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Dr Erik Adriaanse

AFRIKAANS: Maak Ammoniumsulfaat deel van jou bemestingsprogram

Specific applications of Ammonium Sulphate may serve as an important source of Nitrogen (21% N) and Sulphur (24%S), for crop production.

Ammonium Sulphate can generally be regarded as a suitable supplemental source of nitrogen (N), which can address all Sulphur(S) requirements. Under specific conditions the use of Ammonium Sulphate can result in higher grain yields compared to other N-sources when used as the primary N-source.

Before planting
Large quantities of Ammonium Sulphate can be broadcast before planting under Centre Pivot Irrigation, especially when soil pH is high and where considerable quantities of plant residues are incorporated in the soil. The volatilization of Ammonium Sulphate is relatively low compared to other N sources, however volatilization can be significant under alkaline conditions and therefore Ammonium Sulphate should rather be incorporated.

The application of Ammonium Sulphate as the main source of N before planting could result in higher grain yields compared to other N-sources under specific conditions (Figure 1).

Figure1. Greater effect of Ammonium Sulphate on Wheat Grain Yield compared to other N-Sources (95% probability difference from b). N-Sources spread and ploughed in. Hutton Soil Form, Mangano Soil Series with a clay content of 9.3% to a depth of 750mm and a pH (H2O) = 6.4. Results adjusted from Snyman, P. J. du T Burger, R, Laker, M.C., 1985. S. Afr. J. Plant Soil, 1985, 2(4), 175 -178.

Possible reasons for higher yields obtained with Ammonium Sulphate compared to other N-Sources as in Figure 1:

  1. Wheat response to S due to S-deficiency in the soil.
  2. Less leaching compared to other N-Sources.

Under strong alkaline conditions the greater acidifying effect of Ammonium Sulphate can enhance the uptake of other plant nutrients.

In Plant Mixtures
All N in Ammonium Sulphate is in the ammonium (NH4+) form while most crops prefer a combination of both nitrate (NO3) and ammonium. Furthermore the conversion of ammonium-N to nitrate-N is usually very slow when band placed. Relatively small amounts of Ammonium Sulphate are incorporated in some granular and liquid plant mixtures to increase the sulphur and ammonium content. Extra ammonium in the plant mixture will accordingly reduce leaching of N in the plant mixture. The application of Ammonium Sulphate as the only source of N in plant mixtures is not recommended.

After planting
Large quantities of Ammonium Sulphate can be broadcast after planting as the main N-source, either through spreading or Centre Pivot applications. As previously indicated soil incorporation is preferred. The practice of applying up to 100kg Ammonium Sulphate/ha over maize plant rows directly after planting serves as an insurance against possible leaching losses of N in plant mixtures and an effective way of meeting all crop S requirements. The band placement of large quantities of Ammonium Sulphate as the main source of N after planting is not recommended.

Mixtures of LAN and Ammonium Sulphate in a ratio of 75% LAN : 25% Ammonium Sulphate (marketed as KANAS by some local fertilizer suppliers)  combines the optimal benefits of both products and contains 26.25% N and 6% S. This ratio of N : S closely resembles the requirement of many crops, with the added benefit that the N is applied in both the ammonium and nitrate forms. Similarly AS is mixed with Ammonium Nitrate Solution to provide a liquid product called Ammonium Sulphate Nitrate (ASN) containing 18% N and 2.4% S. Relatively large quantities of either KANAS or ASN may be broadcast or band placed without any toxic effects.

In combination with herbicides
The addition of Ammonium Sulphate to Glyphosate herbicides improved efficiency between 44 and 49% when treated with wetter/buffers and by 59% when treated with spreaders/adhesives (Nienaber, H. S. A Grain May 2010). This effect of Ammonium Sulphate should even be more pronounced when the salt content of the water is high.

Consult a qualified agronomist for locality specific applications. The results referred to in this article were obtained under specific conditions and are therefore not generally applicable under all conditions.