Sowing a clover pasture usually brings about a series of chemical and physical changes in the soil, most of which are of benefit to soil fertility. These changes result mainly from a build—up of soil organic matter and the accumulation in the soil of fertilizer residues. Most importantly they bring about increases in the supplies of available nitrogen, phosphorus and sulphur, but they also result in improved physical structure of the soil and often there are increases in the availability of other nutrient elements such as potassium, calcium and zinc.

Unfortunately, associated with these changes there is a gradual increase in acidity of the soil which, in some soils, may eventually prove harmful. Increased acidity in pasture soils has now been reported on a wide range of soils in New South Wales, South Australia, Western Australia and Victoria.

The rate of acidification is quite slow and the time required for a decrease of one unit of pH to occur may range from about 50 to well over 100 years depending on the soil type. In this regard it is probably worth keeping in mind that about three quarters of our improved pastures in southern Australia are less than thirty years old. In general, the rate of acidification will be greatest in light—textured soils of low initial organic matter content.

An example of the increase in acidity under subterranean clover pasture is given in Figure 1.

Figure 1 The relation between period under continuous subterranean clover pasture and the pH (1:5 water) of the surface 10 cm of yellow duplex soils frm Binda N.S.W.

In these soils the pH decreased with time and over the 50—year period the average decrease was almost one unit.

The data presented in Figure 1 concern the surface 10 cm of soil only, but Figure 2 indicates that the increase in acidity can extend to depths considerably greater than this — in these soils to a depth in excess of 60 cm.

Figure 2 The relation between soil depth and pH of yellow duplex soils from the Pejar area, near Crookwell, N.S.W. A= Native unimproved pasture. B = 27 year— old sub. clover pasture. C = 55 year—old sub. clover pasture.

A further example of an increase in soil acidity under clover pasture on a different soil type in a different environment is given in Table 1. In this case the pasture was not permanent but provided short—term leys in cropping rotations.

TABLE 1. The pH of red-brown earths from the permanent rotation experiment, Waite Institute, Adelaide. 0-9” (22.5 cm) depth.

F Fallow, W Wheat, 0 = Oats, B Barley, Pe Peas

Px = Ryegrass pasture, Pxx = Subterranean clover pasture

When the soil had been under a cropping system which did not involve legumes there appears to have been little, if any, change in pH, but when a legume was included in the rotation, either as a crop (peas) or subterranean clover ley, pH decreased.

Consequences of increased Acidity

Chemically, the increase in acidity favours an increase in the solubility of many of the cationic metals (such as zinc,manganese, nickel and aluminium) in the soil and this increases their availability to plants. In the cases of manganese and aluminium this is of particular importance because in some soils available levels may ultimately be attained that are harmful to some plant species. With manganese substantial increases in solubility may occur if the soil pH falls below 5.3—5.4 and with aluminium this may occur at about

4.8—5.0.

Figure 3 shows a relationship between soil pH and the amount of manganese extracted by calcium chloride solution (an index of available manganese) from surface soils from the Crookwell district. Broadly, as pH has decreased the amounts of extractable manganese have increased.

Figure 3 The relation between soil pH and the manganese extracted by 0.01 M calcium chloride in yellow duplex soil from Prejar, N.S.W.

Figure 4 records the levels of extractable manganese in the three soil profiles referred to in Figure 2. Clearly as these soils have become more acid with time the amounts of extractable manganese have increased substantially throughout the entire sampled depth of 60 cm.

Figure 4 The relation between soil depth and manganese extracted by calcium chloride from yellow duplex soils from Prejar, N.S.W. (see Figure 2).

Similar changes have been recorded in the exchangeable aluminium (an index of available aluminium) in these soils. Figure 5 shows the relationship between pH and exchangeable aluminium in the surface soils and Figure 6 shows changes with depth in the three profiles. As with manganese substantial increases have occurred in the pasture soils compared with the very low levels present in the virgin soils.

Figure 5 The relation between soil pH and exchangeable aluminium in yellow duplex soils from Pejar, N.S.W.

Figure 6 The relation between soil depth and exchangeable aluminium in yellow duplex soils from Pejar, N.S.W. (see Figure 2)

Effects on Plant Growth

Plant species vary greatly in their capacities to tolerate metal toxicities. Some species, such as lucerne and rape, for example are highly susceptible to both manganese and aluminium whereas, at the other end of the scale, species such as cocksfoot and cereal rye are quite tolerant. The effects of increasing soil acidity will thus depend upon the plant species grown.

Subterranean clover and many of the grasses commonly associated with it are fairly tolerant of both manganese and aluminium so that generally the pasture remains quite unaffected well after levels toxic to more sensitive species have developed. Quite often problems are first experienced if some change in farming practice involving more acidity increases and the levels of available manganese and aluminium increase, the less susceptible species are increasingly likely to be affected until ultimately the pasture species themselves can be affected.

This, to some degree, can be illustrated by reference to the Pejar soils (cf. Figures 3 and 5). When rape was grown on these soils in pot culture it showed symptoms typical of manganese toxicity which ranged from none at all to severe depending on the soil pH. Only on three soils was there a failure to respond to treatment of the soil with calcium carbonate (lime) and on the most severely affected soil yield was depressed by 94 per cent when lime was not applied (see Table 2).

TABLE 2

yield with no lime

The percentage yield (yield of lime treated x 100) of rape grown in pot culture on yellow duplex soils from Pejar, N.S.W.

That manganese was a major factor contributing to the effects on growth is indicated in Figure 7.

Figure 7 The relation between calcium chloride extractable manganese in the soil and the manganese content of rape grown in pot culture on yellow duplex soils from Pejar, NSW.

When subterranean clover was grown under the same conditions, however, symptoms of manganese excess were only evident on three of the soils, and on these the reduction in yield when lime was not applied ranged from only 18 to 30 per cent (cf. 87—94 per cent for rape).

Discussion

It is clear that in general soils under clover pasture slowly become more acid and, as a result of this, the possibility of toxicity problems (especially manganese and aluminium) increases as the period under pasture lengthens. However, not all soils will necessarily develop toxicity problems and the nature of the problems that do develop will vary from soil to soil. Their nature and the onset of them will be determined by various soil properties, environmental factors and management practices. The most important soil properties are the initial pH of the soil, its buffer capacity, clay content, organic matter content and the amounts and chemical nature of the manganese and aluminium compounds initially present in it. Many soils, for example, do not contain sufficient manganese in forms that can become available to plants to ever generate toxic levels while in other soils the rate of acidification is so slow that 200 to 300 years will elapse before problems are likely.