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A tool for efficient and cost effective use of soils and fertilisers in the humid tropics.

Report by Ross J. Coventry


Min Plus is a finely ground volcanic rock product that consists largely of calcium, magnesium, and potassium silicates and contains a range of elements which are essential to support plant growth. It is a natural mineral replenisher for soils that have low natural fertility as a consequence of the weathering losses of their primary minerals. When applied as a conditioner to mineralogically depleted soils and allowed to react with the soil before fertilising, Min Plus can induce phenomenal improvements in plant growth and yield, and in produce quality.


Minerals occurring in soils are gradually broken down by weathering of process with the passage of time to release their constituent elements into the soil. Some of these elements are dissolved by waters leaching through the soil profile and are lost from the soil, others are removed in the soil particles that form the muddy component of runoff waters after rain, and others are taken up in a dissolved form by plant roots to sustain growth. Elements taken up by agricultural crops are removed upon harvesting and, with leaching or erosional losses, dramatically alter the mineral balance of the soil. Thus, the mineral content of soils used to produce each successive crop undergo a steady decline which must be balanced by inputs of soil conditioners and fertilisers if the system is to remain viable and productive.

Weathering processes, mineral breakdown, and leaching losses are greatest in the humid tropical areas of the world where temperatures are high, there is abundant moisture in the soil, and chemical reactions proceed rapidly. Nutrient elements are lost from the rootzone of the plants; residual minerals of little chemical benefit to plants (mainly kaolinitic clays and iron and aluminium oxides and oxyhydroxides) tend to accumulate in the soil. As weathering, leaching, and element depletion processes proceed, the growing conditions for plants of all kinds become more stressful. Symptoms of the increased stress are diminished plant vigour, poor root development, slow growth, low resistance to pests and diseases, low yields, and declining nutritional value of the produce.

Modern agriculture's use of conventional technology - employing large applications of NPK fertilisers, new plant varieties, a range of agricultural chemicals, and crop rotations - may overcome some of these stresses on a year to year basis. But many modern practices are, at best, expensive quick fixes. They fail to address the fundamental, longer-term problems of highly weathered and strongly leached soils that are, to all intents and purposes, worn-out.

When vital minerals and essential elements are restored to the soil through conditioning and remineralisation strategies, the stresses are reversed, and the plants respond positively. A number of scientific studies have documented the benefits from the use of silicate-based amendments in the highly weathered soils of Mauritius, Hawaii, Florida, and Brazil. They include:

  • Improved mineral content of the soil.
  • Better plant root development.
  • Hardier plant foliage.
  • Faster plant growth.
  • Greater plant resistance to pests and diseases.
  • Increased crop yields and produce quality.

Of particular interest are the publications of D'Hotman de Villiers (1961, 1962), Clements (1965), Fox et al. (1967), Plucknett (1972), Gascho (1978), Leonardos et al. (1987), and the papers by Gillman and co-workers. References to these and other relevant studies are listed in the "Further Reading" section, below.


To understand the way in which soil conditioners work in highly weathered soils, it is important to understand several fundamental characteristics of soils including: cation exchange reactions, soil acidity effects, and phosphate fixation processes.

Cation Exchange Reactions

Soils consist of gravel, sand, silt, and clay particles, often bound together in aggregates, with intervening pore spaces which hold the air and water that are essential for plant growth. Many of a plant's nutritional needs are met from nutrients dissolved in the soil water. These nutrients are present as positively charged ions ("cations") or negatively charged "anions", just like the sodium cations (Na+) and chloride anions (Cl-) that are produced when table salt (sodium chloride NaCl) is dissolved in water.

In soils, the commonly occurring cations are calcium (Ca2+), magnesium (Mg2+), potassium (K+), and sodium (Na+), the “basic cations”, and the first three are essential for growth in all plants. Hydrogen (H+) and aluminium (Al3+ ions), the “acidic cations”, build up in highly weathered soils from which the basic cations have been lost by weathering and leaching. In the very acidic soils (those with a soil pH less than about 5.5), the aluminium cations may be concentrated to such an extent that they may become toxic, causing very poor growth in most plants, or even plant deaths. Only the most acid tolerant plant species such as azalea, camellia, coffee, gardenia, rhododendron, and tea thrive on such soils.

The strongly reactive components of the soil are the "colloids" which are very tiny fragments of mineral or humic materials in the soil. They are finer than 2 thousandths of a millimetre and present a large surface area in the soil upon which many chemical reactions take place. The surfaces of the colloidal particles usually carry a net negative electrical charge. To maintain a neutral electrical state in the soil, positively charged cations are attracted from the soil solution and are held loosely on the negatively charged sites on the surfaces of the colloids by electrostatic forces that operate similarly to the way iron filings may be held by a magnet. Different soil colloids have different abilities to attract and hold cations from the soil solution. The cations may swap, or "exchange" positions between the surfaces of the colloids and the soil solution depending on the mineralogical and chemical properties of the soil. The overall effect determines the "cation exchange capacity" (CEC) of the soil. The more fertile soils will have a high CECs and, conversely, the poorer soils will have a low CECs.

Certain highly weathered tropical soils are at significant risk because their negative charge, hence their CEC, can be diminished very nearly to extinction by natural weathering processes and inappropriate land management practices. Consequently, the basic nutrient cations may be almost entirely leached from the soil by the rain resulting in a marked decrease in soil fertility, an increase in soil acidity, and greater environmental hazards in streams and coastal wetlands through the concentration in drainage water of the plant nutrients leached from the soil.

Role of Min Plus:

Important effects of Min Plus as a soil conditioner is to raise the negative charge, and to increase the CEC of treated soils. Hence, Min Plus improves the ability of the soil to hold nutrient ions supplied from the breakdown of organic matter (humus) or mineral particles in the soil, or those derived from fertilisers applied to the soil. Min Plus also enhances the ability of the soil to release exchangeable cations (retained on colloid surfaces) to the soil solution for uptake in a dissolved form by growing plants.

Soil Acidity

The ability of the soil to release acidic exchangeable cations (H+ and Al3+) to the soil solution is a measure of the soil's acidity. Which is the major control over the amount and kind of nutrients available in the soil to be taken up by plants during their growth. Soil acidity is measured is measured on a pH scale from 0 (extremely acid), through 7 (neutral), to 14 (extremely alkaline). Although most plants prefer to grow in soils with a pH of 6.0 - 7.0, some can tolerate more acid or more alkaline soil conditions. Generally, plant growth is seriously diminished in the strongly acidic soils of the world through the build-up of toxic levels of aluminium and manganese in soils with a pH of less than 5.5.

The pH of acid soils can be raised by additions of lime (CaCO3) or dolomite (CaMg(CO3)2) at rates of the order of 1.5 - 10 t / ha. Bruce (1988) and Cumming and Elliott (1991) described the main benefits of liming acid soils as:

  • Promoting plant growth by adjusting the pH to the range preferred by the specific crop.
  • Reducing the solubility of aluminium and manganese (Al and Mn) to lower than toxic levels in strongly acid soils (i.e. those with soil pH less than 5.5).
  • Providing a better plant response to fertiliser additions by increasing the availability of essential plant nutrients such as molybdenum.
  • Enhancing leguime growth by providing a more suitable environment for microbiological activity that enhances plant growth.

Unfortunately, the beneficial effects of lime or dolomite applications are generally short-lived under the heavy soil leaching processes that prevail in humid tropical conditions, may consume large quantities of lime to raise the soil pH to an appropriate level, and are largely confined to the immediate top soil.

Role of Min Plus:

Research conducted mainly in Mauritius, Hawaii, and Brazil (see the "Further Reading" section, below) has shown that the application of silicate-based rock dusts as a "liming material" can be very effective. Apart from the liming benefits outlined above, the application of Min Plus, a silicate-based rock dust, to highly weathered soils could alleviate another very significant problem - "phosphate fixation" where phosphorus applied as a fertiliser is chemically bound to soil particle surfaces so strongly that it becomes unavailable to plants.

Phosphate Fixation in Soils

Phosphorus, an element that is essential for adequate growth of all plants, is commonly available only in very low amounts in many soils of the humid tropics; it is supplemented from phosphatic fertilisers such as superphosphate, diammonium phosphate, or rock phosphate. The fertiliser dissolves in the soil under the action of rain or irrigation water, and its components will be held in the soil forms that can be absorbed by plants. The phosphorus in the dissolved phosphate anions has an ability to form chemical bonds with the iron or aluminium that form oxides or oxyhydroxides in highly weathered soils. This means that the phosphorus will then be locked onto the solid part of the soil, and will have effectively been removed from the soil solution. The phosphorus that was added in the fertiliser is then no longer be available in the liquid phase, and cannot therefore be taken up by the plant roots. This is what is meant by "phosphate fixation".

Fixation reactions in acidic, iron-rich and clay-rich tropical soils may allow only a small fraction (10 - 15%) of the phosphorus supplied in phosphatic fertilisers and manures to be taken up by plants in the year of application. A major consequence of the widespread phosphate fixation problem in humid tropical soils is that the farmer does not gain the full agronomic benefit from the expensive fertiliser that has been applied to the soil. Consequently, there is a tendency for many farmers to over-fertilise by applying additional fertiliser over the amount that stays in the soil solution and can be taken up by plants. Not only do the surplus fertiliser additions add unnecessary expense to the farming enterprise, but they may also be mobilised by erosion into downstream parts of the ecosystem and contribute to nutrient enrichment in sensitive wetland environments.

Role of Min Plus:

It is thought that silicate ions, derived from a silicate-based soil conditioner, can occupy the sites on the soil to which the phosphate ions might otherwise be attracted. Min Plus consists of approximately 60% of silicate anions. Hence, allowing applications of Min Plus to react in the soil under naturally moist conditions for some time (2 - 12 months or more) before applying a phosphatic fertiliser, may result in the phosphate from the fertiliser being prevented from forming chemical bonds with iron or aluminium on the surfaces of the soil colloids. The phosphorus will then be held in soil solution in Min Plus-amended soils and remain available for uptake by plants. The soil conditioner thereby increases the efficiency of fertiliser use, sustains strong plant growth, and reduces environmental leaching hazards.


International Research

Some of the results from using silicate-based soil conditioners and remineralisers have been known for more than 50 years from studies in Mauritius, but they have not been applied in Australian agriculture. For example, Evans (1947) showed that applications of up to 32 t / ha of crushed basalt to strongly weathered basaltic soils promoted significant increases in sugarcane tonnages and sucrose contents. Feillafe (1950, 1952) and Parrish and Feillafe (1958) also found improvements in sugarcane yields from the application of silicate-based soil amendments. Similarly, D'Hotman de Villiers (1961, 1962) suggested that increased sugarcane yields could be maintained in up to 7 ratoons from one heavy application (54 t / ha) of crushed basalt.

It has been known for some time that silicate-based soil amendments, some with close affinities to Min Plus, may also be used as soil amendments in highly weathered tropical soils. For example, Wong You Cheong and Halais (1970) compared the effects of applications of lime and Portland cement (a mixture of calcium silicates and calcium aluminates) on sugarcane soils in Mauritius; although both treatments had similar beneficial effects on soil acidity, the cement treatment produced a significantly larger increase in crop yield. Similar responses to silicate amendments, generally derived from fly ash or crushed furnace slags, have been observed in sugarcane crops in Hawaii (Clements 1965; Fox et al. 1976) and Florida (Gascho 1978).

Australian Studies

Although a few studies have been made in Australia using silicate-based soil amendments such as calcium silicate slags or Portland cement (Ayres 1966; Hurney 1973; Haysom & Chapman 1975), very little research has been undertaken in this country into the effectiveness of rock dusts to improve plant growth.

Dr Gavin P. Gillman and co-workers have addressed themes of major significance to the use of silicate-based soil amendments in sustaining agricultural production in the humid tropics. Their results are set out in a series of publications which are listed in the "Further Reading" section (see below), including:

  • The exchangable cation status of tropical soils.
  • The maintenance of soil fertility.
  • Soil surface change characteristics.
  • Soil acidification and lime requirements.
  • Grouping of soils with similar charge properties.
  • Leaching of nutrients from tropical soils.

Dr Gavin P. Gillman (1980) showed that crushed basaltic scoria applications to a highly weathered soil in North Queensland, produced a marked improvement in soil cation exchange capacity as the particle-size of the crushed basaltic soil amendment became finer, and as the duration of incubation of the conditioner in the soil increased. Gillman concluded that, "a single large application of the basalt cinders may obviate the need to apply Ca and Mg for many years, as well as reduce the leaching losses of applied NH4 [ammonium] and K over a similarly long period."


No Australian studies addressed the practical issues raised by the use of silicate-based soil amendments in humid tropical soils until 1997 when a group of soil scientists led by Associate Professor Ross J. Coventry at James Cook University, Townsville, began their present research program. These studies have been funded in part by the Australian Government through Project No. UJC-4A of the Rural Industries Research and Development Corporation, entitled "Soil amendments from rock quarry by-products".

The research program is built on the principles and approaches advocated by Gillman and co-workers and is focussed on the 3 main areas, as follows:

1. Mineralogy and properties of Min Plus

Laboratory research to determine:

  • The mineralogy and chemical composition of quarry products used to manufacture Min Plus.
  • The most effective size of grinding of the rock dust to maximise the soil conditioning effect of Min Plus.
  • The effectiveness of Min Plus in supplying nutrients to plants.
2. Properties of Min Plus treated soils

Laboratory, shadehouse, and field trials to determine:

  • The mineralogy and chemical composition of a range of soils both before and after amelioration using Min Plus rock dust.
  • The ability of highly weathered tropical soils to retain nutrients after Min Plus applications.
  • The range of soil type over which the silicate-based amendments may be effective.
  • The length of time over which Min Plus-induced improvements in soil chemical properties might be expected to persist in a variety of strongly leached soils under heavy seasonal rainfalls.
3. Plant nutritional responses to soil conditioning

Laboratory, shadehouse, and field trials to:

  • Determine the growth responses of a range of tropical crop and pasture plants to applications of Min Plus and other amendments (e.g. lime, cement) in specific soils.
  • Establish the most effective methods for using Min Plus in combination with conventional NPK fertilisers to ensure optimal plant growth in highly weathered tropical soils.
  • Compare plant growth responses to Min Plus treatments (calcium and magnesium silicates) with responses to liming (calcium carbonate) and cement (calcium silicate) treatments.
Potential benefits of the current research

The research program is expected to provide major potential benefits to agricultural production and environmental protection in the humid tropics from the use of Min Plus as a soil conditioner through:

  • Improved plant growth in acidic, highly weathered soils of low fertility.
  • Farming strategies to ensure more sustainable uses of soil resources and fertilisers.
  • Reduced fertiliser use, especially in soils with strong phosphate fixing properties.
  • Import replacement as a result of diminished use of high-cost imported fertilisers.
  • Environmental benefits from decreased nutrient losses from farmlands.
  • Reduced nutrient enrichment in sensitive natural wetlands and nearshore marine systems.


Many of the highly weathered soils used for agricultural, horticultural, and forestry production in the humid tropics of northern Australia present significant problems for plant growth including:

  • Strong acidity.
  • Low negative charge on the soil colloids.
  • Low cation exchange capacity.
  • Low contents of calcium, magnesium and other essential plant nutrients.
  • Strong phosphate fixation characteristics of soils.

Min Plus has many properties that make it suitable for the amelioration of the soil problems outlined above, thus explaining the remarkable successes obtained with similar, silicate-based soil amendments used in Mauritius, Hawaii, and Brazil.

Min Plus is well suited as an amendment on the soils used for the cultivation of sugarcane, tropical fruit (especially bananas and pawpaws), and forestry timber trees in North Queensland. Research to date has shown that Min Plus has the capacity to:

  • Raise the soil pH, thereby reducing the acidity of the soil.
  • Increase the negative charge in the soil in a more permanent way than by the reversable process of increasing soil pH by liming.
  • Rase the cation exchange capacity of the soil, hence increasing the soil's ability to retain and supply cations to the soil solution for uptake by growing plants.
  • Deliver balanced quantities of calcium, magnesium, and a range of other essential plant nutrients occupying the increased negatively charged sites in the soil.
  • Supply silicate anions to overcome the phosphate fixation characteristics of the soil.

The research results suggest some simple management practices for humid tropical soils that include:

  • Raising the soil pH and reducing the soil acidity problems by Min Plus applications or liming.
  • Increasing the negative charge in the soil by heavy applications of Min Plus, or by increasing the soil organic matter content through the incorporation of plant residues (especially ley crops of tropical legumes) into the soil.
  • Supplying smaller quantities of nutrients more frequently to Min Plus conditioned soils by splitting fertiliser applications.
  • Overcoming phosphate fixation problems by applying Min Plus at a rate of at least 2.5t/ha, and well in advance of phosphatic fertiliser applications. Better soil conditioning and remineralising results will be obtained from heavier applications of Min Plus (up to 50t/ha), and by allowing sufficient time (2-12 months or more) under naturally moist conditions for the Min Plus to react with the soil prior to applying phosphatic fertiliser.


Ayres, A.S. (1966). Calcium silicate slag as a growth stimulant for sugarcane on low-silicon soils. Soil Science 101, 216-227.

Bruce, R.C. (1988). Soil acidity and liming. In: Fergus, I.F., Understanding soils and soil data, Australian Society of Soil Science, Inc., Queensland Branch, Brisbane, pp. 87-107.

Clements, H.F. (1965). Effects of silicate on the growth and leaf freckle of sugar-cane in Hawaii. Proceedings of the International Society of Sugar Cane Technologists 12, 197-215.

Cumming, R.W. and Elliott, G.L. (1991). Soil chemical properties. In: Charman, P.E.V. and Murphy, B.W. (Eds), Soils: their properties and management - A soil conservation handbook for New South Wales, Sydney University Press in association with Oxford University Press, South Melbourne, pp. 193-205.

D'Hotman de Villiers, O. (1961). Soil rejuvenation with crushed basalt in Mauritius. Part I: Consistent results of worldwide interest. International Sugar Journal, December 1961, 363-364.

D'Hotman de Villiers, O. (1962). Soil rejuvenation with crushed basalt in Mauritius. Part II: Fertility of basalt and nutritional effects. International Sugar Journal, January 1962, 3-5.

Evans, H. (1947). 18th Annual Report. Mauritius Sugar Industry Research Station Report, 42-48.

Feillafe, S. M. (1950). 21st Annual Report. Mauritius Sugar Industry Research Station Report, 25-28.

Feillafe, S.M. (1952). Fertilizing value of crushed basalt. 23rd Annual Report of the Sugarcane Research Station, Mauritius, 19-20

Fox, R.L., Silva, J.A., Younge, O.R., Plucknett, D.L., and Sherman, G.D. (1967). Soil and plant silicon and silicate response by sugarcane. Soil Science Society of America, Proceedings 31, 775-779.

Gascho, G.J. (1978). Responses of sugarcane to calcium silicate slag. I. Mechanisms of response in Florida. Soil and Crop Science Society, Florida, Proceedings 37, 55-58.

Gillman, G.P. (1980). The effect of crushed basalt on the cation exchange properties of a highly weathered soil. Soil Science Society of America, Journal 44, 465-468.

Gillman, G.P. (1984). Using variable charge characteristics to understand the exchangeable cation status of oxic soils. Australian Journal of Soil Research 22, 71-80.

Gillman, G.P. (1985). Influence of organic matter and phosphate content on the point of zero charge of variable charge components in oxidic soils. Australian Journal of Soil Research 23, 643-646.

Gillman, G.P. and Abel, D.J. (1986). A summary of surface charge characteristics of the major soils of the Tully-Innisfail area, North Queensland. CSIRO Australia, Division of Soils, Divisional Report, No. 85.

Gillman, G.P., and Bristow, K.L. (1990). Effect of surface application of urea, ammonium sulfate and lime on exchangeable cation distribution in an Inceptisol in humid tropical Queensland. Australian Journal of Soil Research 28, 39-53.

Gillman, G.P., Bristow, K.L., and Hallman, M.J. (1989). Leaching of amendments from a north Queensland Oxisol. Australian Journal of Soil Research 27, 183-198.

Gillman, G.P., Bruce, R.C., Davey, B.G., Kimble, J.M., Searle, P.M., and Skjemstad, J.O. (1983). A comparison of methods for determining cation exchange capacity. Communications in Soil Science and Plant Analysis 14, 1005-1014.

Gillman, G. P., Shamshuddin, J., and Bell, L. C. (1989). Soil chemical parameters and organic matter in soil management. Soil management and smallholder development in the Pacific Islands. International Board for Soil Resources and Management, 8, 142-153.

Gillman, G.P., and Sinclair, D.F. (1987). The grouping of soils with similar charge properties as a basis for agrotechnology transfer. Australian Journal of Soil Research 25, 275-285.

Gillman, G.P., and Sumner, M.E. (1987). Surface charge characterisation and soil solution composition of four soils from the southern Piedmont in Georgia. Soil Science Society of America, Journal 51, 589-594.

Gillman, G.P. and Sumpter, E.A. (1986). Surface charge characteristics and lime requirements of soils derived from basaltic, granitic, and metamorphic rocks in high-rainfall tropical Queensland. Australian Journal of Soil Research 24, 173-192.

Gillman, G.P., and Yu, T.R. (1986). Cation retention in red soils of the tropics. Proceedings of the International Symposium on Red Soils, Nanjing, China, Nov. 20 - 29, 1983, pp. 251 - 261. (Science Press: Beijing / Elsevier: Amsterdam).

Haysom, M.B.C., and Chapman, L.S. (1975). Some aspects of the calcium silicate trials at Mackay. Proceedings of the Australian Sugar Cane Technologists 42, 117-122.

Hurney, A.P. (1973). A progress report on the calcium silicate investigations. Proceedings of the Australian Sugar Cane Technologists 40, 109-113.

Leonardos, O.H., Fyfe, W.S., and Kronberg, B.I. (1987). The use of ground rocks in laterite systems: an improvement to the use of conventional soluble fertilisers. Chemical Geology 60, 361-370. Murtha, G.G. (1986). Soils of the Tully-Innisfail area, North Queensland. CSIRO Aust., Div. Soils, Divisional Report No. 82. Oades, J. M., Gillman, G. P., and Uehara, G. with Hue, N. V., Noordwijk, M. V., Robertson, G. P., and Wanda, K. (1989). Interactions of soil organic matter and variable charge clays. In: Coleman, D. C., Oades M. J., and Uehara, G. (Eds), Dynamics of soil organic matter in tropical ecosystems, University of Hawaii Press, Honolulu, Hawaii.

Parrish, D.H., and Feillafe, S.M. (1958). Annual Report. Mauritius Sugar Industry Research Station Report, 81-82.

Plucknett, D.L. (1972). The use of soluble silicates in Hawaiian agriculture. University of Queensland, Papers of the Department of Agriculture 1, 203-223.

Uehara, G. and Gillman, G.P. (1981). The mineralogy, chemistry, and physics of tropical soils with variable charge clays. Westview Press, Boulder, Colorado.

Wong You Cheong, Y., and Halais, P. (1970). Needs of sugarcane for silicon when growing in highly weathered latosols. Experimental Agriculture 6, 99-106.

Copies of any of the scientific papers listed above are available on request from International Mineral Consultants.