**4. Limitation**

It is complicated to speculate the agronomic effectiveness of biochar with limited studies being conducted in different soil types, climatic belts and land use conditions. Heterogeneous nature as well as production cost of biochar for research and field application will continue to remain a major limitation until commercialscale pyrolysis facilities are established. Some of the experimental constraints on use of biochar in agricultural systems are [38, 58]:

i.Unavailability of sufficient quantity of biochar for large scale use

ii.Susceptibility of dry biochar to wind erosion

iii.Non-adoption of biochar by local farming communities

iv.Unavailability of adequate farm labour

v.Expensive wage costs incurred for collection and processing of crop residues

vi.Lack of appropriate farm machinery for on-farm recycling of crop residues

vii.Inadequate incentives for recycling of crop residue.

Other limitations involve contamination risk of biochar (PAHs, heavy metals, dioxins) when contaminated feedstocks are used or when inappropriate process conditions are used for biochar preparation such as temperature greater than 500°C.Removal of crop residues from the field for biochar production results in its reduced incorporation into soils, hampering many soil properties. In certain cases, extremely high rate of biochar application produces negative effects on earthworm survival rates [59].
