Biochar is attracting increasing scientific, political, and industry attention for its potential benefits as a soil amendment. Issues such as food security, declining soil fertility, climate change adaptation, and profitability, are all drivers for implement­ing new technologies or new farming systems. Application of biochar to soil has been shown to have effects ranging from very positive, through to neutral and even negative impacts for crop production. It is therefore essential that the mechanisms for action of biochar in soil be understood before it is applied.

The application of biochar to soil can influence a wide range of soil constraints, including low pH and high available Al [51], soil structure and nutrient availability [13], bioavailability of organic [56] and inorganic contaminants [25], cation exchange capacity (CEC) and nutrient retention [33, 46], and organic matter decline [32]. Biochars have a highly porous structure with surface areas sometimes exceed­ing 1,000 m2/g [18]. Like activated charcoal, they adsorb organics, nutrients and gases, and are likely to provide habitats for bacteria, actinomycetes, and fungi [49]. Increases in water holding capacity following biochar application to soil have been well established [11, 41] . and this may influence crop production, soil microbial populations, and population flux during wetting/drying cycles.

Soil constraints where biochar may provide benefits to productivity include:

• Low pH and high Al availability.

• Low CEC and nutrient holding capacity.

• Low water holding capacity, poor infiltration.

• Poor soil aeration, root development.

• Hard setting soils.

• Residual herbicide or heavy metal phytotoxicity.

• Presence of certain soil-borne diseases.

In some cases, biochar application to soil may influence nutrient availability and nutrient use efficiency [54]. The application of a low nutrient biochar derived from timber increased the retention of N in soil and uptake of N into crop biomass [48]. Lehmann et al. [30] showed that biochar reduced leaching of NH4+, maintaining it in the surface soil where it is available for plant uptake. Similarly, the application of charcoal derived from bamboo into a sludge composting system was shown to provide significant increases in N retention in the compost [25] . Increased fertility of soil resulting from biochar application is likely to increase crop vigor, and thus may enhance disease tolerance.

Biochar is also likely to influence a range of soil physical properties. For example, [11, 13] demonstrated significant declines in soil tensile strength following addition of biochar derived from green waste or pecan shells. These declines in soil tensile strength may allow for better crop root penetration (especially during dry periods), and will also reduce costs associated with soil preparation (such as tillage).

Biochar has been shown to increase biological N2 fixation (BNF) of Phaseolus vulgaris [44], largely due to greater availability of plant micronutrients following biochar application. By increasing potential for BNF, and increasing N use efficiency, lower rates of synthetic N fertilizers may be acceptable for maintaining productivity. Synthetic N fertilizers have a significant C footprint, with over 4t CO, emission required per t N fertilizer produced [55].

Although there is a paucity of published data on the effects of biochar on soil-borne pathogens, evidence is mounting that control of certain pathogens may be possible. The addition of biochar (0.32, 1.60, and 3.20% (w/w)) to asparagus soils infested with Fusarium root rot pathogens increased asparagus plant weights and reduced Fusarium root rot disease [19]. Further, Matsubara et al. (2002) (cited in [49]) have shown that biochar inoculated with mycorrhizal fungi are effective in reducing Fusarium root rot disease in asparagus. A study of bacterial wilt suppression in tomatoes found that biochar derived from municipal organic waste reduced the incidence of disease in Ralstonia solanacearum infested soil [38]. The mechanism of disease suppression was attributed to the presence of calcium compounds, as well as improvements in the physical, chemical, and biological characteristics of the soil. Likewise, Ogawa [57] describes the use of biochars and biochar amended composts in reducing bacterial and fungal soil-borne diseases.

The economic value of biochar as an agricultural commodity is largely untested. Although the benefit of biochar in many systems has been described to increase crop yield, the cost-benefit ratio of applying the technology has not been completed. Van Zwieten et al. [52] discusses several mechanisms for valuing biochar as a commodity. Simply, it could be valued based on its nutrient or liming value, replacing commodities such as fertilizer or lime, alternatively, it could be valued according to benefits to productivity or projected productivity. A recent study [6] using biochar derived from Eucalyptus banded at a low rate of 1 ton/ha was shown to have a breakeven valuation of around Aus$170 per ton of biochar in broadacre wheat, assuming yield benefits for 12 years. In the cost-benefit outcome described by Van Zwieten et al. [52], biochar derived from poultry litter waste was valued at $300 per ton, based on the performance enhancement of three crops following the single
application of biochar. Clearly, the economic value of biochar will depend on its properties, but will also be driven by supply and demand, inherent value of the target enterprise, and demonstrated benefits.