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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…
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…
Dung and urine are rich in plant nutrients, and clear evidence of this is the accelerated pasture growth where these products are deposited. Each animal is in a sense a “fertilizer spreader”, and farm management practices need to take this into account in order to make the best use of these nutrients and to avoid unnecessary expenditure on fertilizer and lime. Nutrients in dung and urine The feed consumed by animals – be it pasture, hay, silage, TMR or concentrates – is rich in plant nutrients. Only small proportions of these nutrients are used for meat and milk production, with the remainder being excreted in the dung and urine. In the case of dairy cows on pasture, the approximate percentages of N, P and K excreted are shown in Table 1. The total amounts of nutrients excreted by a cow in a year are considerable (Table 2), with the amounts of nitrogen and potassium being the largest. Attaching a rand value to these nutrients underlines the economic significance of the recycling process. It should, of course, be borne in mind that the monetary value reported in Table 2 does not include that of the secondary nutrients, sulphur, calcium and magnesium, or of micronutrients such as zinc, copper, manganese and boron. And there is also the value of the dung in terms of its contributions to soil health (through improving soil organic matter levels, soil structure and biological health). Why the high fertilizer requirements on livestock farms? Given that the removal of plant nutrients from the farm in milk and meat is minimal, why is there an ongoing need for such large amounts of fertilizer in typical livestock operations? In the case of nitrogen, there is a partial explanation, in that large amounts of this nutrient may be lost by leaching from the rooting zone and by volatilization to the air from urine patches. But phosphorus and potassium are not lost in these ways, and one would expect long-term fertilizer requirements for these nutrients to be low on intensive livestock farms. The principal reason for the ongoing high requirement for fertilizers is that because of the movements of animals and feeds, nutrients get depleted from certain areas of the farm and concentrated in other areas. To illustrate this, let’s take a look at nutrient flows in a typical pasture-based dairy-farming operation (which includes silage and/or hay-making). On a farm of this kind, because of the movements of animals and feeds, there are large-scale flows of nutrients from one area to another. These flows are illustrated in the diagram, and result in a concentration of nutrients in areas where animals spend significant amounts of time and are fed (shown in blue), and a depletion of nutrients in more distant areas (yellow), and in particular in those areas in which feed is grown and removed. Plant nutrients are, of course, brought into the system in the form of fertilizers, lime and purchased feeds. Similar nutrient flows occur on intensive beef and sheep operations….
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
Langebaan Rock Phosphate (Langfos) is a phosphate rock (PR) of organic sedimentary origin that was widely marketed in South Africa until 1994. The original product Langfos Premium had a total phosphorus (P) concentration of 12.6% and citric P concentration of 3%, was milled and screened so that 80% of the particles passed through a 0.149mm sieve. The remainder of larger particles were used in combination with water soluble sources of superphosphate to manufacture granular NPK mixtures. In 2010 a new mine was opened and Langfos is once again available to farmers as an additional phosphate source for consideration. The total P content now ranges from 8 -10% and citric P concentration ranges from 1.8 -3.2%. 63% of the phosphate is in the tri calcium phosphate form and has a 20% calcium content as well as traces of Sulphur (S), Magnesium (Mg), Zinc (Zn), Boron (B), Copper (Cu), Molybdenum (Mo), Iron (Fe), Manganese (Mn) and Cobalt (Co). The product has a Group 2 Fertilizer registration in accordance with Act 36 of 1947. A comparison of Langfos Premium to the Langfos currently available: Langfos Premium Langfos (Pre 1994) 2010 – present Total P: 12.6% 8-10% Water soluble P: 0 0 Citric Soluble P: 3% 1.8-3.2% Particle size: 80% < 0.149mm 23% < 0.149mm 20% > 0.149mm 67% > 0.149mm Langfos is not water soluble and should be considered to be a slow release fertilizer; approximately 50% of the P in Langfos becomes available for plant uptake in the first year of application, the balance over an extended period of time. From personal experience Langfos Premium has proven effective as a long term slow release source of P on acidic soils in Kwa Zulu Natal (KZN) and formed the basis of establishment of many Kikuyu pastures, sugarcane lands, avocado and citrus orchards where soil P test levels were found slowly increase despite limited or no application of superphosphate over periods spanning approximately 20 years. Thibaud et al. cite early researchers who found that the dissolution of PR is determined by gradients in the activities of phosphate, calcium, and hydrogen ions. The rate of PR decomposition is enhanced by soils with a low pH, high levels of reserve acidity and a low calcium status. PR was found to be more effective when applied to acid, well buffered soils than neutral soils and soils that had been recently limed. In addition to this soils with a large capacity to immobilize P from the soil solution generally promote more rapid and extensive dissolution of PR. Thibaud et al. (1992 & 1993) conducted research using a strongly P-fixing Balmoral clay and a weakly P-fixing Avalon sandy loam soil in greenhouse pot trails. They found that Langfos was not an effective substitute for superphosphate. It was however found that Langfos was more effective under strongly acidic conditions and that soils that retain P weakly may be more suited to fertilization with Langfos. The relative agronomic effectiveness (RAE) of Langfos compared to triple superphosphate however was 36% and 14%…
Urea is a cheaper source of Nitrogen than LAN, can farmers justify the economics of paying a premium for nitrates or does urease inhibitor NBPT provide an effective alternative to LAN? Dr Erik Adriaanse explains: Ammonium nitrate results in higher yield and profit over urea +NBPT or urea on its own
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
