An analysis by McHenry (2012a, b) quantified the po­tential of using biochar as a soil amendment to displace annual applications of single superphosphate (SSP)

(0% N, 8.8% P, 0% K, 11% S) in wheat cropping systems in WA. The analysis assumed two biochar applications over a 15-year period, applied in year zero, and year eight. The analysis ignored all production inputs and outputs, and only calculated the difference between us­ing an average "full rate" of SSP (90 kg/ha), and a "half rate" of SSP with deep banded biochar equivalent to 1 t/ha. The 45 kg/ha year half-rate SSP application is approximately equivalent to an annual application of 4 kg of P/ha. The simplified analysis assumed that the use of either method would achieve an identical wheat yield, negating the requirement to model wheat prices. The application cost of deep banding the biochar (tons per hectare per application) was assumed to be $110. The annual application costs of both the rates of SSP were assumed to be $20/ha, goods and services tax (GST)1 inclusive. A range of biochar prices (deliv­ered to farm, per ton) was analyzed: $0, $50, $100, $150, $200, $250, $300, $350, $400, and $450/t. Similarly, a range of SSP costs (delivered to the farm, per ton) were calculated: $250, $300, $350, $1250. A carbon price was included in the analysis, and was analyzed at intervals of $5 tCO2-e, between $0 and $100 tCO2-e. The analysis assumed a 0.8 carbon fraction recalcitrance. A real dis­count rate of 8% p. a. was used, and all capital and main­tenance costs were based on average current prices and were GST inclusive. In summary, the results showed that without a carbon value the "half rate" of SSP (45 kg/ ha year) and biochar (1 t/ha application) were only cost competitive with the full rate of SSP (90 kg/ha year) when the biochar purchase price was unreasonably low (< ~ $20). At 2012 prices of SSP (generally between $200 and $450/t), the choice of using half SSP application rates with biochar additions at the above application rate assumptions were not an attractive option unless

the biochar purchase price was practically zero. The net cost was also calculated assuming the carbon in the biochar was eligible in soil carbon markets, and the various potential prices of carbon were subtracted from the gross biochar purchase price. While the intro­duction of a carbon price would effectively subsidize biochar costs, very high carbon prices (>$100/t) were required for the sequestration value of biochar to simply equal the purchase price and cover costs of soil application (McHenry, 2012a). The low SSP price, the high market prices for biochar, and the high bio­char soil application cost of deep banding relative to conventional broadcasting, all resulted in the option of halving SSP applications by using biochar an unat­tractive practice.