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Soil compaction can be a serious yield-limiting factor in crop production, and indications are that this problem is more widespread than is generally perceived. Since compaction is below ground and, thus, not visible, its role in yield decline often goes unnoticed. Soil types Soils vary in their susceptibility to compaction. Research indicates that soils with higher sand and silt contents are more vulnerable to being compacted than loam and clay soils. In addition, the higher levels of organic matter in humic soils reduce their susceptibility to compaction. Causes Management practices that promote compaction include the following: Intensive tillage. Tillage may temporarily loosen the soil; however, in the long-term regular tillage increases the bulk density of soils through the weakening of soil structure and the depletion of soil organic matter. In addition, implements such as the mouldboard plough and disk harrow compact the soil beneath their working depth. Repeated use of these implements can result in the development of plough pans (compacted zones immediately below the ploughed layer (Figure 1, left). Wheel traffic. The wheels of vehicles used to apply fertilizers, lime and other products, harvest crops and pull implements, result in decreases in soil porosity and yield-limiting compaction (Figure 1, right). In the production of annual crops, 60% or more of the soil surface can be trafficked through cultivating, planting, fertilization, herbicide and other chemical applications (Mitchell & Berry, 2001). The in-field traffic associated with maize silage operations and the production of sugarcane frequently results in severe compaction problems (Figures 2 and 3) and associated yield losses. Animal hooves. The role of animals, particularly in intensive grazing systems, in promoting soil compaction is often overlooked. However, although animals are not as heavy as machinery, their weight is applied to the soil in a relatively small hoof-print, and so can cause significant compaction and damage to pastures, thereby decreasing forage yields and animal performance (Figure 4). Importantly, soils are most susceptible to being compacted when they are wet. Unfortunately, timing of farming operations, such as harvesting, fertilizing and rotating livestock, often makes it difficult to exclude machines and animals from soils when wet. Effects of compaction Detrimental impacts of compaction on soil health and plant growth are due largely to the decrease in the proportions of macro-pores in compacted soils. Effects associated with this include the following: Root penetration into compacted soil layers is restricted, and root and biological soil health compromised through water logging and anaerobic conditions in soils. Water infiltration as well as water-holding capacity of soils are reduced, and effective rainfall is diminished through increased runoff. Ponding on the soil surface after rain or irrigation is usually an indication of compaction. Nutrient uptake by roots is poor in compacted soils. This is due to, firstly, limited root development, and secondly, restrictions on the movement of nutrients to roots by mass flow. Yellowing of plants due to nitrogen deficiency is often associated with poor nitrogen recovery by roots in compacted soils. The picture in Figure 2 (left) provides clear…

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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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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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