Lime rates increasing but more needed
Western Australian grain growers are increasing the amount of lime they apply to paddocks, but even higher rates are needed to achieve appropriate pH levels in the soil.
This is the message from Department of Agriculture and Food, Western Australia (DAFWA) researcher Chris Gazey, who says WA soils are continuing to acidify and a cross-industry approach is needed to promote lime as an essential input in WA and other states.
Mr Gazey, whose research is supported by the Grains Research and Development Corporation (GRDC), said evidence from 20 years of WA soil acidity research, development and extension (RD&E) proved that managing acidity was a contemporary issue.
“It makes sense in terms of profitability as well as environmentally to recover and protect the soil resource,” he said.
“There is compelling evidence that growers, consultants and other stakeholders need to remain focussed on managing this problem, which costs WA agriculture more than $500 million per annum in lost productivity.”
Mr Gazey said growers were increasing their use of lime and appropriate soil sampling to identify and prioritise lime application.
“WA growers applied more than 1.6 million tonnes of lime in 2013/14, an increase of almost 50 per cent on 2012/13,” he said.
“However, the evidence clearly shows that soils are continuing to acidity, so even more lime is required, and its application needs to be guided by objective measurement through soil sampling at depth.
“Lime needs to be applied at higher rates and to more paddocks, and its application needs to be better targeted within paddocks.”
Mr Gazey said mapping across south-west WA showed that the severity and extent of soil acidity, including subsurface acidity, was currently poor.
He said evidence supporting improved soil acidity management in WA included:
- Analysis of more than 69 long-term lime trials demonstrating that long-term yield responses of 10 per cent greater yield are common in years following lime application
- Re-evaluation of a 1994 trial site at Mingenew showing that lime rates, previously considered adequate, do not ameliorate soil pH at depth
Mr Gazey said soil acidity was estimated to erode potential crop yields by 9 to 12 per cent, worth $500 million annually.
“About 14.25 million hectares of WA wheatbelt soils are acidic or at risk of becoming acidic to the point of restricting crop yields,” he said.
Soil pH is a measure of the concentration of hydrogen ions (acid) dissolved in the soil water.
Some trace elements become more soluble at lower pH and so does aluminium (Al), which is toxic to plants when in solution.
If the soil pH is too low, the concentration of Al in the soil water increases rapidly and reduces root growth.
Poorer root systems restrict the uptake of nutrients and water and lead to lower plant biomass and grain yields.
When lime is added to soil, it neutralises hydrogen ions.
As the pH is raised, Al becomes less soluble and plants are able to develop effective root systems.
The management of soil acidity in south-west WA has been a topic of RD&E for more than 20 years.
“The most recent assessment of the current condition of the soil resource in this region demonstrates that soil pH has continued to decline,” Mr Gazey said.
“More than 70 per cent of samples taken from the soil surface (0-10cm from the surface) and almost half of samples from the subsurface (10-20 and 20-30cm below the surface) have inappropriate pH levels.
“Appropriate pH levels are defined as being at least 5.5 at the surface and at least 4.8 at subsurface levels.
“The ongoing acidification of agricultural land is the result of productive farming practices – including product removal and nitrate leaching – combined with insufficient use of agricultural lime.”
Grower surveys and workshops
WA grower surveys and workshops were conducted by DAFWA from 2010 to 2013 as part of a project, funded by Caring for our Country, which aimed to improve soil acidity management by WA growers.
A total of 539 growers were surveyed across the WA grainbelt, representing about 1.6 million hectares of land, or 15 per cent of the grainbelt.
“From the growers surveyed, 74 per cent considered soil acidity to be a moderate or greater problem on their farm and in their district,” Mr Gazey said.
“On a positive note, 90 per cent of respondents considered soil acidity to be manageable.”
The principal barriers to applying sufficient lime to all paddocks immediately were economic constraints such as cash flow and the cost of transport.
Survey results indicated that many growers, particularly those who had taken part in Caring for our Country project activities, intended to markedly increase their subsurface soil sampling.
Mr Gazey said the survey revealed that growers in recent years had applied lime as a single rate 75 per cent of the time.
“But in the future, many growers intend to increase the proportion of split rate or variable rate lime applications to about 50 per cent of total applications,” he said.
“Growers indicated that the most common lime rates would increase from 1-1.5t/ha to 1.5-2t/ha.”
Analysis of response to liming
In 1993, a DAFWA database of 69 small plot and large-scale lime trials in the WA grainbelt was investigated to determine factors affecting crop responses to liming.
Soil information and yields from the trials were collected for the period 1991 to 2012.
Mr Gazey said the analysis showed that the average gain from liming, considering all years and crops in the database, was 0.18t/ha or a 10 per cent yield increase.
“When the analysis removed the year that the lime was applied and the following year (lime takes time to react in the soil and responses in the first two years are not expected) there was an average 0.25t/ha or 12 per cent yield increase,” he said.
“The data is similar to most other trials around Australia, although higher responses were found in trials that included a combination of lime and deep ripping.”
Mr Gazey said that in most situations, yield gains from liming appeared to be relatively constant.
“Analysis of the WA data indicates that liming has a 10 per cent chance of achieving a 40 per cent yield increase, a 30 per cent chance of achieving a 10 per cent yield increase and a 100 per cent chance of achieving a 3 per cent yield increase,” he said.
Long-term liming response at Mingenew
In 2013, a DAFWA small-plot lime trial in a paddock at Mingenew – originally conducted in 1994 – was re-examined to investigate the long-term effects of liming.
In 1994, agricultural lime sand was applied to replicated trial plots at rates of 0, 0.5, 1, 2 and 4t/ha. The plots were 1.8m wide by 30m long.
Following the initial treatments, the grower spread 1t/ha per hectare of lime each year in 1998, 1999, 2003 and 2012 as part of normal paddock operations.
“The cumulative application of lime by the grower in these years increased or maintained the topsoil (0-10cm) pH above 5.5-6 in all plots where lime was applied in 1994,” Mr Gazey said.
“But only the plots that received 4t/ha of lime in 1994 had subsurface pH levels close to the target of 4.8 or higher.
“Where the 1994 treatments were 2t/ha or less, the soil pH below 20cm had continued to acidify.
“Not only had the soil continued to acidify, but the depth to the lowest pH was deeper, meaning it will now require more lime and take longer to recover.
“The treatments receiving the lower rates of lime were so acidic at depth that they will require large amounts of lime and mechanical incorporation to recover the soil pH profile in a reasonable time frame (three to five years).”
Mr Gazey said the wheat yield was 10 per cent higher in the treatment that received a total of 8t/ha of lime since 1994, and 6 per cent higher for the treatment that received 6t/ha of lime, compared with the treatment that received 4t/ha of lime.
“The soil pH of the plots that received 8t/ha of lime over the 20-year period almost meets recommended targets,” he said.
“It is important to note that the quantity of lime applied in these plots far exceeds most current and intended lime application rates in WA.”
Mr Gazey said soil acidity continued to constrain WA agriculture, but crop and soil measurements and analyses from long-term lime trials clearly demonstrated the benefits of managing soil pH.
“Liming increases crop and pasture yields where soil pH is below recommended targets, and the maintenance of soil at an appropriate pH is needed to maintain yield potential,” he said.
“If not managed appropriately, soil acidity will continue to cost growers the opportunity to reach their yield potential.”
Mr Gazey said soil acidity management should be viewed not as an encumbrance but as an economically viable opportunity to improve the yield potential and resilience of cropping systems.
New collaborative research
With the GRDC’s support, Mr Gazey and his colleagues are working closely with WA grower groups to test and improve a range of tools and practices to help growers better manage soil acidity.
The soil acidity research is part of a collaborative research effort ‘Soils Constraints West’ which aims to develop and deliver solutions for a range of soil constraints limiting productive grain cropping in WA.
Soils Constraints West represents more than $33 million of new research aimed at addressing non-wetting soils, subsoil constraints, soil compaction and soil acidity over five years.
The GRDC, DAFWA, CSIRO and Murdoch University are funding the research, which was developed following consultation with WA grain growers through the GRDC western regional panel and the GRDC Regional Cropping Solutions Networks (RCSNs).
This new research is in addition to the GRDC’s already substantial western region investment in other areas of soil research including soil nutrition, soil carbon and soil ‘health’.
For more information:
GRDC Soil Acidity in WA Hot Topic at www.grdc.com.au/SoilAcidityWA
DAFWA soil acidity webpages at www.agric.wa.gov.au/soil-acidity/managing-soil-acidity
Chris Gazey, DAFWA
08 9690 2000, 0429 107 976
GRDC Project Code DAW00236, CSA00033