Sifting soil carbon truth from fiction and theres a lot of it

Soil carbon methodology often leaves something to be desired

A once cropped low fertility paddock with many years of unimproved pasture. Big potential for improving soil carbon.

There is science-based evidence that improving pastures and managing them well is a sound way to build soil organic carbon (SOC).

The difficulty is there are many associated claims, for example using a specific product on a pasture, practising a specific grazing management, or a given multi species pasture mix, that are also critical for building SOC. Claims are commonly supported by prominent people, but what is commonly missing is science to back up such claims.

The challenge, for research and industry, and for those who will pay SOC credits, is reliable and cost effective measurements of changes in SOC. Research is currently focused on these aspects. We, like many farmers, are keen to be involved in accredited carbon improvement schemes. But, even though approached by brokers, we have instinctively cautioned some of the methodology advocated to us.

For example we were told our few periodically taken soil samples would be sufficient for setting the SOC base level. Clearly not. Schemes need to be soundly based on detailed and extensive soil analysis. Schemes also have to take into consideration, even if long term build up is occurring, that SOC varies considerably for factors including variability through the year, variability from year to year because of factors such as seasonal differences, and adverse impacts of droughts and fires.

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Research, which involves detailed soil sampling and analysis, conducted over many years, and published in credible science journals, has noted that upgraded pastures can result in good build-up of soil SOC. Logic and research indicates the biggest margin for SOC build up is likely from run down paddocks, perhaps from many years of prior cultivation and little pasture improvement.

One particular research project, conducted on the Avendano property at Boggabri, noted large increases of SOC on an area with a long history of cultivation and little pasture improvement (mainly unfertilised native grass with no legumes added). Research was undertaken by Dr Rob Banks (Principal soil scientist Soil Futures Pty. Ltd), for his PhD. Major findings included that SOC increased from 58 tonnes/ha on unimproved native pasture to 84t/ha on the tropical grass (Premier digit predominately) in the 0-90cm soil layer.

Farmers inspect a high quality tropical grass plus winter legume pasture on the Avendano properties, near Boggabri.

The research assessed identical sodic duplex soil profiles comparing 14-year-old tropical grass with unimproved native grass. Comparisons were assessed for attributes via analysis of the profiles to 1.5 metre depth. Improved tropical grass pasture had phosphorus and sulphur deficiency corrected by superphosphate and adding of winter legumes, that included sub clover, serradella, biserrula and rose clover. These as part of the pasture contributed to nitrogen build up and greater pasture mass.

There are other than superphosphate ways to correct soil sulphur and phosphorus deficiency. For example feedlot and poultry manure, provided used at high enough rates to supply similar amounts of deficient elements to what products like superphosphate supply at normal rates around 100 to 150 kg/ha.

Another example of research showing SOC benefit via improved pasture was the long term "MASTER" trial near Wagga Wagga. Despite seasonal fluctuations in SOC stocks and severe droughts, on average there was an increase in SOC of 0.5 t C/ha/year in the most productive annual and perennial pastures. These also involved superphosphate use and winter legumes.

Research has often shown that fertiliser products that don't supply sufficient levels of elements like sulphur or phosphorus to correct soil deficiencies neither impact on production or SOC.

Matthew and George Avendano inspect one of their over 30 paddocks of tropical grass pasture. Much of this country has changed over the last 30 years.

A likely scenario, requiring more research, is that improved pastures SOC gradually slows and then reaches its likely upper limit. In other words long term well improved pastures probably have less capacity to build SOC than currently low quality pastures.

A closing note is that Australia has some excellent internationally accredited soil scientists who understand soil carbon issues to the highest level. Governments will surely use their advice for accredited Australia carbon schemes.

Next week: Pasture fertiliser timing.

  • Bob Freebairn is an agricultural consultant based at Coonabarabran. Email or contact (0428) 752 149.

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