Sue Briggs, DEPI Rutherglen
In the last few years, carbon sequestration has been a hot topic in the farming sector. Having high Organic Carbon (OC) levels can, in some soils, increase water holding capacity, improve cation exchange capacity and improve soil structure and reduce slaking.
As dairy farmers in a high rainfall region it is likely that soil OC levels could already be fairly high, particularly compared to other farming sectors. The reason why many dairy farms could have high OC levels is a result of common 'best practices' used such as fertiliser application, rotational grazing and improved perennial pasture species. In some cases the soil OC could already be close to peak levels, in which case the aim would be to try and maintain the level.
Building up the OC levels in soil can take a long time, however they can be reduced relatively fast by certain management practices. It may therefore be more beneficial to focus effort on ways to minimise and avoid reductions in soil OC levels.
What is organic carbon?
Soil organic matter consists of anything in the soil of biological origin (i.e. roots, fungi, bacteria etc). Organic carbon (OC) is a measure of the carbon found in soil organic matter. It is reported as a percentage on your soil test results.
Soils with OC levels less than 1 per cent will show evidence of slaking.
Table 2: OC targets for high rainfall areas:
|Range||OC level (%)|
Organic carbon breakdown
Organic carbon is divided into different 'pools' that are defined according to their rate of decomposition (seefigure 1).
A standard soil test is unable to distinguish between the soil OC pools. However, knowing howthese pools store and lose OC will provide an appreciation of the implications that managementpractices have on organic carbon levels.
Can every dairy farmer achieve high OC levels?
The amount of carbon that can potentially be stored in the soil is determined by soil type, climate and management. Generally the higher the production the higher the potential OC.
The potential storage of OC in soil depends on the soil type. Clay particles make it harder for microbes to breakdown organic matter, therefore the amount of OC stored in soil tends to increase with increasing clay content. In lighter soils (i.e. sandy loam) soil microbes are able to readily access OC which causes greater loss of OC by decomposition.
The potential storage of organic carbon in soil is rarely achieved because climate influences plant productivity. High rainfall soils tend to have greater ability to store OC than the same soil type in a lower rainfall area.
Management practices determine the actual storage of OC in the soil by either increasing inputs or decreasing losses. Dairy farms generally comprise highly fertile perennial pasture and where this has been in place for many years there may be limited options to increase OC inputs.
The following are some management practices which may reduce the losses:
- Plant growth – Acidic soils and pugging are factors that limit pasture growth. Regular soil testing and lime application as well as limiting grazing on wet or waterlogged soil are management options to consider to optimise pasture growth.
- Overgrazing – A higher percentage of OC is found in the top 15 centimetres of the soil. Loss of topsoil from erosion caused by over-grazing can reduce OC levels.
- Soil disturbance – If you work up a paddock prior to pasture renovation you are reducing the amount of OC in the soil from the humus pool which is usually the hardest to add. Direct drilling may be an option to consider.
- Perennial vs. annuals - This may be a risk in the future where climatic variability may lead to farmers replacing perennial pasture species with annual species.
Without continual inputs of OC, the amount stored in the soil will decrease over time because the organic carbon is always being decomposed by soil microbes. Under perennial pasture production soil OC levels will remain relatively stable on dairy farms, however certain management practices can quickly degrade levels. Focussing efforts on preventing losses via best management practice will help to maintain OC levels and the associated soil health benefits.