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Soil Acidity

Soil acidity the limiting factor



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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By Biostimulants, Conferences and Training, Soil Acidity No Comments

Ruth Rhodes previously worked for The South African Sugar Research Institute (SASRI) as a soil scientist and is now a private consultant, delivered a well balanced and objective presentation on the definition of soil health, highlighting some prevalent factors affecting soil health and methods of quantifying soil health. Rhodes initiated the concept of soil health by highlighting “how every soil tells a story” as illustrated below where two soils that started out identically thirty years ago ended up so different due to land use and management: The “term soil health” today is used interchangeably with the terms “soil quality” and “soil condition” and there are various definitions that are used to describe soil health: A state of a soil meeting its range of ecosystem functions as appropriate to its environment. Soil health / quality describes soils that are not only fertile but also posses adequate physical and biological properties to “sustain productivity, maintain environment quality and promote plant and animal health”- Doron 1994. “how well soil does what we want it to do” – USDA Rhodes pointed out that there are many other definitions however we should want our soils to support and  grow optimally yielding crops, “forever” without harming the environment. The soil food web may be used as the starting point in assessing soil health, however there are over forty different factors that determine soil health which can be grouped into biological factors, physical factors, chemical factors and nutritional factors. Rhodes aptly likened these groups of factors to being the pieces of a puzzle and that if on piece was missing then the puzzle is incomplete: Soil health shouldn’t be viewed in terms of biology only as it is comprised by a whole range of different factors, of the these there are only two inherent qualities that we can’t really control and aren’t affected by management easily; soil depth and texture. They are determined by the factors of soil formation such as climate, topography, vegetation, parent material and time which give soils some kind of inherent health or quality for example comparing a loamy soil to a sandy soil. A loamy soil may seen to be more healthy because it has a higher water holding capacity; or referred to as having a higher “soil capability’. Dynamic qualities affect soil quality or condition that we can manage, Rhodes proceeded to briefly highlight some of these factors and how the changing nature of soil properties determining soil health may be affected by management. Chemical and Nutritional factors Soil Acidity is a significant yield limiting factor in dryland agriculture in South Africa especially in KZN and the Eastern Cape and is starting to become a problem in irrigation areas which until recently have not been a familiar with this problem. Soil acidity initially starts off in small patches that expand if not rectified, they can often be identified as areas displaying poor growth (in severe cases not even weeds will grow); “seed vigour” and germination problems resulting from soil acidity have…

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By LIME & LIMING PRODUCTS, Soil Acidity, Sulphur No Comments

Gypsum (CaSO4.2H2O) and its dehydrated forms are relatively common minerals that are distributed worldwide in sedimentary and evaporative deposits and produced as bye products of various industrial processes. The main sources of gypsum are from the production of phosphoric acid from rock phosphate; gypsum produced from this process is a finely grained high purity material and is commonly referred to as phosphogypsum (PG). The main sources of PG in South Africa are located in Modderfontein, Phokeng, Potchefstroom and Phalaborwa which are reported to carry in excess of 15 million tons. Other sources of gypsum are produced from flue gas desulphurization (FGD) in industrial processes in which Sulphur Dioxide (SO2) is scrubbed to meet SO2 emission standards. There are numerous deposits of natural gypsum in the Cape which are mined. Natural gypsum tends to have a larger particle size distribution than gypsum produced from industrial processes unless it is milled finely. The effect of particle size is an important factor in determining the effectiveness of gypsum amendments. Gypsum is slightly soluble in aqueous solution, dissolving to an extent of 2.5 g / l. Natural gypsum may contain traces of calcium carbonate and therefore may reflect slightly alkaline pH levels on analysis, while PG may contain free acid which may reflect as slightly acidic on analysis. Overall Gypsum has very little effect on soil pH. South African agriculture utilizes approximately 200 000 tons of gypsum per year which with an approximate 18% Sulphur (S) content provides a cost effective source of S. Disclaimer: The figures published are not a guarantee of analysis, they are sourced from published Product Data Sheets and provided to serve as an indicator of typical analysis which may vary due to industrial impurities and changes in minerology of the natural sources. Please note: Consult a qualified person (Act 36 of 1947) for specific applications / recommendations

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Save on lime costs and avoid subsoil acidity – use the right N source

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

Soil acidification is an unavoidable process that follows on from fertilization, different fertilizer products differ in their capacity to acidify the soil; substantial savings on lime costs can be made by selecting fertilizer products accordingly. Dr Erik Adriaanse explains how significant savings on lime costs may be achieved through the use of Limestone Ammonium Nitrate (LAN): Save on lime costs  

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