Soil acidity is not caused only by the influence of man. The fact that a large proportion of Australian soils were acid, and some extremely acid, before man imposed agriculture on them is evidence of the acidification which occurs naturally. Natural acidification is a slow process. The reason so many Australian soils are more acid than soils of similar climatic regions in the world, for example Europe, is that our soils are much older and have been exposed to the natural acidifying processes for much longer. The effect of agriculture is to ‘speed up’ the processes of acidification which occur in nature.

Hydrogen ions, acids and bases

The key to soil acidification in nature or as a result of agriculture is the hydrogen ion. Hydrogen (chemical symbol H) is the simplest chemical element. An atom of hydrogen consists simply of a proton and an electron.

If you remove the electron from the H atom you get a hydrogen ion (denoted H+, the plus sign to show that there is a positive charge on the hydrogen ion)*

* Although the hydrogen ion is so important to soil acidification its availability to the plant is usually of little concern. This is because the secondary effects which result from high availability of hydrogen ions (e.g. high availability of aluminium) have more effect on plant growth than the hydrogen ions themselves.

The fact that in solution H⁺ associates with water to form H₃0 (hydronium ion) is ignored in this paper.

Substances which release hydrogen ions are called acids. Hydrochloric acid is a simple acid. It consists of a hydrogen atom and a chlorine atom which, when dissolved in water, readily break apart to give hydrogen ions (HCl à H⁺ + Cl). Substances which tie up hydrogen ions are called bases. The base we are most familiar with is the carbonate ion (C0₃^2- the “2-” shows there is a double negative charge on the ion). This is the base in lime (CaCO₃) and dolomite (CaCO₃ +MgCO₃ ). If you add C0₃^2-to soil, then it will initially tie up hydrogen ions form HCO₃^- (bicarbonate ion):

The bicarbonate ion can also be a base and can tie up another hydrogen ion, eventually to form carbon dioxide and water:

To do complete justice to a discussion of acids and bases would take more time and space than is available here. I have presented the features of acids and bases that are required for an understanding of the causes of soil acidification. In this paper I will use the weight of hydrochloric acid as my measure of added acidity. One t hydrochloric: acid per ha 10 cm is equivalent to 2 meq/100 g soil (bulk density = 1.37) and requires

1.37 t pure CaCO₃ to neutralize it.

If you add various amounts of an acid to a soil the pH will decrease. Figure 1 shows the results we obtained when we added acid to two soils. Soil A originally had a lower pH than soil B. Upon addition of acid the pH of soil A changed only slowly, but that of soil B decreased rapidly. If pH 4.2 had been the pH below which plant growth was affected by soil acidity, then soil A could have had five times as much acidity added (2.5 t hydrochloric acid/ha) as soil B (0.5 t hydrochloric acid/ha) before the soil could be said to be too acid.

Figure 1: pH changes with acid additions for two soils

This paper is concerned with long term changes in the acidity of soils. When discussing acidification it is essential to distinguish between acidification which occurs during a growing season, having no long term effect on the acidity of the soil, and acidification which affects the soil in the long term. Fluctuation in soil acidity during the course of a year is a very important factor influencing the growth of crops and pastures. For example, aluminium toxicity probably affects some plant species less because those species cause the soil adjacent to their roots to become less acid, and so reduce the concentration of aluminium to which their roots are exposed. Plants growing in manganese deficient soils can increase the availability of manganese by making the soil near their roots more acid, especially if fertilizer nitrogen is applied in the form of ammonium.

The hydrogen ions which acidify soils come from a variety of sources. In the remainder of this paper I will discuss the three sources which may account for much of the acidification which has been reported in Australia. Figure 2 is a diagramatic representation of these three sources. In this scheme, applying fertilizer is used as the starting point for increased acidity. The first source of acidity to be discussed is the increase in soil organic matter which can result from fertilizer application (à). The second is the increased rate of addition and removal of nitrogen (----), and the third source of acidity is the removal of produce (....)

Figure 2: Three of the ways agriculture may increase the acidity of a soil

Increasing the organic matter content of the soil can acidify soil

Soil organic matter contains acidic groups

These groups are weak acids which do not behave in as straight forward a manner as strong acids. Whereas strong acids such as hydrochloric or sulphuric acid release all their hydrogen ions when added to a soil, weak acids release only a proportion of their hydrogen ions. Also the proportion of hydrogen ions they release varies according to how acid the soil is - the more acid the soil the fewer hydrogen ions a weak acid releases. So if you add organic matter to a soil which is already very acid then the presence of acidic groups on the organic matter will not cause the soil to become more acid. *

The work of Dr. Williams in the Crookwell district (Donald and Williams 1954, Williams and Donald 1957, Williams 1980) is the classic study which shows how increasing the amount of organic matter in a soil which is not already very acid, can acidify that soil. In two surveys, one 30 years ago and one recently, Dr. Williams collected many soil samples from two adjoining properties in the Crookwell area.

The soils he sampled ranged from undeveloped virgin land to some which had been developed with subterranean clover up to 50 years before and had received superphosphate since that time. Dr. Williams found that the longer a soil had been developed the more acid it was (the lower the pH -Figure 3, and the greater the total acidity - Figure 4). He also found that the acidifying effect was not only confined to the surface soil, but extended into the subsoil (Figure 5).

One result of 50 years of superphosphate and subterranean clover was that an extra 44 t/ha of organic matter had accumulated in the surface 10 cm of the soil compared to undeveloped soil. From Dr. William’s data it can be calculated that for these soils, adding 44 t of organic matter was equivalent to adding 3.7 t hydrochloric acid/ha. This amount of acid is more than enough to account for all the. acidification which had occurred in the top soil, which required the equivalent of about 2.2 t hydrochloric acid/ha, and at least some of the acidification which occurred in the subsoil.

* Here the total acidity in a soil (amount of acidity which requires lime to neutralize it) would be increased by increasing the organic matter content but the availability of that acidity, and so presumably the effect of that acidity on plant growth, would not increase with increased organic matter. Throughout this paper by soil acidity I mean the activity of soil acidity (of which soil pH is an indicator) and not the total amount of acidic material in the soil (which varies with pH and buffer capacity).

Figure 3: The relation between age of subterranean clover pasture and pH (1:5 water) of the surface 10 cm of soil (from Willimas 1980).

Figure 4: The relation between age of subterranean clover pasture and the titratable acidity at pH 7.0 in the surface 10 cm of soil - linear regression. Titratable acidity = 0.82t + 1.34 (from Williams 1980).

Figure 5: The trends of pH (1:5 water) with depth in yellow podzolic soils 50 in apart. The pasture soil had been under continuous subterranean clover pasture for 32 years. (from Williams, 1980).

In the Crookwell area, organic matter, and so acidity, had increased because superphosphate applications increased pasture growth by overcoming a phosphorus deficiency. This has often led many people to suggest that wherever you apply superphosphate you will get acidification. But it is an indirect effect and, if the Crookwell area had not suffered from phosphorus deficiency but, say, severe copper deficiency, then applying copper would have caused an Increase in the acidity by increasing the amount of organic matter in the soil.

Addition of organic acids from plant material was probably a major cause of the acidification found by Dr. Williams in the Crookwell area. Unfortunately, other researchers have not supplied enough information to draw conclusions about what caused acidification in other areas. In particular, the total acidity or buffer capacities of the soils have not been given.

Adding and accumulating, or removing nitrogen can acidify soil

Nitrogen can be added to a soil as fertilizer or compost or from the atmosphere by nitrogen fixing bacteria within a legume nodule converting it into a form available for plants to use. Nitrogen can be lost from a soil by being removed in produce (usually in protein of animals, grain or hay) by being leached out of the soil as nitrate or by volatilising into the atmosphere as gaseous nitrogen or ammonia (Figure 6). If nitrogen accumulates it usually does so in the organic matter in the soil.

Figure 6: Nitrogen Movements in an Agricultural System

Soil can become more acid as a result of nitrogen movements into and out of the soil, depending upon what form the nitrogen is added and in what form it is removed, or accumulates.

Dr. Helyar published an article in 1976 in which he reviewed the nitrogen cycle in terms of soil acidification. I have adapted the results of this review to make up Table 1. For details of the reactions involved I suggest referring to Dr. Helyar's review where they are summarized in a very clear diagram and table.

From Table 1 it can be seen that adding nitrogen as ammonium fertilizer will cause most acidity. The best result you could hope for if you added ammonium is that no acidity occurs. For this to happen the ammonium would have to accumulate in the soil or leach from the soil unchanged. However because of the chemistry of ammonium these events rarely occur. If the ammonium you add is converted to nitrate and leached out of the soil, then a very rapid rate of acidification occurs. If there is no leaching, but the ammonium you apply is taken up by plants, converted to protein and subsequently removed, say in hay or grain, then an intermediate rate of acidification occurs.

The actual acidifying effect of fertilizers in the field has been studied in Western Australia (Mason, 1980). Various types of nitrogen fertilizer were added at 76 kg nitrogen/ha each year for 12 years of continuous cropping trials at two sites in the South West of Western Australia. The maximum potential amount of acidity can be calculated for each fertilizer from Table 1. The ammonium sulphate fertilizer had the potential to add acidity equivalent to 3.6 t hydrochloric acid/ha, the urea. 1.8 t/ha and the calcium ammonium nitrate 0.9 t/ha (Table 2).

Table 1: Soil acidification from nitrogen addition, removal and accumulation Acidity added (+) or removed (-) from a soil (effective kg HCl added for each 100 kg N).

Source of nitrogen	Fate of Nitrogen
	Leaches from soil as nitrate	Lost to atmosphere as nitrogen, nitrous oxide or ammonia gas.	Accumulation in soil as ammonium ion.
			- Leaches from soil as ammonium ion.
		- Removed as protein in produce
		- Accumulates as protein in soil organic matter.	(Neither of these processes occurs to a great extent).
Added as ammonium fertilizer	+520	+260	0
- Fixed from atmosphere by legumes.
- Added as protein in compost, blood and bone etc.
- Added as urea (in fertilizer or urine), liquid ammonia or ammonia gas.	+250	0	-250
- Added as nitrate fertilizer		-260	-520

From the pH changes that occurred and from the buffer capacities of the soils the increased acidity which actually resulted can be calculated (Table 2). A variable proportion, ranging from none to a half, of the maximum amount of acidity present in the fertilizer actually appeared in the soil. Why this proportion varies is the subject of further study.

Fertilizer*	Maximum acidity in fertilizer (kg HCl/ ha)	Change in soil pH**		Actual acidity added (kg/HCl/ ha)		Actual acidity percentage of maximum.
		Site	Site 2	Site 1	Site	Site 1	Site 2
Ammonium sulphate	3600	-0.9	-1.1	640	790	18	22
Urea	1800	-0.2	-0.5	140	360	8	20
Calcium ammonium nitrate	900	0	-0.6	0	430	0	48

Table 2: Acidification due to addition of a high rate of various nitrogen fertilizers at two sites in Western Australia (from Mason, 1980).

* Fertilizer applied at the rate of 76 kg N/ha for 12 years of continuous crop.

** The pH change is the difference in pH after 12 years between the plots which received nitrogen and those which did not, The mean pH of the nil plots at site 1 (Merredin) was 5.4 and at site 2 (Beverley) was 6.3 (pH measured in 1:5H₂0).

Nutrient uptake by plants can acidify soil

Nutrient absorption by plants can affect soil acidity in the short term and, to a lesser extent, in the long term. Figure 7 summarizes the steps that are involved in acidification from nutrient uptake. The pH of the soil near the root surface (the rhizosphere) can alter markedly during the growing season as a result of nutrient uptake (steps 1 to 3, A to C), but if no plant material is removed then there is no long term effect on acidification (steps 4 to 6). If plant material is removed then the acidity of the soil can be affected in the long term (steps D to G).

Figure 7: How nutrient uptake by plants can acidify soils in the short term only (steps 1 to 6) and in both the short and long term (steps A to G).

The actual process by which this acidification occurs is very interesting, so in the following section is a description of the processes involved. Let us take a hypothetical case where a root grows into a region of soil which contains ions with 100 positive charges and 100 negative charges. Assume, for the sake of the argument, that the root initially contains no charged ions. So, we have a situation which may be represented like this:

Say the root absorbs ions with 20 negative charges and 40positivecharges. We would then have this situation.

A question which arises is why a plant would absorb more positive charges on nutrients than negative charges. Plant roots are selective in what chemicals they absorb from the soil water. The root tends to absorb the ions it requires and to keep out ions it does not need. Plants absorb a lot of nitrogen and, if this nitrogen is in the form of ammonium (positively charged NH₄⁺) then often all the negative charges the plant absorbs (mostly on phosphate: HP0₄^2- / H₂P0₄^- and sulphate: SO₄^2-) cannot balance the positive charges on the ammonium absorbed. Potassium (K⁺), calcium (Ca²⁺ ), and magnesium (Mg²⁺ ) are other positively charged nutrients that can be absorbed in large quantities. Even if no nitrogen is absorbed, as in the case of legumes where the plant’s nitrogen is obtained from the atmosphere, the total amount of potassium, calcium and magnesium absorbed is usually greater than the total amount of negatively charged nutrients. Alternatively, a plant can absorb its nitrogen as the negatively charged nitrate (NO₃^-), in which case an excess of negative charges usually occurs in the root.

Returning to our hypothetical case, after absorption of the nutrients there is a net positive charge of +20 in the root. It is necessary for the plant to reduce the net charge. Not to reduce the net charge would result in the plant using a lot of energy trying to take up further positive ions. It requires energy to force a positive ion close to other positive ions (such as into the root here) just as it requires energy to push the north pole of a magnet close to the north pole of another magnet. The positive ions in the root tend to repel other positive ions.

One method plants use to reduce an excess of positive ions in roots starts with the production of organic acids. The plant uses a process similar to photosynthesis to produce these acids (Hiatt and Hendricks 1967)

Carbon dioxide + water + energy à 20 COOH

The organic acids release their hydrogen ions

20 COOH à+ 20 COO^- + 20 H⁺

so that in the root the situation now is:

The root now excretes the 20 hydrogen ions and so balances the charges inside the root and in the soil:

The net effect is that, for every positive charge absorbed in excess of the negative charges absorbed, the plant excretes a hydrogen ion, and so acidifies the soil by that much. The acidification process described so far has occurred while the plant is growing in the soil. If the plant now dies, and all the plant material decomposes back to the water, carbon dioxide and the mineral salts they started as, then there would be no effect on acidification. This can be represented diagrammatically like this:

Organic matter breaks down:

Leaving the same soil conditions as in the beginning:

Excess uptake of positively charged nutrients can affect acidity in the long term if plant material is removed, say be harvesting a grain crop or by removing sheep which have converted part of the plant material they have eaten into body tissue.

When plant material is removed like this the decomposition of the remaining plant material can tie up only a proportion of the hydrogen ions which were originally excreted into the soil. In pictorial form:

Plant dies:

Organic matter breaks down:

Leaving increased acidity in the soil:

Actual acidity additions resulting from the removal of a range of crops are presented in Table 3. These calculations are based on the results from one glasshouse experiment and so the figures given cannot be used as reliable estimates of what will occur in the field.

Whether nitrogen is applied as a fertilizer or is obtained from legumes is an important factor (Table 3). If we restrict the discussion to the situation in which the nitrogen is fixed by legumes, then it can be seen there is a wide range of acidity additions depending on what type of crop material is removed, ranging from two kilograms of hydrochloric acid per tonne of cereal grain to 74 kg per tonne of tobacco plant.

	Acidity added (kg hydrochloric acid per tonne dry plant material removed)
Material removed	Effect of uptake of nutrients other than nitrogen (Protein removal not con-sidered)	Effect of nutrient uptake and protein removal, where nitrogen originally came from
		Ammonium fertilizer	Atmosphere (fixed by legumes) Urea fertilizer	Nitrate fertilizer
Average for corn, sorghum oats, barley
- whole	10	44	10	-25
- grain	2	50	2	-48
Soybeans
- whole plants	51	128	51	-26
Lucerne
- whole plants	47	116	47	-23
Sweet clover
- whole plants	22	70	22	-26
Tomatoes
- whole plants	47	86	47	+ 7
Tobacco
- whole plants	74	166	74	-18

Table 3: Predicted acidity additions to a soil as the result of removing plant material when effects of uptake of excess of positively charged nutrients and removal of protein assumed the protein’s nitrogen come from various sources. (Calculated from data of Pierre & Banwart, 1973).

CONCLUSION

The mechanisms of acidification I have discussed here are among many.₂ On a geological time scale, the gradual leaching of bases (HCO₃^- CO₃^2- etc.) from soil is probably a significant cause of acidification. In agriculture, adding elemental sulphur (not sulphate) to a soil is another well known source of acidification ( 1t sulphur when fully oxidized produces acidity equivalent to 1.1 t hydrochloric acid). ‘Ferrolysis’ (a series of reactions resulting from extreme waterlogging) has been proposed as a mechanism of acidification (Bradley and Vimpany, cited by Lee, 1980).

In Europe and Canada ‘acid rain’ (rainfall containing acids released originally as industrial waste into the atmosphere) is causing acidification of large areas. In the Western Australian wheatbelt many of the older soils have extremely acid subsoils. Ploughing these soils to a depth greater than 10 cm may mix the acid subsoil through the topsoil and so increase the acidity of the topsoil.

More than one of these mechanisms are probably acting on any one soil. We cannot yet generalize to the extent of saying that one mechanism is more important than the others in a particular situation.

To identify the sources of acidification for any soil we need to be able to balance acidity inputs with the change in acidity of the soil. We do not have accurate measurements of how fast particular soils are becoming acid. The reason for this is that the studies which have shown that soils have become more acid have not taken into account the possibility that the treatments which caused the soils to become acid may also have altered the bulk density and the salt content of the surface soil (Uren, 1981). An over estimation of the rate of acidification would result either from increases over time in bulk density of a soil the pH of which rises with depth or from increases in the salt content of a soil when the pH is measured in a soil water suspension.

Acknowledgements

I would like to thank Drs. A.D. Robson, S.C. Jarvis and J.W. Bowden and Mr. R. Cameron for their helpful comments on a draft of this paper and Mr. R. Klemm for his technical assistance.

REFERENCES

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