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8 October 1998 |
Ref 98/240 |
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CSIRO scientists believe they may have found a fresh way to tackle one of the gravest threats to Australia's, and the world's, environment and food supply - acid soils.
Dr Andrew Noble and Dr Peter Randall say that trees, used effectively, can help arrest or even reverse the process in which farmed soils slowly turn acidic, eventually stunting the growth of sensitive crops and pastures.
"When trees are intregrated into farming systems, they provide an additional weapon in the fight against salinity," says Dr Randall. "In some acidified soils, growing trees may be the best option."
Australia has more than 7 million hectares of acid soils which cost the nation a billion dollars in lost income every three years.
Worldwide, the problem is immense, with acid soils affecting food production across Asia, India, Africa and Latin America, as well imposing heavy costs on farmers in Europe and North America.
"Soils may be acid to start with, or they may turn acid as large tonnages of crops and pasture are removed, nutrients cycle poorly and nitrate leaches out of the topsoil," says Dr Noble. "Acidifying nitrogen fertilisers also contribute significantly to the problem."
The process eventually makes the soil so acidic it is hostile to plant growth.
Where production is intensive and profitable the problem can be combatted by farmers spreading lime - but over broad acreages with low profits per hectare or in poor countries, this is not an option.
"We've known for some time that trees play a vital role in preventing dry-land salinity, by pumping rainwater out of the ground, and preventing it from adding to saline aquifers," says Dr Noble.
"But even the problem of salinity, which is high profile in Australia, does not cause losses as great as those arising from acid soils."
The researchers believe that trees can help reverse acidification in four ways:
- by acting as "biological pumps" and recycling nutrients from deep layers in the soil, adding them to the surface via tree litter and so offsetting the chemical changes which lead to acidity
- by neutralising soil acidification during the break-down of organic substances in leaf litter
- the soluble organic compounds in the litter may directly tie up some of the toxic substances in an acid soil
- by preventing nitrates from leaching out of the soil. Tree roots intercept them as they sink deeper underground and eventually they are returned as leaf litter protein to the surface.
The team added leaf litter to soils and then measured the resulting decline in acidity to demonstrate the reduction in acidification.
However they also found that some species of trees are better than others at reversing the process - deciduous trees do it better than eucalypts, acacias and pines. Field tests confirmed that soil at sites under trees was less acid that soil a few metres away.
"The work indicates the potential for redistribution of nutrients and alkalinity by trees to ameliorate the surface layers of acid soils by depositing litter, particularly where there is alkalinity at depth in the soil," Dr Noble concludes.
The work strongly suggests that it is better to leave tree litter to decompose under trees. Burning the litter also returns alkalinity to the soil, unless the ash is blown or washed away. Removing timber and litter will hasten the process of acidification.
The researchers have proposed a system of classifying tree species according to their ability to rectify an acid soil. They also recommend that the new insight into the role of trees in protecting the soil's acid/alkali (pH) balance become part of planning for sustainable agroforestry.
The study was supported by Australia's Rural Industries Research and Development Corporation (RIRDC) and the Vincent Fairfax Family Foundation..
More information from:
- Dr Andrew Noble 07 4753 8555
Dr Peter Randall 02 6246 5119
Margaret Bryant 08 9333 6215
Katrina Nitschke 02 6246 5323
Katrina.Nitschke@pi.csiro.au
(Australia's largest scientific research organisation)
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