Previous PageTable Of ContentsNext Page

Soil Acidification - The Evidence

A.N. Smith and R.A. Falkiner

We hear much about the “Acid Soils Problem”, possibly far too much, so that many primary producers are aware of the phrase and have become uneasy thinking that their enterprise is doomed if their soils test lower than 5.5 pH in soil:water. This, of course, is not so.

We wanted to categorise the extent of natural soil acidity and to measure the change that agriculture has wrought. The ideal is to measure before and after, This is, of course, not possible since no one had the foresight to take soil samples when settlement began and, even if they had, there would always be the problem of changes in storage.

What we did was to sample 73 paired sites, one being uncultivated or “virgin” land - the unused portions of cemeteries and unfertilised parts of state forest and timber reserves; the other from the nearest cultivated ground with adequate history. The sites were taken from the area bounded by Barellan, Savernake, Jindera, Tumbarumba and Wallendbeen, and samples were taken at 0-10 and 10-30cm.

Figures 1 and 2 show the change in pH that agriculture has brought about both in the top soil and the immediate subsoil. Clearly there is a difference, Agriculture has lowered the pH or increased the acidity. Properties involved had had varying periods of cultivation since first clearing so we do not know what proportion of this increase in acidity is due, for example, to the effects of superphosphate or the effects of tillage and cropping.

In an attempt to isolate what effects there might be from the period of agriculture, the change in pH was plotted against years of cultivation. The points were clearly at random. Then the change in pH was plotted against total P applied. The points are shown in Figure 3; little correlation is apparent. When the data are separated into varous soil types, some relationships become clear. For example, solodised solonetz + solodic and red brown earth are shown in Figure 4. The solodised solonetz + solodic shows a slope of 0.4 calcium chloride pH units per 100 kgP per ha and 0.45 water pH units from 100 kg per ha.

Although there appears to be a straight line relationship there are not many points on each curve and we will endeavour to obtain more superphosphate histories so that we can give a more certain interpretation. However, the indicated rate of change is slightly lower than Donald and William’s value of 0.5 pH units per 100 kg P applied as superphosphate to pastures. This coincidence may indicate that the change in pH under a cultivation system results from the indirect effects of the superphosphate applied rather than the tillage practices themselves. In these wheat, sheep, pasture systems phosphorus is applied to both crop and pasture, although in the past the rate on pasture was usually greater. In an experiment at the Wagga Institute on red earth which measured the pH change with different lengths of clover and crop, Furniss found that (Table 1) the magnitude of pH change was directly proportional to the duration of pasture in the rotation. In this experiment both crop and pasture received the same rate of superphosphate each year. This result indicates that the clover is responsible for the pH change, not the superphosphate.

FIGURE 1(a). The distribution of soil pH in 0-10cm samples taken from uncultivated or “virgin” land. Please Note: The pH values given have been measured in CaCl2 not in water. Refer to glossary for explanation.

FIGURE 1(b). The distribution of soil pH in 0-10cm samples taken from areas with a history of cultivation. Please Note: The pH values given have been measured in CaCl2, not in water. See glossary for explanation.

FIGURE 2(a). The distribution of soil pH in the 10-30cm subsoil samples taken from uncultivated or “virgin” land. Please Note: The pH values given have been measured in CaCl2, not in water. Refer to glossary for explanation.

FIGURE 2(b). The distribution of soil pH in 10-30cm subsoil samples taken from areas with a history of cultivation. Please Note: The pH values given have been measured in CaCl2, not in water. Refer to glossary for explanation.

FIGURE 3. The relationship between change in pH and the total amount of phosphorus applied.

FIGURE 4. The relationship between pH change in 0-10cm and total phosphorus applied in a red brown earth and a solodised solonetz + solodic.

TABLE 1. The change in pH (in CaCl2) on a red earth as related to different lengths of subterranean clover and crop (S = subterranean clover; W = wheat)


ΔpH (12 years)


-0 44




- 0.56

S.E. 0.08

The damage to crops is not due to pH, which is merely an integrated measure of total soil condition, but rather the presence of aluminium or manganese. pH is, of course, relatively easy to obtain, but it does not measure the toxic elements directly. Aluminium and manganese are what we wish to estimate, but that is difficult in the field.

Fortunately aluminium availability is closely paralleled by that of iron. The classical indicator for the presence of ferric iron is potassium thiocyanate (KSCN). You will all remember from school the blood-red colour produced by this indicator when iron is present. This was used by Comber in the 1920’s as a measure of suitability of soils for maize. If the soil was colourless there was no iron, therefore no aluminium, therefore the soil was suitable.

We have modified it. The test is now to shake up 1cm of dry soil in a test tube with 5cm of alcoholic saturated potassium thiocyanate solution. Allow it to stand for 15 minutes, re-shake and stand overnight. Then observe the colour. There is a relationship between the aluminium concentration in the soil and the colour. We have taken a cut-off of 1.5 colour units, an example of which is presented at the Conference.

If it is redder than this you have an 85% chance of having an aluminium concentration greater than l0ppm (you also have an 80% chance of having a manganese concentration greater than 4oppm); if the solution is colourless or less pink than the cut-off there is an 80% chance that the aluminium concentration is less than l0ppm. It is difficult to fix on an exact toxic limit for Al in soil, but there is every indication that concentrations of greater than 10 are beginning to have a serious effect on plant growth.

This test gives as reliable an indication of aluminium concentration in the soil as does pH, on our evidence, but it is one that is available to all, simple to carry out and foolproof if the soil is dry and the mixture shaken adequately. Because it is much modified from Comber' s original concept, we have decided to rename it. We took the first two letters from my name and the last three from Allan Smith’s, F-a and i-t-h, therefore we say that if you want an easy, convenient and reliable estimate of aluminium in the soil, you can use “Faith”.

Previous PageTop Of PageNext Page