Soil amendment with specially produced biochars is increasingly being recognised as a desirable practice from the standpoints of sequestration of carbon, reducing the emission of greenhouse gases and improving the soil fertility. While the agronomic benefits of biochars are well articulated in literature, little attention has so far been paid towards some of unintended consequences of biochar amendment to soil, especially in terms of their impact on environmental fate and efficacy of herbicides. Biochars, being highly carbonaceous, aromatic in nature and high in specific surface areas, have extra-ordinary capacity to sorb pesticides and potentially deactivate them. We found that incorporation of relatively small amounts of fresh biochars in soil (0.05% by wt.) can inhibit the microbial degradation of pesticides as well as reduce their plant uptake and efficacy. Indeed, biochar amendments to soils may significantly compromise the bioavailability and efficacy of pesticides. We found that 2-3 times higher than normal rates of pesticide application may be needed to gain effective weed or pest control in biochar amended soils.