A MAJOR threat to soil health and pasture production, soil acidity is increasing in its extent and severity under high-yielding, continuously cropped farming systems in southern Australia.
Naturally acid soils are becoming more acidic and naturally neutral soils with alkaline subsoils are acidifying in the surface layers.
Soil acidification is a natural process, but the rate of acidification is increasing due to the higher rates of nitrogen (N) fertiliser being applied and an increase in cropping/hay intensity and yields.
But the issue is being addressed, with trials comparing new lime sources showing promising results under no-till cropping and tools now available to help grain growers manage soil acidity.
In South Australia, a number of projects have been developed to explore issues and opportunities for improved treatment of soil acidity.
The projects involve input and investment from the Grains Research and Development Corporation (GRDC), farming systems groups, the Agricultural Bureau of South Australia, Landcare groups, various regional Natural Resource Management Boards, the Department of Environment, Water and Natural Resources (DEWNR), Primary Industries and Regions SA (PIRSA) and the Australian Government.
Declining soil health
PIRSA Rural Solutions principal consultant Brian Hughes, who is overseeing the projects, said activities included increasing awareness and knowledge of landholders, agricultural merchants and consultants, soil testing, development and trialling of new liming sources, and development of various computer-based tools.
Mr Hughes said soil acidification had been associated with a decline in soil health, particularly soil biota (including worms and rhizobia), issues with leaching and tie-up of some soil nutrients, and the release of toxic compounds including aluminium which affect root growth.
Poor performing patches in high value crops were often a first sign of soil acidification.
“Identified barriers for growers to treating soil acidity include a general lack of awareness (particularly of subsurface acidification), lack of readily available information on liming products and options, concerns about the cost and economic returns from liming, and the possibility of inducing trace element deficiencies, greater snail numbers or an increase in certain weeds,” Mr Hughes said.
Acidity-prone areas increasing
In SA, areas prone to soil acidity include the historical higher rainfall grazing areas covering much of the Mt Lofty Ranges, Lower South-East and Kangaroo Island, and emerging areas in the mid to high rainfall cropping areas including the Lower Eyre Peninsula, Mid North and Upper South-East.
An estimated 2.1 million hectares in SA are now considered prone to acidification of soil to below pH 5.5 in calcium chloride (Ca).
“Assuming current practices continue, it is estimated that acidity as defined by the potential pHCa <5.5 in the surface will develop over an additional two to three million hectares over the next 10 to 50 years,” Mr Hughes said.
As a generalisation, tolerant plants become affected by acidity around pHCa 4.5; sensitive plants around 4.8-5.0 and very sensitive around 5.0-5.5.
Highly sensitive plants include durum wheat, most barley cultivars, faba beans, lentils, chickpeas, lucerne, medics and strawberry clover, while sensitive plants include some wheats, canola, phalaris, red clover, Balansa clover.
Tolerant plants include interstate wheats, annual and perennial ryegrass, tall fescue, Haifa white and subterranean clovers, while highly tolerant plants include lupins, oats, triticale, cereal rye, cocksfoot and kikuyu.
Trials compare lime options
In an effort to determine the most effective approach to addressing soil acidification on different soil types, trials have been established in SA to compare newer liming products with older products.
The trials are located at Tungkillo on a thick sand over clay with input and support from the Tungkillo Landcare Group and at Wirrabara on an acidic loam over clay with input and support from the Laura Agricultural Bureau.
Mr Hughes said both trials had shown responses to lime two years after application under no-till cropping. At both sites, higher quality limes seem to be having a quicker impact on yield and changes to pH.
Given that most lime products are only slightly soluble, there was no response in year one of the trials, with monitoring continuing for four to five years.
Sub-surface acidity emerging problem
Support for these these trials has come through Natural Resources SA Murray-Darling Basin and Natural Resources SA Northern and Yorke, and the GRDC.
A new trial site funded through the GRDC is being established this year at Koppio where treatment options for sub-surface acidity are being compared.
Sub surface acidity refers to acidification of soil layers below the 0-10cm layer. This is an emerging problem and more difficult to treat than surface layers.
Meanwhile, tests have been developed to measure lime quality in terms of its effectiveness in counteracting soil acidity.
Historically this has included purity (Neutralising value) and fineness (Effectiveness Neutralising value).
In addition, using a scanning electron microscope, CSIRO researchers have found that some lime products have much larger surface area and porosity and potential reactivity.
“These include those formed from coral which have a ‘honey comb’ particle structure that greatly increases the surface area and its effectiveness,” Mr Hughes said.
“Mount Gambier limestone is one such liming material. Other products such as lakebed products are dispersive and break into small particles when applied, which also increases their effectiveness.”
Mapping pH zones
Precision pH mapping is another area of research, with the GRDC, PIRSA, DEWNR, Natural Resource Management Boards and the National Landcare Program, along with private providers/contractors, investing in the validation and demonstration of technologies for paddock-scale precision pH testing.
Through these technologies which produce accurate paddock scale pH maps, growers have the ability to apply targeted lime rates, varied according to pH zones.
Mr Hughes said several decision support tools/computer models had also been developed to assist landholders and advisers to make better decisions for treating soil acidification.
“An auditing tool enables landholders to determine approximate acidification rates on their paddocks given soil conditions, crop rotations and yield and fertiliser inputs. Other tools evaluate the impact of acidification on production (cost of not liming) and compare costs of different liming products. These tools and technologies help deliver more cost-effective solutions, leading to greater adoption of strategies for managing acidic soils.”