Best-practice approaches for liming have emerged from new yield response curves plotting the hip-pocket cost of acidity in soil surface layers, to a depth of 10 centimetres.
Leading the study, Southern Farming Systems (SFS) research and extension officer Lisa Miller measured the effects of lime top-dressed on 14 cropping paddocks, sown to cereal, oilseed and legume crops, from 2014 to 2017.
Ms Miller says a large data set collected from the trials in minimum-till farming operations across south-west Victoria, from the Bellarine Peninsula to Hamilton, was used to develop the yield response curves.
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One of these long-term, surface-applied lime trials was established on the Casanova family's property at Westmere.
To put a dollar figure on grain losses resulting from soil acidity, the research team calculated the yield difference where no lime was applied, and where lime was applied at three tonnes per hectare.
Applied at this rate, liming effectively eliminated topsoil acidity as a constraint to crop productivity, highlighting the cost-benefits of surface liming based on the yield gap between acid and non-acid soils.
Yield losses stemming from acid topsoils were also measured against commodity prices to show the cumulative cost of surface soil acidity over a three-year crop rotation.
For instance, a trial site on the Brown family's property at Drysdale, Victoria, showed that where no lime was applied to acid soils, with a pH of 4.2, the cumulative cost of soil acidity over a three-year rotation of barley/canola/wheat was $734 per hectare.
This figure was based on the study findings that acid soils reduced their 2014 barley yields by one tonne per hectare; 2015 canola yields by 0.4t/ha; and 2016 wheat yields by 1.2t/ha.
Commodity prices achieved for 2014 barley, 2015 canola and 2016 wheat were $278/t, $531/t and $200/t respectively.
"Yield penalties as a consequence of soil acidity can occur each year, and the costs of these penalties can accumulate, especially given the current price of crop inputs," Ms Miller says.
"Yield penalties as a consequence of soil acidity can occur each year, and the costs of these penalties can accumulate, especially given the current price of crop inputs."
Compared with the potential cumulative loss of more than $700/ha over a three-year period, surface liming can 'quickly pay-off', she says.
This is because the $150/ha cost of surface liming, including the price of spreading lime at 3t/ha, can help overcome yield losses resulting from topsoil acidity for seven or more years.
But she adds that these long-term amelioration benefits are limited to the topsoil, to a depth of 10cm, and also depend on the soil's acidification rate.
"Yield responses shown in the response curves reflect where surface liming has ameliorated the topsoil, mainly at depths of five to eight centimetres, after four years," she says.
Although liming can provide benefits for seven years, Ms Miller urges growers to consider applying lime to the soil surface at shorter intervals to help counter the effects of acidity deeper in the soil profile. Amelioration benefits might start to subside after three to four years.
The study also looked at the effects of surface liming on deeper soil layers - from 10 to 20cm, and 20 to 30cm - at 100 crop and pasture trials between 2014 and 2018.
Ms Miller says a 'surprise finding' of this deeper investigation was that acidification is also occurring deeper down in the soil profile.
And for surface-applied lime to reach these deeper soil layers, the topsoil needs to be saturated with lime, at least achieving a soil pH of 5.5 in the top 10cm.
More frequent surface lime applications, as often as every three years, are also encouraged because lime moves slowly down through the soil profile - at a rate of about 2cm a year in medium to high rainfall areas.
So it takes about five years for surface-applied lime to move down about 10cm in the soil profile - where it just reaches the subsurface layer from 10 to 20cm. It takes 10 years for it to move down into the subsoil layer from 20 to 30cm.
For this downward lime movement to occur at all, however, topsoil pH needs to be maintained at 5.5.
"The findings suggest that lime movement beyond a soil depth of five centimetres could lift yields by 10 to 20 per cent," Ms Miller says.
Another finding of the study was that lime moved slowly irrespective of the amendment type. It was previously thought that soft rock limes commonly used in south-west Victoria were faster-moving than hard rock limes.
The research showed that while the coarse particles of soft rock lime were initially more soluble and, in turn, fast-moving than their hard rock counterparts, they ultimately moved down through the soil profile at a similar, longer-term rate.
"Some growers allow their soil pH to drop to 4.5 before liming, but using this approach, it's likely they have incurred yield penalties for some time and driven acidity further down the soil profile," Ms Miller says.
"Acidity leaches to lower depths in the soil profile - its subsurface and subsoil layers - if topsoil pH is less than 5.5."
Ms Miller also suggests targeting a soil pH of 4.8 in deep soil layers from 10 to 20cm, and 20 to 30cm.
Below this pH threshold of 4.8, she says aluminium seems to become soluble, causing toxicity that impacts plant root growth at depth in the soil
Lime's low solubility and slow movement means yield responses can also be slow to emerge in the first season after surface lime applications.
Opting to grow more acid-tolerant crops, such as wheat and lupins, can help to mitigate this sometimes discouraging effect, Ms Miller says.
Incorporating lime with tillage, which can help speed up the amendment's effectiveness, is being tested in a new GRDC-funded trial program starting this year.
GRDC Research Code: 222026
More information: Lisa Miller, firstname.lastname@example.org, 0488 600 226