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There is high pressure on agricultural soils to produce more food to satisfy humanity due to global increase in human population. Thus, practices such as biochar addition to soils to improve crop production are increasingly being adopted, especially in the tropics where agricultural soils are, in general, highly weathered and less productive. This review aimed to explore, using existing literature, the factors that affect the agronomic effectiveness of biochar amendment, the changes in soil properties following biochar application, and the effect of such changes on crop growth. Several studies on biochar have shown that biochar application influences the physical, chemical and biological properties of soils, with most studies reporting increases in soil water holding capacity, pH and microbial activity. Changes in soil properties, following biochar application, are largely influenced by the initial properties of the soil, as well as the characteristics of the applied biochar. Typically, compared to fertile soils, less fertile soils are more positively influenced by biochar, in terms of soil modification for crop growth. Nutrient-rich biochars are also the most effective biochars on these less fertile soils. Crop growth benefits resulting from the use of biochar are generally due to the conditioning effect of biochar on soil properties, direct nutrient addition from the applied biochar and/or improvement of nutrient use efficiency, especially when combined with other amendments. Biochar application could sometimes have no effect or negative impact on the soil and this consequently affects crop growth and development. Long-term field experiments are required to improve research understanding of the mechanisms involved in the use of biochar for crop production purposes.
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