Sulfate of ammonia might be a cheaper source of nitrogen fertiliser, but the risk of volatilisation losses increases when it is applied in close succession to lime.

Key points

• Applying sulfate of ammonia in close succession to surface liming could increase the risk of nitrogen losses via ammonia volatilisation
• The practice is common enough in Western Australia to raise concerns
• Research is underway to quantify the risks

Anecdotal evidence suggests that applying sulfate of ammonia in close succession to surface liming, and prior to seeding, increases the risk of volatilisation.

Although the data is limited, it is feasible that nitrogen fertiliser losses from ammonia volatilisation could be as high as 30 per cent for Western Australian soils under certain conditions. Given the increasing reliance on nitrogen fertilisers in WA, and greenhouse gas emissions associated with their production and use, understanding whether this is a significant loss pathway is important.

A risky business

A short-term study quantifying the frequency of the practice and the level of risk is being led by the University of Western Australia (UWA), with the assistance of Murdoch University as part of the SoilsWest alliance.

An exhaustive literature search did not find any research directed at this practice, although there was enough evidence to demonstrate the risks.

Volatilisation losses depend on the timing and placement of nitrogen fertiliser, soil pH and soil pH buffering capacity, plus environmental conditions. In eastern Australia, research has found volatilisation losses of up to 34 per cent when sulfate of ammonia is applied to alkaline soils that contain free lime.

Given that lime increases soil pH, it is reasonable to expect that applying sulfate of ammonia to recently limed soil would increase the risk of ammonia volatilisation.

Common enough

To assess the extent of the practice, UWA surveyed 13 agronomists whose clients crop a total of 1.6 million hectares throughout WA’s grain growing regions.

The survey showed that the broadcasting of lime and sulfate of ammonia over paddocks is not extensive, but common enough in the WA grainbelt to warrant further investigation.

The majority of advisers (nine of 13) said that that they had clients applying lime and sulfate of ammonia in close succession, although this ranged from five per cent to 100 per cent of a client base. When this practice takes place, it occurs from one year in three to one year in six, usually prior to sowing canola.

Five of the 13 advisers raised concerns about gaseous nitrogen losses occurring when lime and sulfate of ammonia are applied in close succession, while three consultants believed sulfate of ammonia is an inefficient source of nitrogen fertiliser.

Glasshouse experiments are already underway to evaluate potential volatilisation losses from surface applications of lime and sulfate of ammonia. Initial results, which compared ammonia volatilisation losses in response to different simulated rainfall scenarios, put losses in the range of less than one to 20 per cent.

The greatest losses occurred from the limed soils under a ‘low’ break-of-season rainfall, with losses more than halved under a ‘high’ break-of-season rainfall scenario.

The next step in this study is to measure the impact of any losses on the growth and yield of canola and to determine whether the lime continues to promote ammonia volatilisation in a subsequent barley crop grown in the glasshouse.

An assessment of the risk of ammonia volatilisation when lime and sulfate of ammonia are applied in close succession is expected to be delivered to growers and industry early in 2023.

More information: Associate Professor Louise Barton, 08 6488 2543, louise.barton@uwa.edu.au