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Rejuvenating soils with MinplusTM

A rock dust and soil conditioner to improve the productivity of acidic,
highly weathered soils

by R.J. Coventry, G.P. Gillman, M.E. Burton, D. McSkimming, D.C. Burkett, and N.L.R. Horner
James Cook University, Townsville.
December 2001

RIRDC Publication No 01/173 RIRDC Project No. UJC-4A

Executive Summary

The problem: sustaining the productivity of highly weathered soils

Highly weathered soils, characterised by intense weathering and strong leaching of their mineral components, display a range of properties that are indicative of low inherent fertility and conditions detrimental to plant growth, including:

• strong soil acidity,
• low contents of nutrient cations supplied from weatherable minerals,
• very low levels of plant nutrients in the form of exchangeable basic cations, especially calcium, magnesium, and potassium,
• low availability of, or high requirements for phosphorus,
• weak ability to respond to liming,
• rapid oxidation of organic matter leading to degeneration of soil structure,
• restricted soil micro-organism activity.

The present report focuses on the use of a finely crushed basaltic rock dust as a means of overcoming such problems in farming systems on highly weathered soils, especially those of the humid tropics of Australia.

Nature of MinplusTM rock dust

The research was carried out into the nature and soil conditioning capacity of the finely ground basaltic rock dust manufactured and marketed as MinplusTM by our collaborative Industry Partner, Pacific Mineral Developments Pty Ltd, Innisfail. The feedstock, a rock quarry by-product passing through a 7 mm sieve, is finely ground in a ball mill to reduce the particle-size of the product to less than 0.250 mm. We found that this particle-size will release the maximum amount of exchangeable basic cations from the dust.

MinplusTM is a product that is entirely natural and is certified for use in organic farming systems. The rock dust reflects the composition of basaltic rocks, predominantly silicates of calcium, magnesium, iron, and aluminium, and is inherently more fertile than dusts derived from the granitic quarry byproducts that are widely available in Australia.

The product provides abundant amounts of a number of macro- and micro-nutrients that are essential for plant growth (notably calcium, magnesium, sulphur, and trace elements: iron, manganese, zinc, and copper), but no nitrogen, and relatively low amounts of phosphorus and potassium. We found that the use of mineral additives from rock phosphate and alkali granite is an ineffective way of enhancing the phosphorus and potassium contents of the rock dust; such supplements would be best supplied by using conventional fertilisers. On the other hand, gypsum appears to have some potential as a sulphur supplement in crushed basaltic soil amendments.

MinplusTM as a soil conditioner

We have studied the properties of seven important agricultural soils, formed in different soil parent materials in the Innisfail-Tully area (granite, basalt, metamorphic rocks, basaltic alluvium, granitic alluvium, and a sandy beach ridge), and have determined the extent to which MinplusTM applications have modified those soil properties. In these experiments, we used Standard grade MinplusTM, finer than 0.250 mm, and a Superfine grade, finer than 0.040 mm, both at rates equivalent to applications of 0, 1, 2.5, 5, 25, and 50 t/ha. In particular, we have studied the effects of MinplusTM on soil acidity (pH), cation chemistry (exchangeable basic and acidic cation contents and enrichment on different parent materials, charge fingerprints, plant nutrients), phosphorus chemistry (phosphorus fixation capacity, plant available phosphorus), and silicon chemistry (extractable silicon using four different extractants).

We have demonstrated from field and pot trials that MinplusTM rock dust has the capacity to enhance plant growth in difficult, highly weathered soils whose properties combine to inhibit crop production.

Our research has shown that this is accomplished by the ability of MinplusTM to:

• rejuvenate the mineral and chemical properties of highly weathered soils;
• improve the cation exchange capacity of the soil;
• enhance the ability of soils to retain nutrient cations in the soil;
• enhance the ability of soils to supply nutrient cations to plants;
• reduce soil acidity, similarly to the action of lime, but sustained against soil leaching in high rainfall terrains over a long time periods;
• neutralise soil acidity without the liberation of greenhouse gases, unlike lime which releases 400 kg of carbon dioxide from every tonne of lime applied to the soil. We note that some 2 million tonnes of agricultural lime is used annually in Australia (Merry & Janik, 1999, RIRDC Project No. CSO-9A), and this has the potential to release up to 800,000 tonnes of carbon dioxide in neutralising soil acidity;
• reduce phosphorus fixation capacity of the soil, so that phosphates added in fertiliser are kept in soil solution and are more readily available for plant nutrition;
• provide silicon to the soil for uptake into plant cells, which is thought to result in improved plant vigour and quality.

Longevity of the soil conditioning benefit

The longevity of the beneficial effects of MinplusTM application was investigated in a laboratory leaching study of 7 MinplusTM-treated soils, which were subjected to the equivalent of one wet season’s rainfall at Innisfail (2,750 mm) applied over a six week period. MinplusTM applications produced a significant increase in cation exchange capacity of all the soils. The increase was sustained against leaching processes: after leaching, the soils showed no diminution in the increased amounts of exchangeable calcium that were derived from the MinplusTM treatments. Similarly, some 94% and 53% of the exchangeable magnesium and potassium contents were retained in the leached soils.

These results suggest that some of the important benefits of MinplusTM applications to soils may be retained against leaching of rainfall over a number of wet seasons. Continued monitoring of changes in the field trials that have been established on the Innisfail Clay will provide a field verification of these findings on at least one of the studied soils. Long-term trials are needed on other soils and under different crop and vegetation covers to confirm the longevity of the benefits of MinplusTM applications to a wider range of soils and farming systems.

Broader benefits from the use of MinplusTM

From our research results, we conclude that the use of MinplusTM as a soil conditioner is expected to deliver following outcomes for farming systems on highly weathered soils: ! improved plant growth in acidic soils of low fertility; ! reduced fertiliser use in acidic soils with strong phosphorus-fixing properties; ! import replacement as a result of diminished use of high-cost imported fertilisers; ! reduced environmental hazards from decreased use of agricultural lime; ! reduced environmental hazards from decreased erosional losses of nutrients from farmlands and their deposition in sensitive natural wetlands and nearshore marine systems.

Hence, the product is thought to have the potential to provide significant commercial benefits through its ability to:

• enhance soil health and increase crop yields;
• reduce farming input costs in lower rates of application of fertiliser, and possibly pesticides;
• reduce the social, economic, and environmental impact of algal blooms and other evidence of high nutrient levels in waterways downstream of agricultural activities;
• provide divalent cations that may be used as a tool to be used in the amelioration of sodic soils;
• provide import replacement through the need for less fertiliser to sustain plant production on soils that have been treated with MinplusTM applications. This point is particularly significant in Far North Queensland, where fertiliser (70,000 tonnes in 1999) is second only to fuel in the scale of imports into the regional economy through the Port of Cairns;
• underpin the development of new farming strategies to ensure more sustainable uses of soil resources and fertilisers.