First it was liquids, now a new option: suspension fertiliser shows promise by Denys Slee

Suspension fertilisers: showing promise at Minnipa Research Centre, which has led the way developing alternative fertiliser methods under the research guidance of Bob Holloway, seen here with colleague Alison Frischke.

FERTILISER HELD in suspension by clay has produced very promising plant growth and yield responses in innovative research underway on the Eyre Peninsula.

The 'mix' is being trialed by the officerin-charge of the Minnipa Research Centre, Bob Holloway, and his colleagues as an alternative option to the use of granular and fluid fertilisers on phosphorus-responsive calcareous soils.

These trials follow research that has shown that highly alkaline calcareous soils, such as those found in large areas on the Eyre Peninsula, and in the Victorian and SA Mallee, have a high capacity to fix phosphorus and trace elements such as zinc, making these nutrients less available to crops.

Research so far

Field trials on the Eyre Peninsula have demonstrated that fluid fertilisers such as ammonium polyphosphate (APP) are more effective, particularly in drier years, on these soils than granular fertilisers such as MAP and DAP. In recent preliminary trials in Victoria, yet to be confirmed in the field, greater efficiencies have been observed in fluid fertilisers compared to granular fertilisers on a range of soils.

The mechanisms that make fluid fertilisers more effective in some circumstances are under further investigation but are thought to include greater spread and less fixation of phosphorus, compared to granular forms.

"Ammonium polyphosphate has been a solid performer in experiments comparing fluid and granular fertilisers, but it is expensive," Dr Holloway said. "APP is a clear liquid but it is difficult to mix zinc and manganese sulphate with it, which are the cheapest sources of trace elements.


"Most farmers will not convert their machines to fluids until the price drops but the price of fluids is not likely to drop significantly until the demand increases."

He suggested an answer to the problems of cost, and the mixing of other nutrients which can cause precipitates in clear liquids, might be provided through the use of suspension fertilisers.

These are generally based on powdered or even granular MAP or DAP mixed in water and held in suspension by clay added to the water.

In trials last year at three highly calcareous sites on the Eyre Peninsula, suspensions produced yields equal to or better than APP, and APP generally produced better yields than granular fertilisers.

"The possibility that suspensions will have the same benefits as clear liquids is exciting given the expectation that they should be significantly cheaper, in terms of cost per unit of P," Dr Holloway said. "There may be some application difficulties compared with clear solutions, but these should not be difficult to solve."

The research is continuing this year to compare the performance of suspension, clear fluid and granular fertilisers on a range of soils.

Ongoing issues for research include product compatibility; the availability of macro- and micro-nutrients in various mixtures to plants; ratios of ingredients, including clay and water; storage and transport issues; the need for dilution with water at sowing; placement effects; on-farm storage and handling; and distribution to individual tines using material that cannot be filtered.

Contact: Dr Bob Holloway 08 8680 5104