The buried cost of soil carbon credits

Carbon credits from carbon stored in the soil on farms may not be as valuable to farmers as hoped, according to CSIRO.

Carbon trading will generate extra costs for agriculture, including increased fuel and fertiliser costs. Soil carbon credits have been seen as a possible way to offset these costs.

Carbon is locked up in soil in humus, a stable form of organic matter. However humus also locks up nitrogen (N), phosphorus (P) and sulphur (S) – elements essential for healthy plant growth.

Using relevant research done over 50 years ago Dr Mark Peoples and other CSIRO scientists determined the value of N, P and S locked up in humus.

They estimate that to replace nutrients stored in a tonne of humus farmers would have to add about 60kg of N, 12kg of P and 9kg of S – about $200 worth of fertiliser.

If an estimated 2.2 tonnes of carbon dioxide is stored in each tonne of humus and if carbon dioxide is valued at $20 a tonne, the value of carbon dioxide stored in a tonne of humus is therefore about $44.

Thus the overall cost of additional fertiliser, $200, will outweigh the value of the soil carbon credits, $44.

Nutrient systems are complex and CSIRO acknowledges this figure could be out by a factor of two, but the figures do cast doubt on the value of soil carbon trading.

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