Lime applications – How much should I apply?
The common rules of thumb of applying liming at 1 tonne/acre may commonly result in under-liming, however it can also cause over-liming in light textured soils creating other issues.
Lisa Miller, from Southern Farming Systems who is managing soil acidity projects in South West Victoria, supported by the Australian Government and GRDC, explains “Lime calculation depends on two main factors; treating acidity throughout the soil profile 0-20cm and soil pH buffering capacity.”
“Soil pH buffering is a natural soil process that tries to stop pH change. It is influenced by amount of organic carbon and clay content. Hence topsoils and heavy clay soils need more lime to create pH change and subsurface layers and sandy soils need less” said Ms Miller.
Calculations of lime requirement are frequently based on soil texture which is a surrogate for clay content.
In the 0-10cm soil layer approximately 1 t/ha of pure lime raises pH by:
- 26 units in clay soil
- 37 in clay loam
- 57 in sandy loam
- 67 in sand.
The lime rate is then calculated for the desired pH change. For example, to raise pH by 1 unit, from 4.5 to 5.5 in clay loam soil approximately 3t/ha of lime is needed, in a sandy loam soil it is about 2t/ha.
If soil organic carbon measures above 1.2% which is common in the topsoil, then add an extra 0.4t/ha of pure lime.
As the calculations are based on treating a 10cm soil layer, repeat calculations and add up what lime is required for the 10-20cm soil layer.
Lime calculations also require adjustment based on the lime’s quality. Pure lime has a neutralising value of 100%, most Aglimes are 80 to 90%, and effectiveness can be further reduced by moisture content of 5 to 10%.
Higher rates of lime generally in excess of 2t/ha on sandy soils and 4t/ha for a clay loam can be used if you are incorporating it into the soil compared to top dressing. Top dressing large amounts of lime in sandy soils may cause micronutrient deficiencies, especially if they are already marginal.
To avoid this issue either split high-rate applications over a period of three or four years or monitor and apply appropriate granular fertiliser or foliar sprays as required.
To check out a lime calculator go to: https://grdc.com.au/news-and-media/news-and-media-releases/national/2013/01/online-tools-can-assist-lime-planning