FARMERS say the Coalition is dramatically underestimating the cost of greenhouse reductions from soil carbon, which makes up 60 per cent of its direct action plan to cut emissions.

Alongside warnings the Coalition could need to pay up to $500 million a year to subsidise power prices, the new estimates raise questions about whether the Opposition Leader, Tony Abbott, could achieve a 5 per cent cut in emissions by 2020 within his capped $10.5 billion budget for the direct action plan.

Same for less ... Tony Abbott's sums come under the spotlight. Credit:Alex Ellinghausen

As the government begins its $12 million advertising campaign tomorrow night to combat voter opposition to its incoming carbon tax, Mr Abbott is assuring voters his direct action fund could achieve the same emission cuts without extra costs for businesses or families.

But Michael Kiely, the chairman of the Carbon Farming and Trading Association, said: ''Australian farmers won't line up to sell an Abbott government soil carbon at bargain basement prices.''