine when, where and how various fertilizer materials can be used. Nitrogen fertilizers are one of the most common fertilizers that we use nowadays.
Anhydrous Ammonia
At normal temperature and pressure, anhydrous ammonia is a gas, but it is stored and transported under sufficient pressure to keep it a liquid. The word anhydrous means “without water.” It may be injected in soil or applied with irrigation water in sprinkler or drip systems. When applied to soil, ammonia is converted to ammonium (NH4⁺) and then to nitrate (NO3¯). Anhydrous ammonia (82%-N) is the slowest of all nitrogen fertilizer forms to convert to nitrate nitrogen.
Urea
Urea, containing 46 percent nitrogen, is a synthetic organic material. This form of fertilizer nitrogen usually undergoes a three-step change before it is taken up by crops. First, enzymes in the soil or plant residue convert the urea nitrogen to ammonia nitrogen (NH3). Next, the ammonia reacts with soil water to form ammonium nitrogen. And finally, through the action of soil microorganisms, the ammonium is converted to nitrate nitrogen. Urea (46%) converts to nitrate nitrogen fairly quickly, usually in less than two weeks in the spring. Like nitrates, urea dissolves in and moves with soil water and thus can be lost through leaching if not converted to ammonia and then ammonium. Leaching can be a problem in coarse soils. Sulfur-coated urea can minimize the risk of losing nitrogen due to leaching because nitrogen is released very slowly over a period of time.
Urea Ammonium Nitrate
Urea ammonium nitrate (UAN) solution is widely used as a source of nitrogen for plant nutrition. The nitrate (NO3¯) portion is immediately available for plant uptake. Under conditions of a well-aerated soil and a pH 6 or above, the ammonium (NH4⁺) fraction is quickly nitrified to nitrate because it is rapidly oxidized by soil bacteria. Soil enzymes hydrolyze the remaining urea portion to form ammonium, which subsequently transforms to nitrate in most soil conditions. The nitrate in this product is subject to leaching and denitrification from the time it is placed in the field. UAN solutions are commonly injected into the soil beneath the surface, sprayed onto the soil surface, dribbled as a band onto the surface, added to irrigation water, or sprayed onto plant leaves as a source of foliar nutrition.
Ammonium Sulfate
Ammonium sulfate, consisting of 21 percent nitrogen is a good source of sulfur when it is needed. It is a dry fertilizer. Ammonium sulfate has intermediate volatilization potential, with higher rates in calcareous than acidic soils because sulfate dissolves some calcium carbonate, increasing pH around the fertilizer granule. Losses from ammonium sulfate should be low when soil pH is less than 7.0, because low pH suggests calcium carbonate is absent.
Ammonium Nitrate
Ammonium nitrate is a popular fertilizer since it provides half of the nitrogen in the nitrate form and half in the ammonium form. The nitrate form moves readily with soil water to the roots, where it’s immediately available for plant uptake. The ammonium fraction is taken up by roots or gradually converted to nitrate by soil microorganisms. Many vegetable growers prefer an immediately available nitrate source of plant nutrition and use ammonium nitrate.
Calcium Nitrate
Calcium nitrate contains no ammonium and therefore does not have an acidic reaction in the soil (it actually has a slight alkaline reaction). It contains about 15 percent nitrogen and 19 percent calcium. It is preferred for sidedressing vegetable crops that are sensitive to calcium deficiency.
Calcium Ammonium Nitrate
Calcium ammonium nitrate (CAN) is a granulated nitrogen fertilizer. The combination of ammonium nitrogen and nitrate nitrogen makes CAN an all-rounder for all fertilizer measures. CAN is considered near-neutral in its effect on soil pH and therefore can be used on soils that have a low pH without lowering further. This also means it is most suitable for using on perennial fruit crops (where soil incorporation of lime is normally difficult to achieve).
Nitrogen Fertilizer Volatilization
As discussed, the primary forms of nitrogen found in nitrogen fertilizers are ammonium, nitrate, and urea or combinations thereof (Section 8.2). The potential is greatest with urea and fluids containing urea such as UAN. Gaseous nitrogen loss via ammonia volatilization is a major potential pathway of loss. Ammonia volatilization occurs as ammonium is converted to ammonia gas at the soil surface and transported to the atmosphere. Therefore, ammonia volatilization can potentially reduce a grower’s economic return and have negative impacts on the environment.
Factors Affecting Ammonia Volatilization
In general, alkaline soils (pH >7.0) are more susceptible to ammonia volatilization, while neutral to acidic soils (pH 5.0–7.0) result in reduced ammonia volatilization losses. In the soil, urea changes to ammonium carbonate which may temporarily cause a rise in pH, sufficient enough to increase volatilization loss. The pH increase is higher in poorly buffered soils. Although the increase in pH is temporary, it can result in substantial volatilization loss from soils with an initial pH as low as 5.5. Anhydrous ammonia and aqua ammonia also temporarily raise pH around the fertilizer, increasing volatilization potential, but for a different reason. Ammonia volatilization largely depends on the texture of a soil along with its buffering capacity. A soil with a higher buffering capacity (because of high CEC, high clay content, high soil organic matter, or a combination of those) has a reduced rate of ammonia loss because rapid pH change is restricted or because of increased adsorption of ammonium to clay particles.
Enhanced Efficiency Fertilizers
Enhanced efficiency fertilizers (EEF) are a term used for formulations that control fertilizer release or alter reactions that lead to nutrient losses. Thus, reducing loss to the environment and/or increase nutrient availability compared to conventional nitrogen fertilizers. Enhanced efficiency fertilizers can spread out the release of nitrogen over the growing season, ideally matching nitrogen supply to plant nutrient demand over time. Adequate and consistent nutrient availability reduces plant stress and may result in better yield. Matching nitrogen released with nitrogen uptake rather than having high levels of nitrogen in the soil solution immediately after fertilization can reduce the risk of excessive vegetative growth in crops such as wheat.
Urease Inhibitors
If the urea or UAN is applied to the soil surface, and especially if the soil pH is above 7.0, the ammonium ions (NH4⁺) will readily convert to ammonia gas (NH3) and volatilize from the soil. The amount of total nitrogen loss from fertilizers containing urea due to ammonia volatilization can vary considerably, from 10 percent loss to 50 percent or more of the applied nitrogen, especially if the soil pH is above 7.0.
Nitrification Inhibitors
Nitrification inhibitors are another type of products for use with nitrogen fertilizers, which suppress conversion of ammonium to nitrate by soil microorganisms. Ammonium is strongly held by negative charges on soil particles (the cation exchange complex) and does not leach from soils, while the negatively charged nitrate ion can wash through the soil when a lot of rain occurs.
Slow-Release Fertilizers
A third type of product, similar to nitrification inhibitors, focuses on controlled release by using a coating on fertilizer material that causes it to slowly dissolve and release the nitrogen fertilizer. In this way, ammonium and nitrate concentrations may be better synchronized with plant nitrogen uptake requirements, thus avoiding excessive nitrogen losses.
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