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Secondary salinity and acidity (continued)

Pressure (continued)

Changes in soil acidity by regions [L Indicator 3.3]

More than 70% of Australian soils have either a very high pH (alkaline) or low pH (acidic), and both the soils and their underlying rocks store massive amounts of salts-not only common salt (sodium chloride) but also carbonates, sulfates such as gypsum, and others. This has arisen from the very long geological period of increasing aridity (White 1994). No recent glaciations have scraped clear the old weathered soils and rejuvenated the landscapes, and in most regions no lush tropical vegetation has added deep organic matter to the soil. Most of these soils present constraints to plant growth, particularly to exotic, domesticated species that are not adapted to very high or low pH, toxicities and deficiencies of plant nutrients, or hard soil layers.

Australian soils are acidic because they are geologically old and have been leached of most of their minerals except sands and metal oxides, and in many cases they are residual sand deserts from earlier periods of aridity. They contain few of the minerals required for domesticated crop and pasture species. Many native species are adapted to low nutrient availability, but in agricultural land use acidity merely equates to dysfunctional systems, leaking water and high nitrate levels if the rainfall is substantial.

Naturally acidic soils occupy about one third of Australia, but many agricultural soils in the Intensive Land-use Zone become more acidic as the result of removal of calcium in harvested product, leaching of nitrate and calcium from nitrogen-producing pastures, and use of acidifying fertilisers. The process is particularly rapid in high-rainfall pasture-based systems, and regions that are particularly susceptible have been identified through research and monitoring studies (AACM International 1995).

Recent work for the National Land and Water Resources Audit has provided up-to-date detail of surface soil pHs in agricultural regions over the period 1990-1999. Their spatial distribution is given in Figure 54. The total area of soils with a pH below 5.5 in the ILZ is now 46.7 million hectares, or 13 million hectares more than was estimated at the beginning of the decade (Chartres et al. 1990).

Figure 54: Surface soil (0-0.1 m) pH distribution in agricultural soils of the Intensive Land-use Zone.

Source: Spatial Technologies Unit (2001)

The proportion of agricultural soils of low, optimum and high pH in each state and territory has been calculated from this information. Over half the agricultural soils in New South Wales, Western Australia, Victoria and Tasmania have a surface pH of between 4.3 and 5.5. In Victoria 35% of agricultural soils have a surface of less than pH 4.8. Below this point aluminium and manganese become soluble and toxic, while calcium and magnesium salts, especially phosphates, become less available. Fertiliser efficiency also declines as pH decreases, so fertiliser used on acid soils without the application of lime may be wasted. This is substantiated by the distribution of plant-available phosphorus from the same soil sample records, which shows a close spatial correlation between low phosphorus and low pH, particularly in southern New South Wales and Victoria (Reuter 2001).

Liming is the only effective solution to this problem. Liming has been used in north-western Europe for many centuries, so it would have been reasonable to expect European settlers to have transferred the practice to Australia, but it has never been common practice in rain-fed agricultural regions of Australia.

The National Collaborative Project on Indicators for Sustainable Agriculture reported that only a small proportion of agricultural land that would benefit from lime is treated each year (SCARM 1998). Out of a potential 90 million hectares of agricultural land that are at risk from severe (pH <4.8) or moderate (pH 4.8-5.5) acidity, only 600 000 hectares were treated in 1995-96. That is, only 2% per year of the most acid agricultural soils received lime.

Trends in lime application do show some increase over the decade, particularly since 1995. These may have resulted from concerted efforts by research and extension agencies to publicise the adverse effects of not applying it, and to demonstrate the financial advantages of application (AACM International 1995). However, there still appears to be a prevailing perception that most of the benefit from liming will be private, accruing to the landholder alone (Evans 1991, Virtual Consulting and Griffin NRM 2000).

The potential for improving water utilisation by adequate liming to assist in salinity control has yet to be fully exploited. Current pasture production in such regions is low compared to the potential biomass yield. Increasing the pH of pasture soils to 5.5 or above would increase the availability of phosphorus, improve the rhizobial fixation of nitrogen by legumes, and stimulate the bacterial portion of the microbial biomass that is suppressed at a low pH. Valuable deep-rooted perennials such as lucerne, and macrofauna such as earthworms, will not grow or persist in acid soils, so the decomposition of carbon compounds to release nutrients proceeds slowly (Pankhurst et al. 1997). Water use studies of high-rainfall grazing sites have shown that unlimed sites are much more leaky than those limed to a pH where good plant growth and water use occurs (White et al. 1998).

Nevertheless, this may still be a more practical way of reducing dryland salinity in some areas than attempting to replant trees that will have difficulty in surviving the low pH in wet, acid soils. State government agencies have recognised the importance of encouraging all farmers, not solely those with high input systems, in having a liming program on acidic soils. For example, under the New South Wales government's Acid Soil Action Program, industry, government and the community are implementing a range of liming initiatives for agriculture. A related strategy is addressing the management of acid sulphate soils, that occur widely in the northern coastal regions of the state (EPA NSW 2001). Similar acidity action plans have been promoted in Western Australia and South Australia. In South Australia, where acid soils are not as widespread as in other states, an estimated 300 000 hectares of agricultural land would benefit from liming, while current figures for agricultural lime use suggests that 100 000 hectares are being treated (PIRSA 2001). A similar situation occurs with sodic soils (heavy soils with a large amount of sodium in their clay), which swell and disperse in water. These soils occupy nearly a third of Australia. In agricultural situations the infiltration of rain is very slow, and plant growth is restricted by both the lack of water and the high pH. The structure of sodic soils is improved when they are conditioned with gypsum, yet the amount of gypsum applied is, like lime, small relative to the potential benefit (CRC Soil and Land Management 1999).

Implications

Recent public attention has focused on tree replanting as the preferred solution to dryland salinity. This has distracted attention from the related issue of disrupted water and nutrient balances that have arisen through acidic agricultural soils.

Without financial incentives, only the most intensive and profitable land uses, such as horticulture, can afford the cost of applying conditioners such as lime and gypsum. Yet the benefits from improved crop and pasture production by large-scale application of lime would seem to offer more certain advantages in controlling leaking catchments and providing an income than trying to plant vast areas to trees. Really large-scale tree and shrub planting also depends on the plants being able to survive the current acid soil conditions. The problem is more complex when the potential sources of gypsum are considered.

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