Dissolution of K, Ca, and P from biochar grains in tropical soils

Many biochars contain considerable amounts of plant nutrient elements in both water soluble and insoluble compounds but little is known of the rate of release of these elements to soil. This laboratory research incubated nine biochars produced from tropical plant wastes in ten tropical soils for eight weeks and measured changes in the amounts of K, Ca and P remaining in the biochars. Some K (8–64%) was soluble in water and 0–75% of this rapidly diffused into the soil together with 0–40% of the K present in poorly soluble compounds. Calcium in biochar was mostly insoluble and was conserved within grains. Considerable soil Ca was adsorbed by biochar due to exchange of Ca from soil induced by the large amount of K released to the soil from biochar. Some (0–75%) of the P in the biochars dissolved in soil including some of the water insoluble P. These various behaviors of plant nutrient elements can be explained on the basis of the diverse crystalline compounds present in biochars and their location within the micro-porous fabric of biochar. Clearly the behavior in soil of these plant nutrient elements in biochar is dissimilar to their behavior in common chemical fertilizers so that appropriate agronomic management strategies must be developed for biochar fertilizers. Soil properties cannot be used to predict the movement of elements from and into biochar grains.