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The Acid Soil Problem Defined

Peter Cregan

Pasture Specialist, New South Wales Department of Agriculture

The nature of the acid soil problem

The acid soil problem is associated with poor establishment and persistence of lucerne and phalaris, reduced barley and wheat yields and the thinning out of sub clover and other pasture legumes leading to a substantial decline in carrying capacity (Cregan et al. 1979).

Problem acid soils have a pH* of less than 5.6 and usually below pH 5.0. The low soil pH is associated with a number of soil chemical and biological characteristics that manifest themselves as the components of the problem acid soil syndrome. These components may adversely affect plant growth.

The following specific problems are associated with problem acid soils:

• aluminium toxicity;

• manganese toxicity;

• molybdenum deficiency;

• legume nodulation failures;

• increase in plant disease and

• calcium and magnesium deficiency.

Hydrogen ion toxicity, decreased phosphorus availability and toxicities of some other trace elements and heavy metals have also been reported.

Plants vary considerably in their tolerance to some of the components of the problem acid soil syndrome. Differences in the reaction of both plant species and varieties within a species to varying levels of aluminium and manganese have been measured under field conditions as well as in laboratory experiments. The expression of some acid soil problems therefore is not merely one of soil chemical characteristics but rather the resulb of a complex interaction between the plant and the soil.

The most common problems associated with acid soils are aluminium and/or manganese toxicities, molybdenum deficiency and legume nodulation failures. In southern New South Wales, northern Victoria and south western Western Australia fungal disease problems are also common on subterranean clover.

* Soil reaction is expressed on a scale of 1-14 known as the pH scale. 7 is neutral while below 7 is acid and above 7 is alkaline. One unit of the scale represents a 10 fold increase or decrease in acidity. Mostplants prefer to grow in the p11 range 5.5 - 6.5.

Note: All pH values refer to the 1:2 soil:water pH test unless specifically stated otherwise.

The aluminium problem

At pH 5.5 and above the concentration of aluminium in soil solution is low (Russel 1950, Coleman 1959). As pH drops, the aluminium concentration increases, especially below pH 5.0. Aluminium saturation of the cation exchange capacity (CEC) is directly related to the amount of

aluminium in the soil solution. The influence of pH on the aluminium concentration in the soil solution and its effect on lucerne yield has been demonstrated by Helyar and Anderson (1970), see figure 1.

Figure 1: Effects of soil pH on aluminium ion activity in soil solution and on lucerne growth

Manganese (Mn) toxicity

Soil manganese exists in an equilibrium between plant available 3+

manganous manganese (Mn2+) and unavailable forms of manganic manganese (Mn3+ ). Plants suffer from toxicity when they absorb too much Mn2+.

The balance between available and unavailable forms of manganese is influenced by:

The chemical nature and amount of manganese in the soil.

Aerobic biological activity in the soil, which lowers Mn2+ levels.

Conditions unfavourable to biological reduction are caused by waterlogging or hot dry periods. Anaerobic conditions resulting from waterlogging or poor drainage enhance Mn2+ availability by favouring low redox potentials and the reduction of Mn3+

The pH - low pH values favour reduction of Mn3+ to Mn2+

Environmental conditions cause a peak or pulse of Mn2+ which often lasts for about three weeks. The longer the pulse of high soil Mn2+ , the greater the chance of toxicity effects developing.

At pH 4.6 and below, toxicity can be a continuing problem. Where high soil Mn2+ is primarily caused by low pH, liming to pH 5.6 will usually reduce soil Mn2+ to non-toxic levels. However, where Mn2+ toxicity is primarily the consequence of environmental conditions liming will only reduce the peak and duration of the pulse (Siman et al., 1974).

Molybdenum (Mo) deficiency

Mo deficiency is most likely to occur when pH is less than 5.6 but can occur between pH 5.6 - 6.0.

Increase in plant disease

Strongly acid soils favour the development of fungi (Russel, 1963). Stovold (1974) found both sub clover establishment and growth were affected by ~ fungi in acid soils on the southern tablelands and slopes. The fungi are well adapted to survive in these soils and cause disease in cold, wet conditions. Pythium spp. commonly cause damping-off of seedlings and necrosis of feeder roots of established plants.

Legume nodulation

Legume nodulation and nitrogen fixation, are frequently affected by one or more of the deficiencies or toxicities commonly associated with acid soils. Even though effective nodulation has occurred, acid soils limit nitrogen fixation by reducing growth of the host plant and thus in turn limiting the growth of nodule rhizobia.

Low pH, Ca and Mo deficiencies, and Mn and Al toxicities have all been shown to affect nodulation. The higher the soil Ca concentration, the less the effect of low pH and Mn and Al toxicities.

Clovers and medics are most sensitive to acid soil conditions during infection by the inoculated rhizobia. This occurs usually at the initial stages of root growth. Later stages of plant and rhizobial development are less sensitive (Munns, 1968). Molybdenum (Mo), a trace element essential for plant growth, is frequently deficient in acid soils. Mo deficiency affects the nitrogen fixation of the nodulated plant because it is a component of nitrogenase, the enzyme central to the process of nitrogen fixation. Mo deficient plants show symptoms of nitrogen deficiency while nodules are very small and numerous.

The extent of problem acid soils

pH distribution. The majority of agricultural soils in eastern New South Wales are acid, but acidity, per se, is relatively unimportant for most commercial species prefer to grow in a slightly acid soil; i.e., pH 6.5 - 5.6. However, the degree of soil acidity is important, for, as mentioned previously, acid soil problems are likely to arise once pH is below 5.6 and become more common when pH is below 5.0.

Some indication of the area and distribution of problem acid soils can be gained from Hudson and Hawkins’(1979) survey of the results of District Agronomists’ soil tests. Part of that survey is summarised in the following figure.

Figure 2: A general indication of the occurrence of acid and highly acid soil in New South Wales

Aluminium toxicity problems

The importance of aluminium toxicity over large tracts of agricultural land can be more clearly defined by considering:

(a) the distribution of exchangeable aluminium levels from District Agronomists’ soil test results; and

(b) the tolerance of commercial temperate crop and pasture species to different levels of exchangeable aluminium. The data for seven selected districts is contained in the following two tables:

Table 1: Distribution of percent exchangeable aluminium for seven* selected districts (Hudson and Hawkins, 1979)



Median pH













Table 2: Critical levels and sensitivity of temperate plants to aluminium



Exchangeable al level above which yields are reduced (% al saturation of cec)




5 Highly sensitive





Oilseed rape


Red clover

10 Sensitive

Clare sub clover




Sub clover


Trikkala sub clover


Woolly pod vetch

20 Moderately tolerant

Rose clover


White clover




Fodder rape


Some oats


Tall fescue





30 Highly tolerant



Cereal rye


Within species, varieties can vary significantly in their re action to Al toxicity

* The samples tested from each district are from District Agronomists’ samples and thus are not a true random sample.

Potential problem acid soils

Some soils are naturally acid while others have acidified to become problem acid soils as a consequence of agricultural practice. When reviewing the extent of problem acid soils it is therefore necessary to consider those soils that are not now problem acid soils, but which have a potential to be so.

The process of soil acidification is influenced by many factors: the most important of these are -initial PH;

cation exchange capacity of the soil; the rate of organic matter accumulation; the presence of nitrate nitrogen and environmental conditions.

Consideration of these influences and soil test trend data indicate that soils are likely to acidify rapidly if they are initially acid, have a low CEC (< 5 me %) and support a nitrogen-producing legume pasture under conditions of high to moderate rainfall (> 450 mm p.a.).

The area of subterranean clover pasture in southern Australia on light-textured soils in which the pH may, in time, decrease to 5.5 or less is estimated as 1.0 x 10 ha in New South Wales, o.5 x 10 ha in Victoria, 1.0 x 10 ha in South Australia and 4.5 x 10 in Western Australia.

In addition, subterranean clover leys are an important part of cereal growing in southern Australia and similar, although slower, pH trends are to be expected in the lighter-textured soils where this form of farming is practised (Bromfield, 1978).

Acid soil problem definition

To enable you to establish whether an acid soil problem exists and then to determine the nature of the problem(s) a well-tested and calibrated soil testing service is essential. A soil pH test should be used first. However, as a p~ test cannot he diagnostic they should only be used as a filter - a first step, to tell you whether a potential for acid soil related problems exist.

Once the pH test tells you that soil pH is low enough for acid soil problems to be expressed, you then need to have more detailed soil and/or plant analysis undertaken. These will help diagnose the specific manifestation of the acid soil problem. This approach is outlined diagrammatically in figure 3.

(See figure 3 overleaf)

Figure 3: Acid soil problem definition (Cregan, 1980)


1. Bromfield, S.M. (1978) Soil acidity, microbes and manganese availability. CSIRO Plant Industry Annual Report: 36.

2. Coleman, N.T., Weed, S.B. and McCracken, R.J. (1959) Cation-exchange capacity and exchangeable cations in Piedmont soils of North Carolina. Soil Science Society American Proceedings 23; 146 - 149.

3. Cregan, P.D., Sykes, J.A. and Dymock, A.J. (1979) Pasture improvement and soil acidification. Agricultural Gazette of N.S.W. 90 (5): 33.

4. Cregan, P.D. (1980) Soil acidity and associated problems - guidelines for farmer recommendations, AGbulletin No. 7, New South Wales Department of Agriculture, Sydney.

5. Helyar, K.R. and Anderson, A.J. (1970) Some effects of the soil p11 on different species and on the soil solution for a soil high in exchangeable aluminium. Eleventh International Grasslands Conference, Surfers Paradise Australia: 431.

6. Hudson, A.W. and Hawkins, C.A. (1979) Soil acidity in New South Wales: The extent and severity of the problem. Workshop on Acid Soils, Sydney: p. 7.

7. Munns, D.N. (1968) Nodulation of Medicago sativa in solution culture; I, Acid sensitive steps. Plant and Soil 28:129.

8. Stovold, G.E. (1974) Root rot caused by Pythium irregulare Buisman, an important factor in the decline of established subterranean clover pastures. Australian Journal of Agricultural Research 25:537.

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