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SOIL ACIDITY AND ITS MANAGEMENT IN CROP PRODUCTION

1. The nature of soil acidity and its diagnosis

Neil Miles, Consultant, Mount Edgecombe, South Africa

Mart Farina, Consultant, Howick, South Africa

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.

Figure 1: Increasing saturation of a clay particle with acidic cations (aluminium (Al+++), manganese (Mn++) and hydrogen (H+)) with increasing soil acidity from left to right (the typical pH values shown are for pH measured in a potassium

chloride solution).

What causes soils to acidify? Although, as noted earlier, acid soils occur widely in nature, the following human activities may markedly accelerate acidification:

pH ≈ 4.8 pH ≈ 4.0pH ≈ 4.3

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1. Acid rain, resulting from atmospheric pollution by industry. This has been shown to be a major contributory factor in some Highveld areas.

2. The use of nitrogenous fertilisers, particularly when applied in excess of immediate crop requirements. 3. The removal of basic nutrients (calcium, magnesium and potassium) in harvested crops and animal

products. 4. 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 leaf symptoms as are often apparent with deficiencies of, for example, N or K. Specific detrimental effects of soil acidity include the following: 1. Aluminium and manganese toxicities

Aluminium, and less frequently, high amounts of manganese, released from clays as the soil acidifies (Figure 1) are toxic to plant roots and result in poor and abnormal root development. Typically, roots become thick and stubby and there is little development of the all-important fine roots (Photo 1). The resultant inefficient root systems limit yields through poor water and nutrient uptake. Importantly therefore, in seasons of reduced rainfall, aluminium toxicity exacerbates the effects of drought. Where subsoils are also acidic, the situation with regard to moisture supply is aggravated further in that moisture reserves in the subsoil are largely unavailable to plants because of restricted root penetration into the lower soil layers (Photo 1). Under these conditions it is not uncommon to see plants displaying symptoms of drought stress even on relatively moist soils. Manganese toxicities occur typically on soils that are not naturally acidic, but have been acidified through long-term agricultural activities. Temperate legume crops, such as dry beans, are particularly sensitive to high levels of soluble manganese in soils (Photo 2), and reports suggest that manganese toxicity is an increasingly widespread problem in Southern Africa.

Researchers have found that the degree to which plant roots are affected by aluminium toxicity depends on the relationship between the quantity of aluminium and the quantity of bases (principally calcium and magnesium), rather than on the level of aluminium alone. The aluminium toxicity hazard is, therefore, represented on soil test reports by the ‘soil acid saturation’ index, with this being a far more reliable indication of the acidity hazard than soil pH.

Photo 1: Maize roots exhibiting severe aluminium-toxicity symptoms (left), and poor penetration of sugarcane roots into

a severely acidic subsoil (right).

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Photo 2: Dry beans exhibiting manganese-toxicity symptoms (left), and field evidence of severe manganese toxicity in dry

beans (right).

2. Deficiencies of calcium and magnesium

Levels of calcium and magnesium in acid soils are often very low, and may pose a limitation to plant growth. A major consideration in this respect is that adequate supplies of calcium in the soil are of critical importance for root growth. The combination of high aluminium and very low calcium levels in subsoils is a widespread yield-limiting factor in grain-producing areas of South Africa.

3. Micronutrient deficiencies

Deficiencies of micronutrients, in particular molybdenum and zinc, frequently limit plant growth on acid soils. In the case of molybdenum, most soils contain adequate reserves of this nutrient for plant growth, but its availability for plant uptake is minimized under acid conditions.

4. Soil biological activity

Acid soil conditions impact negatively on soil biological activity. Evidence of this is the retardation in the break-down of surface-applied organic matter (e.g. trash) on acid soils. Interestingly, most earthworm species are sensitive to soil acidity, and liming frequently results in a rapid increase in earthworm populations.

4. Species differences in tolerances to acid soil conditions

Agricultural crops do not all respond in the same way to acid soil conditions, but vary widely in their tolerances to soil acidity (Figure 2). Certain horticultural crops and temperate legumes and grasses are highly sensitive to acid soil conditions, while crops such as cowpeas and sugarcane are remarkably tolerant. Nevertheless, severe soil acidity has been shown to limit the growth of all species, including the highly tolerant ones. Thus, the management of soil acidity is an essential component of sustainable production practices, regardless of the crop grown.

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Figure 2: Relative tolerances of crop, pasture and horticultural species to soil acidity.

DIAGNOSIS OF THE PROBLEM

Soil acidity is not easily detected from symptoms in the above-ground crop, and soil analysis remains the only reliable means of identifying the problem and of determining lime and gypsum requirements. Since the incorporation of lime and gypsum is possible only before planting, soil samples should be taken well before the planting operation; this will allow sufficient time for the samples to be processed in the laboratory, and for products to be ordered and incorporated into the soil. In terms of the actual soil sampling procedures, topsoil samples should be taken with a Beater auger, while subsoil samples should be taken with a screw-in type auger, or from the faces of soil pits. In this article, space does not allow for a detailed description of sampling procedures; nevertheless, the importance of obtaining truly representative samples for laboratory analysis cannot be over-emphasised. It is worth noting that initial evidence of excessive soil acidity is frequently in the form of patches of weak crop growth in an otherwise healthy (good) stand (Photo 3). For the optimization of production, it is obviously essential that the acidity problems in the weak areas are corrected. With this in mind, soil samples should be taken from these weak patches during the early growth stages of the crop when the poor growth is clearly visible. Samples taken from the whole field after removal of the crop may be biased by the more favourable soil conditions in the “good” areas. Once the problem has been accurately diagnosed, the “poor” areas may be treated, either by liming them separately or liming the field as a whole at the rate recommended for the “poor” areas. With the aid of GPS technology, grid soil sampling of fields is a particularly powerful tool for managing variability in soil acidity and other soil fertility factors. Using computer mapping techniques, grid soil sampling data are converted into treatment maps, and variable rate applications of lime and nutrients can then be imposed. As a result, higher yields from hectares that were being under-limed or under-fertilized, coupled with reduced input costs for the hectares with adequate fertility, translate into greater profits.

carrot, cabbage, tomato, lucerne, white

clover, macadamia, banana, avocado,

litchi

sugarcane, cow peas

potato

tall fescue

maize, sorghum, buckwheat,

babala, teff, cocksfoot

lupin, velvet beans, sunn hemp, soyabean

Highly

sensitive

Highly

tolerant

sunflower, dry bean, cotton, barley

Moderately

sensitive

Moderately

tolerant

sweet potato, kikuyu, Eragrostic curvula

oats, Digitaria eriantha

perennial ryegrass

Italian ryegrass

red clover

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Photo 3: Excessive soil acidity often shows as patches of poor growth in fields. In the case of the maize field, good and poor areas had pH’s of 4.35 and 3.83, and acid saturations of 9 % and 62%, respectively.

In the second article in this series, we will consider the properties of lime and gypsum, together with practical aspects of managing both top and subsoil acidity.

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