Clay spreading and delving has transformed thousands of acres of sandy soils in the Upper South East throughout the past decade or two into productive paddocks.
But a recent study – funded by a national Landcare grant – has highlighted soil acidity may be diminishing some of these yield gains.
Rural Solutions SA-PIRSA soils consultant Mel Fraser said incorporating clay into sandy top soils had helped overcome water repellance in non-wetting sands with the added benefit of raising soil pH. But many highly productive flats, which had been continuously cropped for a decade or more, had experienced a drop in soil pH, and were below optimal levels, down to pH 4.3 (in Calcium Chloride).
She says they could benefit from the relatively cheap input of lime every few years.
“Acidity was naturally an issue in these sands and when we applied clay the pH rose to neutral, but over the years with agronomic practises and crop removal have caused the pH to drop again,” Dr Fraser said.
“We have seen the introduction of canola where sulphate of ammonia fertiliser is more commonly used and, while legumes fix nitrogen, they also contribute towards acidifying the soil.”
In the past few years she says some growers have noticed patches of non-performing or lower production in their clayed or delved paddocks.
The study, involving 10 landholders, found that acidity was present in more than 80 per cent of the paddocks surveyed and in many cases it was negatively impacting production.
“We had an aerial photo of each of the paddocks to identify the different soil types in the dune swale landscape that is typical in the SE and we used this to conduct strategic soil sampling,” Dr Fraser said.
“Up on the hills where the yields have been lower it is closer to neutral but on the flats, where they have grown three to five tonne a hectare crops, the pH was lower – invariably the pH was linked to soil type.”
The study also looked at a few paddocks set up as permanent pasture but found these had not experienced the same drop in pH.
Dr Fraser encourages croppers to identify their non-performing areas using yield maps, aerial or satellite NDVI images and then soil test these areas with their agronomist.
In cropping systems Dr Fraser recommends ensuring the top 10 centimetres of soil remains above pH5.5 (in Calcium Chloride) and reiterates the importance of testing below this depth too.
“We need to consider the pH in the subsoil too because, as the season progresses, plants draw moisture from further down but if it is a hostile environment this can be constraining for yields,” she said.
Paddock pH mapping is another tool for finding areas that need lime most, rather than applying the same rate across the paddock.
Rural Solutions SA has a Veris on-the-go pH mapper that they use for projects and a number of commercial operators offer precision soil mapping services across the state.
Dr Fraser said tools to assist growers to calculate the amount of lime required to treat soil acidity were available on the AgEx Alliance website.
- Details: agex.org.au/project/soil-acidity
Liming addresses potential yield drop
Sherwood farmers Trevor and Trish Menz were unaware of the drop in soil pH in some areas of their farm until participating in a recent study.
But they have been quick to act, applying 1.5 tonnes a hectare of lime earlier this year to most of the acidic areas.
In the past 15 to 20 years they have clay spread or delved nearly all their 1130ha property Naroo. The results have been impressive and long-lasting, lifting yields by about 20 per cent with fewer frost damaged crops.
But it was not until Rural Solutions SA soil consultant Mel Fraser soil tested a cropping and permanent pasture paddock for pH that they realised some of the heavier cropping ground, which had been closer to pH neutral, was dropping.
“The soil test was a high as pH 5.5 to 6 (in water) and down to 4.5 (in calcium chloride),” Mr Menz said.
“A 1 to 1.5 pH difference may not sound a lot but it really is – it can really affect yields. We want to thank Mel or we would have kept going along thinking everything was rosy.”
In March the couple spread 450t of lime and are looking at further soil testing to generate variable rate maps to put out more lime where it is needed most.
“Hopefully we have got it early so we don’t need to be putting out huge amounts but we will keep monitoring it,” Mr Menz said.
