Since the 1981 Riverina Outlook Conference on acid soils we have recognised that soil acidity is a real problem in this region. Yield increases resulting from lime application have been recorded in both pastures and cereals and crop tolerance differences have been confirmed in the field.

Our thinking has now moved to a group of “second generation” questions -those questions which only assume importance after the initial demonstration of acidity problems. One such problem is the importance of subsoil acidity. This section covers the evidence for local subsoil acidification, a brief overview of subsoil acidity and the local evidence for a problem. The final discussion will be on methods of amending the subsoil or, alternatively, of minimising the impact of subsoil acidity on production.

Acidification

There is evidence that our agricultural system not only acidifies surface soil (say 0 to 10 cm) but can also cause acidification deeper in the soil profile.

Dr Murray Bromfield and others have documented the acidification of soil under long term subterranean clover pasture (Bromfield et al. 1983). On podsolic soils similar to those east and south of Wagga they found acidification to 60 cm depth. They tested a virgin site, a second site after 26 years and third site after 55 years of subterranean clover and found acidification (0 to 60 cm depth) of one to one and a half pH units after 55 years.

This effect of subsurface acidification is known to extend into the wheat belt on red earth and red-brown earth soil. A long term crop/pasture rotation experiment at the Agricultural Research Institute, Wagga (Kohn et al . 1986) found acidification to 30 cm after 18 years of production. The pH changes are shown in Table 2.1 (pH in 1:5, soil:0.01M CaCl₂).

TABLE 2.1 Acidification over 18 years at Wagga (after Kohn et al. 1986)

DEPTH	INITIAL pH	FINAL pH	pH CHANGE
0 - 10 cm	5.3	4.6	-0.9
10 - 20 cm	5.3	4.8	-0.5
20 - 30 cm	5.4	5.1	-0.3

At the 1981 Outlook Conference, Smith and Falkiner presented data for 73 paired sites. The paired sites were virgin or uncultivated land and the cultivated paddock immediately adjacent. Averaged over all sites, the virgin soil had pH 4.96 in the 0 to 10 cm and the cultivated soil was pH 4.68. In the 10 to 30 cm layer, the figures were pH 5.08 and pH 4.98 respectively.

All the data presented so far illustrates that the process of acidification is occurring both in the 0 to 10 cm layer and deeper into the soil. However, the rates of pH change vary.

World Scene

The idea that subsurface acidity may restrict plant performance is not unique to our situation (see Adams, 1984). Basically subsurface acidity may reduce root growth and so impair uptake of nutrients and water.

Adams (1984) drew attention to a series of experiments in southern USA. The following points are noted:

(i) subsoil acidity restricted root growth in the cotton and reduced yields by one-third (Adams et al. 1967);

(ii) on the same site but in a different season peanut roots and subsequent yields were not affected by the subsoil acidity;

(iii) frequent irrigations of cotton gave good yields although root growth in the subsoil was still restricted (Doss and Lund, 1975).

This summary introduces two ideas: firstly, that plant species (and perhaps varieties) vary in their reaction to subsurface acidity; and, secondly, that seasonal conditions and presumably nutrition can be so favourable that plants may not need to access the nutrients and moisture in the subsurface acid soil.

At another extreme, responses to lime may not occur until subsurface acidity has been corrected. Porter and Wilson (1985) reported increased wheat yield at Merredin, WA, when the soil was limed to 160 cm but they observed few responses to shallow (0 to 10 cm) lime incorporation (see Table 2.2).

TABLE 2.2. Effect on Wheat Yield of Liming Deep Acid Soils to 160 cm (after Porter and Wilson, 1984)

	GRAIN YIELD t/ha
TREATMENT	SITE 1	SITE 2
Drums with no lime	0.89	0.64
Drums with lime	1.50	1.76
Undisturbed area	1.15	0.48

In some situations yield increases after liming are improved the deeper the profile is amended. Doss et al. (1979) incorporated lime to depths of 15, 30 and 45 cm and found an advantage in incorporation to 30 cm compared to 15 cm in yield of both cotton and corn. There was no advantage in incorporation to 45 cm over 30 cm.

A similar yield increase in corn after lime incorporation to 30 cm compared to 15 cm is shown in Figure 2.1. The data were accumulated yield over three years from an experiment in Brazil.

Figure 2.1. Cumulative yields of maize over three years at a site in Brazil, as affected by lime rates and depth of lime incorporation (CPAC 1976).

Clearly the ability to exploit the subsoil can be important for yield in many field situations.

Local Evidence

Local evidence of the importance of subsurface acidity is confirmed in some glasshouse experiments on soils similar to those which occur locally and to limited field observations.

Pinkerton and Simpson (1986) and Bromfield (unpublished) have demonstrated that the subsurface soils on some podsolic soils can restrict root growth and yield in some species and cultivars. The soils they tested in the glasshouse were from the southern tablelands of NSW.

Limited field evidence has been obtained. In my own research I have found that the yields of Egret wheat are low even after the amendment of the surface 10 cm, when compared to Olympic. This is despite the late sowing times on my experiments which would be expected to favour Egret and disadvantage Olympic. An example of the yields of Egret and Olympic (averaged over 5 years) at a site near Borambola (east of Wagga) is given in Figure 2.2. I believe that subsurface acidity at this site (at the depth of 10 to 30 cm) restricts the root development of the acid soil sensitive Egret (and Clipper barley) and so limits yield compared to the more tolerant Olympic wheat and Tyalla triticale.

Figure 2.2 Average yields and the effect of lime on wheat (Olympic, Egret), barley (Clipper) and triticale (Tyalla) over five seasons (1981-1985) at an acid soil site 30 km east of Wagga. The lime was applied as a single dressing in 1981, prior to sowing and was incorporated to about 10 cm depth.

How to Correct the Problem

Firstly, the simple approach would be to lime the soil to whatever depth was necessary to “fix” the problem. Clearly this is an extremely expensive operation and would, in most situations, be prohibitively expensive. As a research approach, however, it would enable some assessment of the importance of subsurface acidity on crop and pasture performance. I have established a single experiment of this type liming the soil in 10 cm layers and reconstructing the profile to 40 cm. Also in this experiment are comparisons of lime mixed to 10 cm and to 20 cm, both of which are closer to the “possible”. No results from this experiment are yet available.

An alternative is to partly amend the profile by liming “slots” of soil using a soil ripper and a means of liming the soil in the zone of the rip. A number of experiments of this type have been established recently, but no results are yet available. Such “slots” may permit some exploitation of the subsoil by plants or, alternatively, they may provide escape channels for plant roots which can then exploit deeper more favourable soil layers.

Other approaches to the amendment of subsoil acidity involve strategies using shallow applied treatments such as lime and waiting for the effects to move deeper into the soil. Such a strategy involves the sustained use of 0 to 10 cm applied lime and the slow correction of subsurface acidity - essentially the problem of subsoil acidity must be “lived with” for some time into the future.

The rate of lime movement in the soil is known to vary with lime application rate, rainfall and soil type with high lime rate, high rainfall and sandy soils giving the most rapid movement.

In the local situation Mark Conyers and myself have begun to assess the movement of pH, Ca and Mg effects from both lime and dolomite applied with shallow (6 to 8 cm) incorporation. There is little evidence of any effect going beyond 2 cm after 5 years at the single site we have already examined. As 2 cm increments were used in this study, it is possible that any movement may be even less than 2cm. The time required for the amendment of subsurface acidity by sustained shallow incorporation varies with the depth of the subsurface acid layer but could be of the order of 20 to 100 years. During this time, acid soil tolerant species and cultivars would need to be used even though lime was being applied and shallow incorporated.

The use of fertiliser such as calcium nitrate has been demonstrated as being able to correct acidity problems (Adams and Pearson, 1969) and its action in correcting subsurface acidity may be more rapid than lime. Essentially the nitrate is mobile in the soil and its uptake by plants has an alkaline effect on the soil. This would not appear to be attractive in our situation due to the cost of the product and to the fact that plant roots need to be present in the acid soil when the nitrate is leached to the acid layer.

Conclusions

1. We believe that acid subsurface soils are likely to constrain agricultural production on many, if not most of our problem acid soils. In many situations this deep acidity has resulted from acidification due to agricultural production.

2. The impact of subsurface acidity on agricultural production in our situation has not been well documented but research is currently under way to attempt to evaluate the problem.

In the current situation the application of lime to 10 cm depth combined with the use of plant tolerance can substantially restore production although the use of tolerant plants (oats and subclover) excludes some plants (e.g. barley, lucerne) from use.

References

Adams, F. and Pearson, R.W. (1969). Neutralising soil acidity under bermuda grass sod. Soil Science of America, Proceedings 33:737-742.

Adams, F., Pearson, R.W. and Doss, B.D. (1967). Relative effects of acid subsoils on cotton yields in field experiments and on cotton roots in growth chamber experiments. Agronomy Journal 59:453-456.

Adams, F. (1984). Crop response to lime in the southern United States. In “Soil Acidity and Liming” 2nd edition (Ed. F.Adams), pp2ll-26S.

Bromfield, S.M., Cumming, R.W. , David, D.J. and Williams, C.H. (1983).

Change in soil pH, manganese and aluminium under subterranean clover pasture. Australian Journal of Experimental Agriculture and Animal Husbandry 23:181-191.

CPAC (1976). Relatorio Tecnico Anual do Centro De Pequisa Agropecuaria Dos Cerrados. 1975-76. EMBRAPA, Brazilia, Brazil.

Doss,B.D. and Lund, Z.F. (1975). Subsoil pH effects on growth and yield of cotton. Agronomy Journal 67:193-196.

Doss, B.D., Dumas, W.T. and Lund,Z.F. (1979). Depth of lime incorporation for correction of subsoil acidity. Agronomy Journal 71:541-544.

Kohn, G.D., Raison, J., Furniss, K. and Taylor, A.C. (1986). The response of total nitrogen, organic carbon and pH to intensity of cropping and length of rotation in six clover ley farming systems. In “Proceedings of a symposium on nitrogen cycling in agricultural systems of temperate Australia”,Wagga Wagga, July 1986. (Ed. P. Bacon). Australian Society of Soil Science, Riverina Branch.

Porter, W.M. and Wilson, I.R. (1984). Soil acidity in the eastern wheat belt. Journal of Agriculture, Western Australia 25(4):132.