Is it true that anhydrous ammonia makes soil hard?
There are some situations where anhydrous ammonia could reduce permeability, but these situations are the exception rather than the norm. Anhydrous ammonia, at the point of injection, can dissolve a portion of soil organic matter, allowing it to move with soil water. On low organic matter soils, where any loss or alteration in organic matter is detrimental, this effect can make the soil seem hard and impermeable. On other soils, this organic matter dissolution and movement actually improves soil structure.
Doesn't anhydrous ammonia damage the soil more than urea (46-0-0)?
Although urea and anhydrous ammonia are physically different prior to application, in soil, urea is rapidly converted into ammonia and has properties identical to anhydrous ammonia. Any of the beneficial or detrimental aspects of anhydrous ammonia on soil are equally applicable to urea since they form the same chemicals in the soil.
*See the Notes section below for details on the chemistry of urea transformation in soil.
What effect does anhydrous ammonia (or urea) have on soil micro-organisms?
Immediately after fertilization, high concentrations of ammonia temporarily inhibit soil microbes in the fertilizer retention zone. Bacteria are sensitive to ammonia and are affected by this to a greater extent than fungi. Over time, ammonia concentrations gradually decline as the ammonia adheres to soil particles or moves with water away from the application site. As ammonia concentrations decrease, soil microbes repopulate the affected area.
Isn't it bad to kill soil microbes?
Whether they realize it or not, many farmers take advantage of this aspect of anhydrous ammonia to maximize nitrogen fertilizer efficiency. When nitrogen is applied in the fall, one of the goals is to minimize the amount of fertilizer that goes into the winter in the nitrate form since nitrate can move with water out of the root zone or can be lost back to the atmosphere as nitrogen gas and other greenhouse gases in the spring. When urea or anhydrous ammonia are banded, the concentrated ammonia temporarily hinders bacteria that normally convert ammonia to nitrate, and the fertilizer remains in a form that cannot be lost from the soil.
Have there been any long-term trials with anhydrous ammonia in western Canada?
A ten-year (1983-92) experiment comparing the effects of anhydrous ammonia and urea was performed at Scott Saskatchewan by Agriculture Canada. The experiment revealed that high rates of anhydrous ammonia or urea had some long-term effects on soil pH. However, the researchers concluded that nitrogen applied according to soil test recommendations or closely matching crop requirements had minimal long-term detrimental consequences for soil microbes, soil biochemical properties, or soil structure.
Benefits of urea or anhydrous ammonia fertilization.
Often when people question the effects of anhydrous ammonia on soil, an underlying assumption seems to be that fertilizer is bad for the soil and the decision between urea and anhydrous ammonia is a choice for the lesser of two evils. This assumption is far from the truth, as responsible nitrogen fertilization can improve soil properties and have positive yield and economic benefits.
Elston Solberg and Marvin Nyborg and associates summarized the results of a 12-year experiment performed at the Breton Plots southwest of Edmonton. On this Gray Luvisolic soil with low organic matter content and low native fertility, they found that annual additions of 22 – 67 lb N/acre as urea increased barley yield by up to 36 bushels per acre lowered the bulk density of the top 3" of soil, and increased the stored carbon content of the top 12" of soil by about 7 lb of carbon for every 1 lb of applied nitrogen. In this study the benefits of annual nitrogen fertilization were dramatic for both crop yield and soil quality.
The link below provides a summary of the impacts of anhydrous ammonia applications on soil.
Urea (46-0-0) transformations in soil (urea hydrolysis)
Chemical formula for urea: CO(NH2)2
CO(NH2)2 + H2O => H2NCOONH4 => 2NH3 + CO2
The first step in this reaction requires urease, an enzyme present everywhere in soil.
H2NCOONH4 (ammonium carbamate) is an unstable intermediate product that rapidly degrades to ammonia and carbon dioxide. Essentially the transformation of urea in soil could be written as:
CO(NH2)2 + H2O => => 2NH3 + CO2
The above reaction requires moisture and is heat dependent. In conditions that are favourable for crop growth, the reaction is rapid.
Urea and anhydrous ammonia have the same beneficial and detrimental effects on soil properties because they are so similar chemically once they have been applied to soil.