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P.D. Cregana and K.R. Helyarb

aSchool of Agriculture, Riverina-Murray Institute of Higher Education
Wagga Wagga NSW 2650

bAgricultural Research Institute, NSW Department of Agriculture
Wagga Wagga. NSW 2650


The rate of soil acidification, characteristic of the various farming systems of NSW, varies from very low rates to rates equivalent to 150 to 250 kg CaCO3/ha/year (Southern Tablelands and South West Slopes pasture and crop! pasture systems), and to very high rates (equivalent to 300 to 600 kg CaCO3/ha/year) in some nitrogen-fertilised intensive cropping, horticultural, and pasture systems.

The mechanisms of soil acidification vary between production systems. The major sources of acids are those produced in the biological carbon and nitrogen cycles. Because of the nature of cyclical processes, net acidification only occurs when carbon or nitrogen leave the soil/plant system in a different form than that in which they entered the system, and then only when an acid was produced in the interim (e.g. the formation of nitric acid during nitrification of organic matter, or an organic acid during plant growth, or the humification of soil organic matter).

On the basis of this understanding of acidification processes, modifications to existing farming systems are proposed. The potential for reducing the rate of acidification centres around improving the efficiency of nitrogen and water use to minimise acidification resulting from the nitrogen cycle. Acidification resulting from carbon cycle acids can be minimised by minimising unnecessary waste product removal and the accumulation of excessive levels of organic matter.


Soil acidification occurs naturally but this is a very slow process - on a geological time scale. However, soil acidification has been rapidly accelerated by many of the farming practices that are common in southern Australia. Frequently the acidification is sufficiently rapid to cause acid soil induced fertility problems in as little as 20-50 years. As a result, species susceptible to soil acidity and once able to grow well on certain soils, now produce very poorly and have often been replaced by more tolerant species. Good examples are the failure of lucerne and barley and their replacement by acid tolerant wheat varieties and subterranean clover. It is important to remember, however, that the use of tolerant species has not stopped the continuing acidification process - it has given us a breathing space’ and hopefully sorie cash flow to take corrective measures. If no corrective action is taken the capacity to produce even the tolerant species will be lost.

In this paper the acidification rates typical of the present farming systems in NSW are briefly discussed. These acidification rates can be used to estimate the long term annual lime requirements of these systems. Secondly, management practices designed to reduce the long term acidification rates and lime requirements are outlined. Such practices are important because the profitability of lime use is more sensitive to changes in the soil acidification rate than to other factors such as the shape of the lime response curve, the soil pH buffer capacity, or the initial soil acidity level (Hochman et al., in press).

TABLE 8.1 Rates of acidification and soil pH change typical of various NSW farming systems (based on Cregan and Helyar, unpublished data)


Lime Required to balance acid added (kg/ha/yr

Estimate of years for soil PhCA* to fall from 5.5 To 4.2 (Problem Level In Top 30 cm
Soil pH buffer capacity (k moles H +/pH unit/10 cm soil)


100 (high)

(usually clay loam,

4% organic matter)

50 (medium)

(usually loam,

2% organic matter)

20 (low)

(usually sand,

0.5% organic matter)

Wet winter/dry summer;
annual fertilised legume-based
pastures, and pasture/crop areas


7-900 mm rain 150-250





5-700 mm rain 100-200





Slightly summer dominant rainfall,
significant dry periods; phosphate fertilised clover/perennial grass pastures


750- 1200 mm rain





Summer dominant rainfall


infrequent dry periods


>1200 mm





>1200 mm rain + 300 kg



50 - 300

50 - 300

25 - 150

10 - 60

Acidification Rates In Present Farming Systems

The rates of acid addition to some of the major farming systems in NSW have been estimated by Cregan and Helyar (unpublished data). In Table 8.1 these acid addition rates have been converted to the amount of lime required to balance the acid addition. Estimates of the number of years before acid soil problems are likely to be observed can be made using these values, knowledge of the soil pH/buffer capacity, and the depth of acidification of the soil. Some examples of estimates for particular soil types are included in the table. These data clearly establish that accelerated acidification is occurring in a wide range of environments and unless soil acidity is ameliorated or the acidification rate is retarded, acid soil problems will become even more common in the near future.

Causes of Soil Acidification

The processes responsible for accelerated acidification have been comprehensively reviewed by Helyar (1976) and Porter (1982).

In southern Australia the most important causes of accelerated acidification are:

(a) the nitrification of soil organic nitrogen compounds and their subsequent leaching from the root zone;

(b) the removal of products and waste products (e.g. manure), high in residual alkalinity;

(c) the use of acidifying nitrogen and elemental sulphur fertilisers;

(d) the use of legumes that fix nitrogen leading to increased nitrate leaching with resultant acidification; and

(e) increases in soil organic matter levels if derived from organic material grown on the soil.

Most of these processes result in acids being produced either within the plant (plant organic acids) or in the top soil (humification and nitrification of soil organic matter - both acid processes, and the acid reactions of some nitrogen and sulphur fertilisers). Since these reactions are in the top soil or in the living plant, then it is tempting to suggest that most acidification should occur in the surface soil with which it is practical to mix lime to neutralise the acidity. However, some of our important farming systems are characterised by plants that tend to absorb more cations than anions from subsoil layers. Because of this, the roots excrete into the subsoil some of the acid derived from organic acids produced within the plant. Subterranean clover and possibly lupins are typical examples. Thus in many of our soils, the whole root zone is acidifying at a similar rate. Some typical examples of acidified soil profiles are shown in Figure 8.1. Management systems aimed at avoiding subsoil acidification are therefore most important. Otherwise subsoil acidification, very difficult and usually uneconomical to correct using lime, will reduce the productive potential of large areas of land.

The Potential For Slowing Acidification Rates And Avoiding Subsoil Acidification

Reduction of the acidification rate is dependant on the manipulation of those factors that lead to acidification. Comparisons between farming systems that are acidifying rapidly and those that show no, or little, acidification, together with estimates of acidification based on knowledge of the main acidification processes (Porter and Helyar, in press), indicate that the greatest scope for retarding acidification without unduly disrupting farming

Soil profile (cm)

Figure 8.1a Profile pH of a virgin and acidified soil on the Southern Tablelands (Bromfield et al., 1983) contrasted with a naturally highly acid soil from the Pilliga (Doyle and Bradley, 1982)

Soil profile (cm)

Figure Profile pH of a red-brown earth at Wagga under clover ley/cropping rotations (Furniss et al., unpublished)

practices and profitability is through more efficient use of soil nitrogen and water, and careful fertiliser selection. The attractive aspect of this point is that water and nitrogen are frequently the factors most limiting to plant growth in Australian agriculture. Therefore, their efficient utilisation should lead to both lower acidification rates and lime requirements, and to higher plant yields..

(a) The role of nitrogen in acidification and evidence for nitrate leaching

(i) The role of nitrogen in acidification

The fate of N influences acidification (Helyar, 1976). Practices that increase plant use of nitrate per unit N addition and minimise leaching,

Figure 8.2 Acid and alkaline processes within the nitrogen cycle. Estimates ranges of magnitude of the various N pools are within the brackets.

will reduce acidification. In general, most of the nitrogen in an agricultural soil/plant system is in the form of soil organic nitrogen (N). This is illustrated in Figure&2 where the sizes of the nitrogen pools in the system are illustrated by the diameter of the circles representing the different forms of N. In a given year a small proportion of the organic N is mineralised. That means it is converted to ammonium, then rapidly (within a day or two) to nitrate. As illustrated in Figure 8.2 this process contributes one unit of acid to the soil for each unit of nitrogen mineralised. Figure 8.2 can be used in a similar way to show why ammonium fertilisers are more acidifying than urea, biologically fixed nitrogen, aqua ammonia, anhydrous ammonia and ammonium nitrate, while nitrate fertilisers usually decrease soil acidity. Follow the acid and alkaline processes from where these forms of nitrogen enter the cycle to where they end up (in soil organic matter), to convince yourself of the ranking given above.

The fate of the nitrate formed during organic matter mineralisation is critically important to the amount of acidification that occurs. If the nitrate is absorbed by the plant or denitrified, the acid produced during nitrification is neutralised. However, if the nitrate is lost by leaching or runoff, the acid contributed by the nitrification process is not neutralised - the soil is acidified. Ideally the demands of the plant N sink and the nitrogen supply should be matched to eliminate the leaching of nitrate and associated cations from the root zone.

(ii) Evidence for nitrate leaching

A limited number of reliable measurements of nitrate losses from our soil-plant systems have been made. We have used some direct nitrate measurements and some indirect data to convince ourselves that nitrate leaching occurs. Examples of measurements of soil nitrate that indicate leaching losses are provided by Simpson (1962) for clover-annual grass pastures (note loss of soil nitrate with heavy summer rains, Figure 8.3; and Osborne and Taylor (unpublished) for measurements indicating nitrate leaching under lupins

Figure 8.3 Soil nitrate levels in the surface soil of an annual grass-clover pasture (adapted from Simpson, 1982)


Figure 8.4 The relationship between wheat yield in a fallow! wheat/barley rotation and rainfall before and just after sowing on a red brown earth at Adelaide (adapted from Piper and de Vries, 1964).

but not under wheat in a wheat/lupin rotation experiment. Indirect evidence comes from the need to involve nitrate leaching to explain soil acidification rates under pastures and crops in southern NSW (Porter and Helyar, in press); and from the low yield of wheat recorded in a fallow-crop rotation experiment on a red brown earth soil near Adelaide, in years when heavy rain fell on the fallow and within one month of sowing. In the latter case, Piper and de Vries (1964) attributed the reduced yields in the high-early rainfall years to the leaching of nitrate mineralised during the fallow period (Figure8.4).

(b) Management to minimise nitrate leaching

In a wide range of environments in southern Australia with moist winters and dry summers, soil nitrate levels in the surface soil increase over summer due to net mineralisation, and are low during plant growth due to leaching and absorption by plant roots (e.g. Figure 8.3). Management to minimise leaching losses involves minimising summer nitrate accumulation, and minimising leaching losses during the critical early wet season period when nitrate can be leached beyond the root zone.

(i) Minimising nitrate accumulation in the dry season

The soil incorporation of low N crop residues (e.g. cereal straw), may have potential for reducing the amount of N mineralised during fallowing. The soil micro organisms absorb any soil nitrate while attempting to maintain a constant C:N ratio during stubble breakdown (Russell, 1961). This N may subsequently become available at a time when plant growth can use it, and the lowered available N level at planting may still be sufficient for seedling requirements.

There is evidence that lower rates of plough layer soil acidification have occurred with stubble incorporation (Taylor, unpublished). In a wheat/lupin rotation experiment, stubble retention and direct drilling was compared with early stubble incorporation throughout the plough layer. The decreased soil acidification in the incorporation treatment (Table8.2) indicates less nitrate leaching due probably to lower fallow nitrate accumulation level s.

TABLE 8.2 Changes in soil pH over 6years under a continuous wheat/lupin rotation, as affected by the degree of stubble incorporation (Taylor, 1986 - unpublished data for a red earth soil at Wagga Wagga)


Change in soil pH over 6 years




Direct drill, stubble retention



Standard cultivated seedbed,


early stubble incorporation



A second technique that may be useful in reducing dry season nitrate accumulation is to use summer active perennial species to absorb the nitrate as it is formed. This is difficult in cropping areas with short rotations, but is probably practical in much of southern Australia wherever pasture periods exceed about four years. The use of lucerne, cocksfoot, and phalaris in higher rainfall areas, and possibly species such as lovegrass and native grasses in lower rainfall areas, are possibilities. The more widespread use of perennial grasses on the Northern compared with the Southern Tablelands of NSW is probably a major reason for the lower acidification rates in the former region (Table 8.1).

(ii) Minimising nitrate leaching early in the wet season

As illustrated in Figure 8.5 the danger period for nitrate leaching is early in the wet season. If heavy early rains leach the soil nitrate (accumulated during the dry season) faster than the roots of the establishing plants penetrate the soil, then nitrate is lost. Several principles can be stated which should favour plant uptake of nitrate over leaching:

- Perennial grass and grass/clover pastures are more effective year-round nitrogen sinks than pure legume pastures or annual pastures, and should minimise the potential for N03-N leaching (Hood, 1976 and Kilmer, 1974). The gramineous plants may also raise the rhizosphere pH through excess anion absorption (Israel and Jackson, 1978).

- Deep rooted annuals (e.g. wheat compared with subterranean clover) have a greater ability to recover nitrogen leached from the topsoil early in the season. More vigorous development of deep roots could explain the apparently lower nitrogen leaching rate under wheat compared with lupins (Osborne and Taylor, unpublished).

- Earlier sowing of crops following the opening rains should lead to more complete absorption of soil nitrate before it is leached below the root zone. For crops and pastures grown under cycles of wetting and drying, minimising N03-N loss can be equated with maximum utilisation of moisture, as N03-N moves through the profile with the wetting front (Olsen et al., 1964).

- Improved crop or pasture growth that enables the plant to be a larger N sink should also result in minimising N leaching. Many factors might influence this, from nutrition and soil physical condition to the choice of varieties and species.

Figure 8.5 Diagrammatic representation of the likely influences of rain, time and pasture type on nitrate use.

- All these methods of encouraging plant uptake of nitrate are likely to be less effective, where the early season rains are particularly heavy (e.g. Figure 8.4).

- The use of summer active deep rooted grasses and early sowing to maximise plant use of soil water, reduces leaching by maximising the proportion of water that passes through the plant, compared with the proportion percolating below the root zone.

- Use limited nitrogen fertiliser levels at sowing coupled with side dressings where possible and where application costs are not excessive. That is, tailor the nitrogen fertiliser regime to match the plant absorption capacity, avoiding high soil N levels when there is a risk of high leaching rates.

The acidifying potential of different nitrogen fertiliser forms

The form of the N fertiliser strongly influences acidification rates (Tisdale and Nelson, 1975). Given the assumptions outlined in Table 8.3 the acidifying potential of the various nitrogen fertiliser forms is shown for the extreme situations of zero or 100% of the applied fertiliser being leached as nitrate. Note the mandatory acidification caused by the ammonium forms even without leaching, and the increase in acidification with leaching for all forms other than pure nitrate fertiliser. The negative value for nitrate fertilisers, where no leaching occurs, results from the alkaline incorporation of this nitrate into the organic form or from alkaline denitrification (Figure 8.2).

Therefore, the choice of the nitrogen form influences the way soils acidify. Pure ammonium forms are associated with an acidification cost and pure nitrate forms can be used as a means of decreasing soil acidity. The choice of the appropriate fertiliser for use in a given situation should take these costs and benefits into account as well as the fertiliser cost itself.

TABLE 8.3 Soil acidification rates expected from various forms of nitrogen fertilisers

Fertiliser and Acidification Class

CaCO3 (lime) to balance acidification where leaching is: (kg lime/kg N)



100% of N applied

(A) Most acidifying - ammonium fertilisers:


Sulphate of ammonia (ammonium sulphate) MAP (monoammonium phosphate)



(B) Medium acidification:


DAP (diammonium phosphate)



(C) Low acidification:


Ammonium nitrate
Aqua ammonia
Anhydrous ammonia
Biologically fixed (legume) N



(D) Alkaline fertilisers:


Sodium and calcium nitrate



* Assumptions:

(i) At any time most ecosystem N is in the NH3 or protein forms as organic matter and plant material.

(ii) The phosphate in DAP converts to the monovalent ion after application (i.e. this assumes the soil pH is less than 7.0).

(d) The role of the carbon cycle in acidification and management to minimise acidification from this source

(i) The carbon cycle and acidification

The acid and alkaline processes in the carbon cycle are illustrated in Figure 8.6.Acid is produced when organic anions are formed from neutral carbon compounds in both the soil and the plant. The reverse alkaline process occurs when the organic anions are oxidised to carbon dioxide and water (Figure 8.6). Thus, acidification from carbon acids results from the accumulation of organic anions as soil organic matter, and from the export. of organic anions in products and waste products.

Figure 8.6 Acid and alkaline processes in the carbon cycle

As in the section on causes of acidification, the acids produced by these processes may be deposited in the surface soil or the subsoil. Most organic acids produced during humification are formed in the surface soil. Acid may also be excreted by roots in this soil layer when cation absorption exceeds anion absorption (e.g. during periods of low nitrate levels in the surface soil).

Acid is excreted by roots into the subsoil whenever cation absorption exceeds anion absorption in this layer. When the plant excretes acid it retains the residually alkaline organic anion. This is likely whenever nitrate levels in the subsoil are low. However, root excretion of alkali can occur when subsoil nitrate levels are high. Thus, it is important to have actively absorbing roots in the soil layers below the surface whenever nitrate movement downward can be expected. These roots are critical both to minimising nitrate leaching and to maintenance of the subsoil pH. This is an important interaction between the carbon and nitrogen cycle acidification processes. In practice probably the most economical way to maintain or increase subsoil pH values is to design management systems that utilise the plant root capacity to exchange alkaline compounds for nitrate in the subsoil.

(ii) Management of carbon cycle acidification

In addition to managing the soil layer in which the acids produced in the carbon cycle are deposited (i.e. the carbon/nitrogen interaction discussed above), the following options are also available to the farm manager to manipulate soil acidification rates:

- Removal of organic anions in products (an acidifying process) varies greatly with the product (Table 8.4). Thus removal of one tonne of good quality lucerne hay is about twenty times more acidifying than cereal grain removal. Clearly different forms of product removal change soil acidification rates, so choice of the form of production should include consideration of soil acidification effects.

Removal of animal products, except for milk, will usually not result in significant lime requirements.

-In grazed pastures sheep and cattle frequently tend to ‘camp’ in certain areas of the paddock (Hilder, 1966) and this behaviour is more marked in hilly than flat paddocks (Cornforth and Sinclair, 1982). Where strong ‘camping behaviour’ occurs this represents net transport of organic anions in the manure from most of the paddock to the camp site. This will lead to increased pH at the camp site and lower pH elsewhere.

Management possibilities include fencing to reduce camping tendencies, and reducing the lime application rates on the camp compared with the remaining area.

TABLE 8.4 Lime required to balance the acidity resulting from removal of one tonne of various products from a soil-plant ecosystem (values based on plant ash alkalinity levels collated by Porter and Helyar, in press)


Lime required to balance
acidity (kg CaCO3/tonne product)

Lucerne and clover hay

55 - 65

Grass hay


Cereal hay


Cereal grain


Increased levels of soil organic matter represent increased acidification if there is a net increase in the organic matter cation exchange capacity. This is usually the case. Thus, restricting the build up of soil organic matter using, for example, pasture/crop rotations rather than permanent pastures, can reduce acidification resulting from this source. However, for there to be net benefit, it is important to utilise the nitrate released during the organic matter breakdown phase. If nitrate leaching is allowed to occur (e.g. in a fallow crop system such as illustrated in Figure 8.4), then all that may occur is that the acidification resulting from nitrate leaching may be balanced by the alkaline oxidation of soil organic matter. The fact that Reeves and Ellington (1985) found soil pH did not change with time in an organic matter depleting continuous fallow/ wheat system, probably reflected this balance.

The feeding out of hay in a particular paddock, or the common dairy practice of using a night paddock, often results in substantial accumulation of alkalinity and balancing acidification in donor paddocks. Feeding out in hay cut paddocks, and rotating night or holding paddocks, may minimise such trends.


There are three approaches available to combat the effects of acidification:

(a) the use of lime;

(b) the use of acid tolerant species;

(c) the adoption of more efficient and less acidifying agricultural practices.

These methods are not mutually exclusive. In practice hest results will he achieved by using all approaches to the extent that is possible given the agricultural system and the economic constraints within which a farmer operates.

In this paper we have highlighted the many options that are available to the farmer to manipulate soil acidification rates. Some are likely to be very cost effective (e.g. encouraging more efficient and complete water and nitrogen utilisation), while others have balancing costs (eg. substitution of ammonium by more expensive nitrate fertilisers). However, consideration of the principles and suggestions outlined in this paper should assist farm managers to manage soil acidification in an economically efficient way.


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2. Cornforth, I.S. and Sinclair, A.G. (1982). A model for calculating maintenance phosphate requirements for grazed pastures. New Zealand Journal of Experimental Agriculture 10:53-61.

3. Doyle, A.D. and Bradley, J. (1982). Lime for cereals on acid soils in northern New South Wales. Proceedings of the Second Australian Agronomy Conference:259.

4. Helyar, K.R. (1976). Nitrogen cycling and soil acidification. Journal of the Australian Institute of Agricultural Science 42:217-221.

5. Hilder, E.J. (1966). Distribution of excreta by sheep at pasture. Proceedings Xth International Grasslands Congress Helsinki:977-98l.

6. Hochman, Z., Scott, B.J. and Godyn, D.L. (in press). “Lime-It”. A Computer Based Framework for Integration of Data on Lime Use in Long Term Subterranean Clover Pastures. In “Soil Acidity and Plant Growth”, A.D. Robson, J.S. Yeates and W.M. Porter. Academic Press, Melbourne.

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8. Israel, D.W. and Jackson, W.A. (1978). The Influence of Nitrogen Nutrition on Ion Uptake and Translocati on by Leguminous Plants. In “Mineral Nutrition of Legumes in Tropical and Sub Tropical Soils”. Ed. C.S. Andrews and E.J. Komprath, pp.113-129. CSIRO Melbourne.

9. Kilmer, V.J. (1974). Nutritional Losses from Grassland through Leaching and Runoff. In “Forage Fertilization”. Ed. D.A. May, pp.34l-362. American Society of Agronomy, Madison, Wisconsin, USA

10. Olsen, R.A., Frank, K.D. and Dreier, A.F. (1964). Controlling losses of fertilizer nitrogen from soils. Proceedings 8th International Congress of Soil Science, Bucharest, Rumania:1023.

11. Piper, C.S. and de Vries (1964). The residual value of superphosphate on a red-brown earth in South Australia. Australian Journal of Agricultural Research 15:234-272.

12. Porter, W.M. (1981). Soil acidification - the cause. Proceedings Riverina Outlook Conference:3l-46.

13. Porter, W.M. and Helyar, K.R. (in press). Acidification of Soils. In “Soil Acidity and Plant Growth”. A.D. Robson, J.S. Yeates and W.M. Porter Eds. Academic Press, Melbourne.

14. Reeves, T.G. and Ellington, A. (1985). Soil acidification in N.E. Victoria. Proceedings of the 3rd Australian Agronomy Conference, Hobart:223.

15. Russell, E. (1961). “Soil Conditions and Plant Growth”. Longmans, London.

16. Simpson, J.R. (1962). Mineral nitrogen fluctuations in soils under improved pastures in southern New South Wales. Australian Journal of Agricultural Research 13: 1059-1072.

17. Tisdale, S.L. and Nelson, W.L. (1975). “Soil Fertility and Fertilisers (3rd edition). Macmillan, New York.

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