The Risks of Surface Spreading Nitrogen Fertilizer on Grass Pastures

By: Bill Manning, DPI Agronomist Gunnedah
March 5, 2010

Previously in this column we have talked about the production improvements possible from the use of nitrogen (N) fertiliser on tropical pastures. Many farmers are concerned at the risks of losing Nitrogen to the atmosphere (volatilisation), if rain doesn’t follow soon after spreading.  Obviously this is a waste of product as well as a contribution to greenhouse gases.

The most commonly used products for surface spreading onto grass pastures are urea and ammonium sulphate (or sulphate of ammonia). Urea has a high concentration of N (approximately 46 %N) and comes as a prill making it easy to spread. Ammonium sulphate contains about 20 % N and 4% Sulphur and is mostly supplied as a crystalline form requiring a belt spreading for application. Higher quantities of ammonium sulphate are required to achieve a certain rate of N compared to Urea. 

Plants cannot use N in the form of urea or ammonium sulphate, it has to be converted to nitrate, and these processes cannot start until there is moisture available (rain or dew or wet soil). 

It is necessary to explain some basic chemistry to understand this process. After urea is spread and rain follows soon after (we hope) the urea is converted to ammonium which is then converted to nitrate. Once it is converted to nitrate it is safe from volatilisation.

The problem is the ammonium stage. Ammonium as well as being converted to nitrate can also be converted to ammonia which is a gas and can escape to the atmosphere. The key to avoiding losses is to get the product into the soil which can only be achieved by rainfall.

The process for sulphate of ammonia is similar except that it does not have to undergo the first step as the N is already in the ammonium form (ammonium sulphate).

This risk of volatilisation occurs whether the ammonium comes from urea or ammonium sulphate.

The problem occurs when there is enough moisture to start this process, say heavy dew or 2-5 mm rain but not enough rain to wash the product into the soil profile where it is safe from volatilisation. If rain does not occur soon after application the ammonium is prone to volatilisation while it is sitting on the soil surface waiting for rain. Applying to a wet soil that is drying also increases the risk as there is moisture to start the reaction without moving the product into the soil. Soil pH is a major factor in determining losses, higher pH increase losses in both urea and sulphate of ammonia as higher pH encourages the conversion of ammonium ions to ammonia gas. Urea as it converts to ammonium creates an alkaline (high pH) environment around the prill which increases the risk of loss. Sulphate of ammonia does not have this effect and so is a bit safer in this regard especially on low pH soils (5-7)

Other factors that increase the risk of loss include daytime temperatures over 20C and high wind which removes ammonia gas encouraging more production of ammonia at the soil surface. A good pasture canopy will reduce air movement and potential losses. These factors act in the same way for ammonium sulphate. Organic matter on the soil surface can also increase losses from urea.

There has been a common belief that ammonium sulphate is much less prone to volatilisation than urea, this is not completely correct.

Because ammonium sulphate does not have to undergo conversion to ammonium it can start volatilisation sooner, whereas urea cannot volatilise until it has undergone the first step. In this case it could be argued that urea provides a safety margin for rain to occur. This however has to weighed against the effects of urea on pH mentioned above. If, however you do not receive the required rain (10-15mm or greater) both products can be lost. 

The critical factor is receiving rain after application. Obviously rain can never be predicted with certainty and there will always be an element of risk. Many growers are however successfully using this technique and the risk of losses should not deter growers from trying this technique on a small area with either product.

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