THE simplest, cheapest and most reliable way of overcoming soil acidity is lime.
This might sound obvious, but as soil acidity expert Mark Conyers told a healthy soils field day in Gippsland recently, lime applications must be tailored to local conditions.
"My key message is look at local trial data to see how much lime (is needed) and how often it is required, by the species you are growing," he said.
"It is very specific to the soil type and crop type, so local data is essential."
Mark, of Industry and Investment NSW at Wagga Wagga, said limestone or calcium carbonate were the best fix for the problem, but calcium nitrate also worked.
"I thought calcium nitrate would leach too quickly, but that has not been the case with the recent run of dry years," he said.
"One tonne of calcium carbonate might give you a 0.5 of a pH unit lift.
"One tonne of calcium nitrate might give you only 0.2, but you are getting nitrogen as well.
"Unfortunately calcium nitrate is about three times the cost of urea, so it is not yet practical. But once the demands increase, I'm sure it will be another management option."
Mark said importing hay or compost also helped reduce acidity.
"A tonne of manure contains about 65kg of calcium, so if you spread 14-15 tonnes of manure per hectare, that's about one tonne of lime per hectare. It takes a lot of it, but it will, over time, help lift pH."
There were other techniques for minimising acidification using resources from the farm, Mark said.
"If you cut hay and then feed that back in the same paddock, then you'll return some of the alkali you harvested," he said.
Cutting and exporting lucerne hay, for example, removed 60kg of calcium for every tonne of hay. For clover hay it was 40kg and grass hay 20kg.
Washings from dairy sheds, or water from an effluent pond, were also alkaline, Mark said.
A more biological approach was to use perennial grasses.
"Their roots can grab the nitrate before it leaches and they act year round," Mark said.
He said a lot of nitrate was leached when crops and pasture were establishing and there were strategies to minimise this leaching.
"If you are using bagged nitrogen, don't put it all on at once, as there is more nitrogen than the plant needs," Mark said.
"Split applications reduce the nitrate leaching and allow you to hedge your bets on a follow-up application as the season unfolds.
"For the more biologically inclined, legume nitrogen is very good, because it is a slow-release nitrogen more synchronised with the needs of the plants."
Mark said that in the natural world, plants balanced their positively charged interior with the negatively charged soil.
The problem was that farming came along and sold off portions of the plant as hay, grain, milk, wool, beef or lamb.
"Farming, by nature, removes alkali from the soil and leaves the soil more acid," Mark said.
Add the geological weathering that has gone on for tens of thousands of years and the soil is left with acidic top layers and lime lodging somewhere down the profile.
"So right from the outset you are dealing with a high degree of natural acidification," Mark said.
"And the higher the rainfall the quicker that takes place."
Then there was the leaching of nitrate nitrogen, produced by the bacterial breakdown of organic matter or ammonium forms of nitrogen.
Nitrate nitrogen was very soluble and easily leached, leaving the topsoil more acid, Mark said.
"The net result is the same as geological weathering, it just seems to be unavoidable," he said.
In acid soils the acid breaks down clay and releases aluminium, which leads to aluminium toxicity and an infertile soil.
Mark said soils varied in their susceptibility to acidity. The more clay in the soil, the more resistance to leaching.
Plants also varied in their tolerance.
"Barley, lucerne and clovers hate it, but perennial and annual ryegrass handle acidity well," he said.
The Healthy Soils, Sustainable Farms project is sponsored by the Caring for our Country program.
