The application of lime to address issues with soil acidity, stratification and toxicity is essential for the sustainability of many farming enterprises throughout Australia.

Dr Jason Condon, senior lecturer in soil science at Charles Sturt University in Wagga Wagga, said Australian soils were quite old and tended to be low in carbon and structurally sensitive.

He said in many areas mixed farmers have dropped livestock enterprises and moved towards continual cropping.

“In the short term that might have had some financial benefit to it, I think longer term it is going to create some real challenges to our soil fertility,” he said.

“With minimum-till farming, continual cropping and so very little cultivation, what we are finding is that we've got soil acidity in the few centimetres down from the soil surface. It is a stratified profile.”

This means changes to pH levels at different depths, which an “average” reading doesn’t account for.

“Plants don't see averages,” Dr Condon said. “In soil that is not cultivated, there are layers. If seeds are planted three to five centimetres down they are actually going into that acid zone.”

The acid zone has been caused, in part, by application of ammonium fertilisers.

Low pH is one of the largest limiting factors to production because of aluminium and manganese toxicity. Aluminium affects root growth and manganese toxicity affects shoot growth.

“So if you are minimum tilling, that acid layer stays there,” Dr Condon said. “There's nothing that is going to stop that from happening.

“We recommend liming and then incorporate that lime down so you are mixing the lime in with the soil that is acid.

“You just need to know what the pH is on each interval. That is the first step. Second step is then to look at your rotation. Have a look at which crops will be most susceptible to conditions of low pH. Some people will put the lime on in the year of sowing that crop.”

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However, the preferred method is to know crops going forward and apply lime in the year prior to growing a sensitive crop like canola, faba beans or lucerne.

“Put the lime on this year and then it's got a whole year to dissolve and fix the pH,” he said. 

“Ideally you would have incorporated it, you would have worked it down 6-7cm into the top soil, so it's working in that time to raise that pH. Then you come in with your sensitive crop and the conditions are perfect.

“That's when you are going to get the greatest agronomic benefit from that lime addition and you minimise the risk of failure of that sensitive crop. 

“You know what is coming, get the lime in the ground, get the pH up before you sow that sensitive crop. That is the most sensible thing to do."

He said growers should not allow pH levels to fall below 5.2 and they should lime above that level.

“Aim higher; go to 5.5,” Dr Condon said. “If you lime to 5.5, you'll get more years where aluminium is not in that system and you'll get aluminium-free productivity.

“The other thing we know from-long term trials is if you keep the pH in the topsoil greater than 5.5, then that alkalinity does tend to shift down.”

“Large applications of lime, infrequently, will do that but probably a better way of doing it is more economically sensible amounts of lime every five to 10 years, depending on soil tests and pH decreases with time.  To do that, it is important that we monitor the pH.

“That is a long term strategy – soil testing, liming to target pH 5.5."

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