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Mart Farina



To read article 1/2 click: ARTICLE 2/2 THE USE OF LIME AND GYPSUM IN MANAGING SOIL ACIDITY In the first article in this series, we discussed the nature of soil acidity. Particular attention was drawn to the harmful effects of soluble aluminium on root growth and function, and how crop species differ in their ability to tolerate aluminium toxicity. In this second article, we focus on practical aspects of soil acidity management. LIME AND GYPSUM — HOW DO THEY DIFFER? Lime and gypsum are chemically very different products, and consequently their effects on the soil are quite dissimilar. In the agricultural context, ‘lime’ refers to any product in which the calcium and magnesium compounds are able to neutralize soil acidity. Carbonates of calcium and magnesium are the most widely used for this purpose. Dolomitic lime contains a minimum of 15% magnesium carbonate, while calcitic limes have less magnesium carbonate than this. In addition to natural carbonates, various by-products of industrial processes are frequently used as liming materials; these include calcium oxide (burnt lime), calcium hydroxide (slaked lime) and calcium silicate (slag). Gypsum, on the other hand, is calcium sulphate, a neutral salt. It is a valuable calcium and sulphur fertilizer and is much more soluble than lime. In addition, it leaches readily into the subsoil and, in highly weathered (naturally acidic) soils, the sulphate component displaces OH- ions from the clay surfaces. These, in turn, convert soluble aluminium to unavailable aluminium hydroxide. The effectiveness of various liming materials varies widely, with the following factors being particularly important in this regard: Chemical purity ─ the presence or otherwise of non-reactive materials such as sand and clay greatly affects the neutralizing value of the lime (importantly, the colour of the liming material is not a reliable indicator of its quality!). Chemical composition ─ the nature of the calcium and magnesium compounds present. Fineness ─ the finer the lime particles, the faster will be their reaction in the soil. Lime particles larger than 0.84 mm in diameter (about the size of a match head) are of little value. Very coarse liming materials are completely ineffective. Hardness ─ the solubility, and hence neutralizing value, of lime depends on whether it is derived from hard crystalline material or from softer relatively unconsolidated material. Where uncertainty exists as to the quality of a particular liming material, a sample should be submitted for analysis. ACTION OF LIME AND GYPSUM IN SOILS The major effects of lime on soil properties are: an increase in soil pH; a decrease in soluble aluminium and acid saturation levels; an increase in calcium and magnesium levels. The value of dolomitic lime as a magnesium fertilizer is often overlooked. Although several magnesium fertilizers are commercially available, they tend to be prohibitively expensive, and dolomitic lime remains the most cost-effective way of increasing soil magnesium levels. The neutralizing effect of lime on soil aluminium and hydrogen is illustrated in Figure 1. Importantly, the soil must be moist for lime to react. The solid aluminium…

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Dr Mart Farina consulting soil scientist

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Dr Mart Farina started his career as a Land Development Officer in Rhodesia and then worked as a soil fertility researcher in the South African Department of Agriculture and the Agricultural Research Council from 1965 to 1998. Since then, he has consulted to Omnia Fertilizer, the FAO in Ethiopia, Nordic Fertilizers in Central America, and to government agencies and individual farmers in Kenya, Tanzania, the DRC, Zambia, Madagascar, Malawi, Zimbabwe and the Republic of Guinea. During the course of his career Mart has authored or co-authored 66 refereed scientific papers, 23 of which were published in international journals. In the interests of technology transfer to the agricultural industry Mart has also published over 50 semi-scientific and popular articles and addressed some 300 farmer’s days; he has also presented over 70 papers at scientific conferences and technical symposia. Mart was a Research Fellow at the University of Georgia, U.S.A., during 1978 and has participated in 10 international conferences on soil fertility, on five occasions as an invited speaker. Mart has been the recipient of numerous awards including six from the Fertilizer Society of South Africa (FERTASA), the BP Scholarship in Agriculture, the Gold Medal for Research from the South African Society of Crop Production, and the Sanachem Prize. In 1988 and 2005 was the Natal Agricultural Writer’s Association Agriculturist of the Year, in 1984 the National Maize Producer’s Maize Researcher of the Year, and in 1994 the Summer Grain Centre’s Researcher of the Year. Mart served as the standing member for Africa, on the International Steering Committee for Symposia on Plant-Soil Interactions at Low pH from 1993 until 2001. Mart has Honorary Membership of the South African Society of Crop Production and is a 40-year Emeritus Member of the Agronomy Society of America and the Soil Science Society of America. Contact details:  

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