Manganese sulphate mono-hydrate (MnSO4.H2O) is perceived to be the cheapest method of correcting deficiency and is still widely used, although its market share has been on the decline for some years. The elemental content ranges from 28 - 32 % w/w as Mn depending on the manufacture.

The most common source of Manganese sulphate is Chinese because it is cheap. The Mn content is stated to be 31% but the analysis can be variable even within the same batch. It contains high levels of impurities some undeclared and often the Mn content is unstable with the sulphate partially converting to the insoluble and unavailable dioxide (MnO2). These impurities form a dark brown sludge which frequently causes mixing and compatibility problems as well as blocking filters and nozzles.

Other sources of Manganese sulphate are more expensive but have a higher consistency of quality than the Chinese. It is important to be aware of the differing qualities of Manganese sulphate, since the quality of any formulated products is only as good as the source used in their manufacture.

Regardless of source, Manganese sulphate applied directly through the sprayer does not perform as well as formulated products particularly in terms of tank mixing compatibility with agrochemicals. It also does not contain any adjuvants such as pH regulators, buffering agents, wetting agents, stickers and spreaders which combine to improve mixing, compatibility, rainfastness, availability, uptake and absorption.

During active periods of growth, the maximum uptake by the crop per application is approximately 500 grams of Mn per hectare. However Manganese sulphate has to be applied at 5 kg/ha just to provide sufficient Mn to correct the deficiency. This supplies 1500 g/ha of Mn which is 3 times greater than the crop can utilise. The excess may cause scorch or is wasted either as ecologically suspect run-off into water tables or made unavailable due to colloidal soil interactions.

Adding appropriate adjuvants to the MnSO4 could improve performance but the optimum blend is not commercially available to the end user and the costs would be such that a good formulated product is preferable.

Quality of Manganese Sulphate Products Sold in the UK

Problems can occur with MnSO₄ and spraying water volumes. Recommended rates should not be applied in less than 230 l/ha to avoid mixing and scorch problems. Modern sprayers with induction hoppers and application rates down to 100 l/ha are difficult to use with Manganese sulphate.  These problems are largely diminished by the use of good quality liquid formulations but medium water volumes are still recommended especially when tank mixing with a number of products.

Unrefined inorganic powders - Pros and Cons

• Manganese sulphate Chinese Belgian (Sedema)

• Cheap

• Large but declining market

• Elemental content 28 - 32 % Mn

• High insolubles

• Variable analysis

• Mixing problems

• Compatibility problems

• Not formulated

• Inefficient uptake

• High application rates required relative to crop uptake