“Limestone needs to be incorporated into the soil or else it does little to ameliorate acidity,” Dr Mark Conyers told Stock & Land after speaking at the GRDC Grains Research Updates in Bendigo last month.
Is it true? Should we be routinely incorporating lime, rather than just topdressing?
The issue lies with the slow movement of lime.
While we see good pH change in the top 5cm after one year, acidity amendment is slow at 5-10cm depth. This is the case in New South Wales when using superfine hard limes, and is similar to our Victorian GRDC-funded trials using softer but coarser rock limestones.
The chemical breakdown of lime requires acidic conditions, compounding lime movement.
Topdressing the lime quickly increases surface pH up to around 6.0 and this slows further breakdown.
Untreated acidic layers exist from 5cm to about 15cm – or the depth of the topsoil (A horizon). Acidity often extends further down the profile on sedimentary, granite or alluvial derived soils.
Lime movement can be influenced by leaching rainfall, of which we had plenty of last year.
The fineness of lime is another factor which affects lime movement.
Although finer lime might create quicker pH change, the fact remains that it is generally not going to ameliorate sufficient acidity to depths of 10cm within two years if it is top-dressed.
Trials are showing different rates of lime movement across different soil types, which is related to the pH buffering capacity of soils. Lime movement is slower in soils that contain higher organic carbon content, which acts to resist pH change.
Trial results of lime movement after the wet year will be presented at the SFS Trials Results Day at Lake Bolac on March 30.
Evidence presented will help farmers decide when they might need to incorporate or manage topdressing of lime to minimise yield declines.
Trials are supported by GRDC and Corangamite CMA through the federal government.