Plants vary greatly in their tolerance of acid soil problems so that the impact of acidity will depend upon the crop or pasture species involved.

As plant problems in acid soils are frequently due to aluminium (Al) and/or manganese (Mn) excess, tolerance to acid soils is associated with tolerance to these elements. The following table lists the tolerances of some temperate pasture and crop plants to high levels of Al and Mn.

Species	Al Sensitivity
Lucerne Barley Phalaris Oilseed rape	Highly sensitive
Red clover Wheat	Sensitive
Subterranean clover Woolly pod vetch Rose clover White clover Ryegrass	Moderately tolerant
Tall fescue Cocksfoot Oats Cereal rye	Highly tolerant
Species	Mn Sensitivity
Lucerne Oilseed rape Subterranean clover Wheat Cereal rye Oats Barley Rye grass Some wheat	Highly sensitive Sensitive Moderately tolerant Highly tolerant

TABLE 1 Sensitivity of temperate plants to aluminium and manganese

It can be seen that some species are sensitive to both Al and Mn (e.g. lucerne and oilseed rape) while others are reasonably tolerant of both mineral excesses (e.g. ryegrass and oats). Barley however is sensitive to Al and tolerant of Mn so the reaction of barley on an acid soil will depend on the nature of the problem (i.e. either Al or Mn or both problems together). Moreover, not all cultivars within a species react the same way to the problems discussed. This makes predicting reactions of a species difficult but opens the possibility of breeding tolerance into crops and pastures.

The different reactions of species to acid soils has long been established but only recently (since about 1960) has the possibility of breeding for tolerance to Al and Mn, within a species been recognised.

The initial step is to identify tolerance to manganese or aluminium in a variety. Once this is accomplished the genetic control and heritability of the character need to be defined and a suitable screening technique developed. At this stage a plant breeder should be able to incorporate the character into locally adapted varieties of the crop or pasture.

The most successful selection for tolerance is probably in Brazil where wheat hes been selected for tolerance to aluminium for about 60 years. For the first 40 years of this time the Brazilians selected for adaption of wheat to their acid soils. In the last 20 years it has become clear that this selection is for tolerance to the high levels of soluble aluminium found in their soils. Interest in Australia is relatively recent.

In my project a method of glasshouse screening of plants for tolerance to manganese and aluminium has been developed. The method uses subsurface irrigation of a gravel bed with a nutrient solution. This system is considered to be superior to solution culture in that plants can germinate under the gravel and the legumes can nodulate better then in the solution culture. Using this method crops such as rape, wheat, oats and barley can be screened for reaction to high aluminium and manganese.

Wheat - Al and Mn tolerance

Australian cultivars tested were all very or moderately susceptible to damage by aluminium. Brazilian cultivars Maringa, Cotipora, Carazinho and Toropi were the most tolerant varieties used. These results have been confirmed using a haematoxylin stain test (Crop Science 18:823).

When tested for tolerance to manganese five cultivars showed no yield depression at relatively high manganese levels. These include Cotipora, Carazinho, Benvenuto Inca (Argentina), Egret, Warigal, and Lance (Australia). The most sensitive varieties tested were Teal, Isis and Durati (Australia). Yield of tops of sensitive cultivars were reduced by 50 to 75% and root yields by 90 to 95% at high Mn rates although these cultivars did not show the most pronounced toxicity symptoms. Catcher and Spica (Australia) had severe symptoms and substantial yield reductions. Tolerant varieties had the highest levels of manganese in both roots and tops.

Within the three tolerant Australian varieties, “Warigal”, tested only once, appears to have good tolerance. “Egret” appears after much testing to be a mixed population when tested at 180 ppm Mn in our system but to be uniformly tolerant at 90 ppm. “Lance” appears tolerant to 90 ppm but not as tolerant as its parent (Collafen) at 180 ppm Mn.

This Mn tolerance can be traced to “WW15” (Mexico) a parent of Egret and Warigal, and “Collafen” (Chile). The tolerance in “WW15” appears to originate in “Lerma Rojo” (Mexico).

From this we draw the following general observation:-

i) If tolerance can be traced in this way its genetic control is probably relatively simple.

ii) The “partial” tolerance of “Lance” cf. “Collafen” may indicate that a number of genes are involved.

iii) Mn tolerance has only recently been introduced into Australian varieties from “WW15” and “Collafen”. Early Australian varieties tested are all Mn susceptible.

iv) Mn tolerance has been selected for, when available in the parents of a variety, in South Australia’s winter rainfall environment (Warigal and Lance) and under irrigation in N.S.W. (Egret). This may indicate selection for Mn tolerance as “vigour” when winter water logging occurs. The use of a screen for Mn tolerance in solution culture would make breeding for a uniformly tolerant variety an easier task.

Breeding for acid soil tolerance in wheat

A breeding programme was commenced in the spring of 1979 to incorporate Al and Mn tolerance from Carazinho into an Egret/Avocet background, obtaining speckled leaf blotch resistance from an Avocet backcross line. At present the final cross has been completed and Fl plants are in the field this season. It will be at least two more seasons before lines from this programme are available for testing in Department of Agriculture trials conducted on farmers’ properties.

Cereal rye and triticale - Al and Mn tolerance

A number of cereal ryes (3), durum wheats (3) and triticales (30) were sown in the gravel bed with solutions containing 0, 5, 10, 15, 20 and 40 ppm aluminium. Four wheat varieties with a known range of reactions were sown as controls.

The cereal ryes tested were found to be very tolerant cf aluminium, while the durum wheats were very sensitive. The triticales showed a range of tolerances with the most tolerant being equal to the cereal ryes. Of the commercial varieties Ningadhu was most tolerant and Growquick was least tolerant. As the triticales are the result of a cross between cereal rye and durum wheat it seems possible that their aluminium tolerance is derived from cereal rye. This tolerance, based on cereal rye, was apparently superior to the tolerance to be found in Brazilian wheat.

As the recommended wheat varieties for southern N.S.W. are all sensitive to aluminium, incorporating the tolerance from Brazilian wheat is an obvious first step. However, the improved cereal rye tolerance could be used by selecting triticales for aluminium problem soils or by attempting, at some future time, to incorporate the rye tolerance into wheat.

A group of cultivars was also screened for tolerance to high manganese. The gravel beds were used with solutions at pH 4.8 and manganese rates as 150 ppm.

The most outstanding cultivar in this experiment was the Brazilian wheat Carazinho which was used as a known tolerant. The dry matter yield of Carazinho was unaffected by manganese.

The triticales showed a range of tolerance to manganese with the best tolerance being found in experimental lines TNY 29 and TNY 31. The species origin of this tolerance was not clear as all cereal ryes and durum wheats were seriously damaged by manganese at the rates used. Most triticales however, did not shown good tolerance, so triticales would not be expected to perform well in soils with high levels of Mn.

Oats - Al and Mn tolerance

A set of oat cultivars (34) was tested for tolerance to aluminium in the gravel bed.

Most of the oats were more tolerant than the majority of wheat varieties. The most oats were Saia, Swan and Alber. Saia (Brazil) was the most tolerant but is diploid and not immediately useful in a breeding programme. Swan (Australia) and Alber (Uraguay) may be useful as parents when breeding for tolerance. The cultivar Coolabah, appeared to be aluminium sensitive.

A similar group of varieties was tested for tolerance to high levels of manganese. The most tclerant varieties were Suregrain (U.S.A.) Stout (U.S.A.) and Moore. Algerian and Sual (a selection from Algerian) were very sensitive to Mn damage.

Oats is frequently grown on soils “too wet for wheat” and a generally high level of tolerance to manganese was expected because of the association of manganese toxicity with waterlogging on many soils. However, the oats tested showed a range of reactions including some sensitive varieties.

The scope for breeding aluminium and manganese tolerant oat varieties would appear to be similar to the scope in wheat.

Other Crops

Acid soil tolerance is also being investigated in CSIRO, Canberra where Dr. Rex Oram is working on barley and phalaris and Dr. Ross Downes is pursuing a lucerne variety with improved acid soil tolerance.

In the project at Wagga I have also tested a range of varieties of oilseed rape, subterranean clover and barley for tolerance. Today I have described the results in the most encouraging areas.

Plant tolerance is not likely to be a complete answer, particularly where soils are strongly acid with severe toxicity problems. In this situation a combination of plant tolerance and soil treatment with either lime or dolomite would probably be the most rewarding approach.