GROWERS need to be wary that the extent and severity of soil acidity is increasing under high yielding continuously cropped farming systems, according to Rural Solutions SA sustainable agriculture principal consultant Brian Hughes.
“Soil acidification is a natural process, but the rate of acidification is increasing due to higher rates of nitrogen fertiliser being used and increased cropping and hay intensity and yields (product removal),” he said.
“Acidification of subsurface soils (at 10-20cm) is also increasing and where this is a problem, the best strategy is to maintain the soil pH (0-10cm) at or above 5.5 pH.”
Mr Hughes said acid soils generally had too much aluminium, manganese and iron and not enough soil magnesium, calcium, potassium and phosphorous.
“Aluminium toxicity is the most common type, which results in poor root development and can restrict rhizobia from performing,” he said.
“About 20pc, or 2.1 million hectares, of ag soils in SA are prone to acidification of soil to below 5.5 pH, mainly in high rainfall and high yielding areas, and if we keep farming the way that we are, that area will increase to 3mha in the next 10-50 years.
“In the past 12 months, we have seen soil acidity pop up in patches on the Yorke Peninsula in lentil crops and on sandier soils north of Mallala. Given the high yielding year we have just had, growers have to be aware that a lot of lime has been taken out of the soils that needs to be replaced.”
Mr Hughes said in a 6 tonne/ha wheat crop, up to 54 kilograms of lime equivalents could be removed, while oaten hay at 4t/ha could remove up to 100kg LE.
“Acidity from nitrogen fertiliser can also occur directly where the fertiliser contains ammonia or indirectly from nitrate leaching,” he said.
“In most continuous cropping operations, with high N inputs, acidification can occur at up to 250-350kg of lime/ha/year, highlighting that cropping soils could acidify rapidly if lime is not applied.”
In SA trials – a no-till barley crop sown into sandy soil at Tungkillo in 2015 – the application of lime showed a yield increase of 0.5t/ha two years after spreading.
Mr Hughes said a response from lime generally only occurred in the second year after application, but would last for several years.
“Lime is slow moving into soils, about 20 millimetres a year,” he said.
“It pays to apply a decent amount of lime when you do it, as the cost of spreading and carting can be more expensive than the lime itself.
“More than 2.5t/ha of lime is required on an acid topsoil to get any subsurface impact past 10 centimetres, although on sandy soils rates may need to be kept lower if manganese is marginal.
“Growers should take a 10-20cm sample to test for subsurface soil acidity (test pH) to really know if there’s a problem.”
Prior to seeding, Mr Hughes said growers needed to be mindful of plant sensitivities when looking at sowing in areas where acidity was becoming an issue.
Some wheats, subclover and vetch were tolerant of acidity, while oats, triticale and lupins were very tolerant.
But Janz wheat and canola were sensitive to acidity, while durum wheat, most barley cultivars, faba beans, lentils and chickpeas were highly sensitive.
“Some cropping trials have shown up to 40 per cent response in some situations when lime was applied, particularly with sensitive species, such as canola, barley and intolerant varieties of wheat,” Mr Hughes said.
Mr Hughes said it paid to obtain purity information from each source of lime.
“There are many types of lime sources in SA, from crushed rock sources (dolomite and limestone), lime sands in coastal areas, lime/dolomite scraped from lake beds, such as the Agricola products, and other formulated products, such as Calcipril, Magprill and liquid limes – the formulated products can be expensive,” he said.
Mr Hughes said new online decision support tools had been developed to assist in comparing treatment options.
“There is also precision pH mapping technology available – Rural Solutions SA offers a mapping service using its Veris pH and EC mapper – which can help provide information for more targeted lime applications,” he said.
- Details: agex.org.au/project/soil-acidity