Rutherglen Research Institute, Department Of Agriculture And Rural Affairs, Vic.
Acid soil infertility is known to limit the production of a wide range of agriculturally important plants. In this introductory session, the problem; caused by soil acidity for dryland farming systems in southern Australia will be considered. The scope of the talk concerns the background and extent of soil acidity in southern Australia, recent research on this problem and the possibilities for practical solutions arising from these results.
Acid soil infertility can be caused by toxicities of H, Al and Mn and deficiencies of Ca, Mg and Mo. Al and Mn toxicities are usually the cause of reduced plant growth.
Australian soils are of great age and many have been strongly weathered and leached and are now acid. Many of these soils have become more acid, due to changes brought about by the shift in land-use to dry-farming systems with legume based pastures.
The survey of soil pH spanning a number of years in the Crookwell region of New South Wales provides the most complete report of increasing soil acidity brought about by a change in land use. This work by Donald and Williams shows the gradual decline in soil pH which occurs under pasture improvement.
The processes that acidify the soil involve acid producing systems associated with increased organic matter, nitrate leaching and product removal. The use of legumes in pasture has resulted in an increase in the soil organic matter far above original levels. The legume is also associated with nitrate leaching as, in most cases, the addition of nitrogen to the system has been from fixation of atmospheric nitrogen. Plants usually absorb more cation nutrients than anions so the loss of plant products from the site of growth, or removal of animal waste to camps, results in a depletion of cation nutrients and a pH increase.
The areas in south-east Australia in which excessive soil acidity is affecting agricultural production is more than 10 m ha. These are mainly dryland farming areas which utilise legume based pasture. Thus, one of the main factors in the success of our dryland system - the input of nitrogen into the soil by symbiotic fixation - is responsible in the long-term for acidifying the soil.
The productivity of both crop and livestock enterprises based on subterranean clover pasture is known to have declined due to soil acidity.
Note: The symbols used here are:
H - hydrogen; Al - aluminium; Mn - manganese; Ca - calcium; Mg - magnesium; Mo - molybdenum; Cu - copper; Zn - zinc; B - boron
In some acid soil areas, wheat grain yields have always been poor. These areas include the Pilliga and Narromine regions in New South Wales and certain parts of the Eastern wheatbelt in Western Australia. In these areas the soils are naturally strongly acid both in the topsoil and the subsoil.
There are also vast cropping areas in New South Wales, Victoria and Western Australia with neutral to acid soil where acidity had not been previously considered as a factor limiting wheat production. It is within these regions that wheat yields on many farms are declining, and growth problems are obvious in the crop. The symptoms observed in wheat crops are patches of stunted growth in a chlorotic condition, and where these symptoms occur, plant and soil survey data have indicated that acidity may be a factor limiting growth.
There were virtually no reports on the use of lime in cropping areas in Australia until recently, when it was recognised that acidity may be involved. In the late 1970s, work commenced with lime in cropping areas in New South Wales, Victoria and Western Australia. The results obtained so far can be broadly divided into three groups, dependent on the acidity through the soil profile and the responsiveness to lime application- These groups are:
* surface soil acidity - lime responsive
* subsoil acidity - lime responsive
* subsoil acidity - lime non-responsive
Many of the acid soils in north—east Victoria and southern New South Wales have the potential for high concentrations of plant available Mn and Al and the acidity is mainly confined to the top 20 cm of soil. On these soils, grain yield responses have frequently been obtained when the soil is limed.
Large differences were obtained in growth and grain yield where a Rutherglen soil was limed and deep ripped. In the five seasons studied the range of grain yield increases were 31-103% with lime and 11-41% with deep ripping. At this site, where the soil was not limed, Al toxicity was responsible for the poor growth and yield of wheat.
Lime responses with wheat have also been obtained where acidity extends into the subsoil, for example in south-west slopes and Pilliga region in New South Wales. In these situations, the use of plant tolerance, together with lime application have given the highest yields.
However, the application of lime to the surface soil of the acid eastern sand plain in Western Australia rarely improved wheat yields. These acid soils have high Al levels but low Mn, and the subsoil is usually more acid than the topsoil. Two sites in Western Australia, which were non-responsive to lime, were used to examine the effect of liming the subsoil. This was done by excavating the soil, liming and then repacking the soil. When the whole profile was limed wheat yields were increased.
Since 1981, 12,000 t of lime has been spread over 17,500 ha in the Corowa-Rutherglen region where the surface acidity is mostly confined in the surface 20 cm. This has resulted in considerable yield increases on a whole paddock scale. With this raised soil pH, it may also be possible to grow a wider range of crops, for example, large yield increases are obtained with rapeseed both on farms and in experimental plots making it a profitable crop after soil treatment.
There is considerable variation in tolerance amongst crop species and between cultivars within species to acid soils. The knowledge of this tolerance often dictates which crop and what variety is grown in an acid soil region. As lime use is expensive, and in some situations subsoil acidity restricts the benefit of lime, then plants with better tolerance to acidity have immediate application.
To get yield improvement from lime where subsoil acidity is the problem the depth of lime incorporation has to be increased. An implement for the injection of slurries of lime into the subsoil has been built at Rutherglen Research Institute and is currently being evaluated in the field.
In Australia, the concept of liming a soil to any set pH has not been developed, and although some lime requirement tests have been introduced, they have not gained wide acceptance. As acid soils vary considerably, there is no all-inclusive formula for predicting whether a soil will be lime responsive, or even for determining the lime requirement on lime responsive soils. Thus the requirement still exists for a test applicable for use by agronomists or farmers in the different regions. In the short-term the use of soil tests regionally developed and validated by field trials probably remains the best option.
More information is required on the nutrient needs of crops on acid soils. In particular the use of Mo for crops has to be evaluated. Liming acid soils can also affect the phosphate availability and uptake by plants. Some consideration may also have to be given to Cu, Zn and B where soils are limed.
In the past decade there have been many reports that pasture productivity is declining, and that this decline is associated with increased acidity. In many areas farmers are having problems with the establishment, growth and persistence of subterranean clover in both permanently grazed and clover-icy farming systems. There are now many reports of herbage yield responses to surface applied lime.
There has been considerable work done in Australia on the effects of soil acidity on the legume/Rhizobium association and on legume growth and this work has greatly influenced agricultural practices. Much is known on the biology of Rhizobium inoculation procedures, nodulation and the nutrition of legumes on acid soils. The main focus of the recent field work is identifying the aspect of the symbiosis and legume nutrition affected by the increased soil acidity. This recent research is important, as it will bear on the possible corrective measures taken to restore pasture productivity to that experienced in the 1950s-60s.
The soil populations of Rhizobium trifolii associated with subterranean clover pasture usually range from 101 to io6 per gram of soil, and should be adequate to ensure the continued nodulation of clover. In soil, the Rhizobium population is a mixture of strains and these strains may vary in their infectivity and nitrogen fixing efficiency.
In a survey over two seasons of 44 permanent dryland pastures in central Victoria it was found that only 72% and 84% of subterranean clover plants were nodulated, despite adequate populations of Rhizobium trifolii. This result suggests problems with the infectiveness of the Rhizobium strain. A different situation exists on the clover-ley soils in north-east Victoria where the soil populations of Rhizobium trifolii were frequently inadequate following cropping. The inoculation of clover seed is recommended irrespective of the soil pH when re-establishing pastures.
The pH of the surface, or organic horizon, is invariably higher than the mineral soil deeper in the profile, and this organic horizon is likely to be the habitat for colonisation and survival of Rhizobium. With the disturbance of this layer as occurs in a crop sequence, then the Rhizobium survival may be jeopardised.
The nodulation of legumes can also be restricted by acidity and, where clover is growing poorly the plants usually have few nodules. Restricted nodulation need not be a problem as with many legumes an inverse relationship exists between nodule number and nodule size, which allows for the maintenance of the total volume of nodule tissue. However, the nitrogen fixation and growth of subterranean clover on acid soils is optimum when the plants have a large number of small nodules. Sparsely nodulated plants (even those with large nodules) is indicative of some problem with the symbiosis.
The requirement of many Australian soils for Mo is well known. With the progressive acidification of these soils, it is likely that there will be an increased number of permanent pasture and ley soils which require Mo fertiliser.
Acidity can affect the growth of the legume independently from the symbiosis, al though, with subterranean clover, the establishment and function of the symbiosis is usually more sensitive to most of the acidity components than is the function of the plant. The one possible exception is Mn toxicity
which shows readily identifiable leaf symptoms, but is probably not agrononniical ly important.
1. Increased acidity on many of our agricultural soils is now affecting both crop and pasture production.
2. At the present time lime and plant tolerance are the management options used for countering the harmful effects of acidity.
3. More work is needed on management systems directed at slowing the acidification process and recognising and then minimising the impact of the toxic soil components.