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SOIL ACIDITY AND ITS MANAGEMENT IN CROP AND PASTURE PRODUCTION

By LIME & LIMING PRODUCTS, PLANT & SOIL NUTRITION, Soil Acidity No Comments

To read article 1/2 click: http://agrispex.co.za/soil-acidity-and-its-management-in-crop-production/ 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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SOIL ACIDITY AND ITS MANAGEMENT IN CROP PRODUCTION

By FERTILIZER PRODUCTS, LIME & LIMING PRODUCTS, PLANT & SOIL NUTRITION, Soil Acidity 2 Comments

ARTICLE 1/2 THE NATURE OF SOIL ACIDITY AND IT’S DIAGNOSIS Acid soil conditions restricting crop growth occur widely in the eastern parts of South Africa. In the higher rainfall areas, soils are often naturally acidic; however, human intervention may accelerate acidification. It is worth noting that soil acidity problems are by no means unique to this country: worldwide, approximately 30% of the land available for cultivation is acidic. Farmers frequently have difficulty in getting to grips with the various soil acidity parameters listed in soil test reports, and furthermore, may be presented with conflicting advice regarding the use of products such as lime and gypsum. The purpose of these articles is to provide scientifically sound and practically useful answers to questions such as: “What exactly is soil acidity?”, “How does it impact crops?”, and “How is it best managed?” SOIL ACIDITY – WHAT IS IT, AND WHAT CAUSES IT? In order to gain a working understanding of soil acidity, there is a need to touch on some basic soil chemistry. Clays and organic matter in the soil carry a negative charge. In a soil that is not acidic, this negative charge is balanced by the positive charge on certain plant nutrients, in particular, calcium (Ca++) magnesium (Mg++) and potassium (K+). As soils acidify, concentrations of other non-nutrient elements, in particular hydrogen (H+) and aluminium (Al+++), as well as manganese (Mn++), increase, and they take the place of nutrients such as calcium and magnesium on the clays and organic matter (Figure 1). Under non-acidic conditions, the aluminium and manganese are contained in the clay and other soil mineral particles, but as acidity increases, clay edges start dissolving, releasing soluble aluminium and manganese into the soil. Importantly, from the perspective of managing soil acidity, it is the soluble aluminium, and sometimes manganese, which are the most important growth-limiting factors in acid soils. Furthermore, it must be borne in mind that pH measures only the concentration of hydrogen in the soil, and not that of aluminium and manganese. These considerations are of cardinal importance in terms of the development of economically sound recommendations for the correction of acidity problems. What causes soils to acidify? Although, as noted earlier, acid soils occur widely in nature, the following human activities may markedly accelerate acidification: Acid rain, resulting from atmospheric pollution by industry. This has been shown to be a major contributory factor in some Highveld areas. The use of nitrogenous fertilizers, particularly when applied in excess of immediate crop requirements. The removal of basic nutrients (calcium, magnesium and potassium) in harvested crops and animal products. Accelerated decomposition of soil organic matter as a result of tillage. SOIL ACIDITY – EFFECTS ON CROP GROWTH The effects of soil acidity on crop growth tend to be insidious, in that it is in the root zone where the major impact occurs. Damage caused to the root system and the unfavourable soil chemistry associated with excessive acidity are translated into poor crop growth, with there frequently being no classical…

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