Hardwood and Poultry Litter Biochar Effects on Phosphorus Sorption of Soil: II.

Land application of biochar prepared from animal and plant sources is report to increase phosphorus sorption as well as have beneficial effects in supplying nutrients for plant uptake. The chemical behavior of the soil after biochar application is mostly unexplored although much has been mentioned about the importance of biochar in reducing the excess nutrient loss from various P sources. The objective of this study was to determine the effects of commercially available biochars on P sorption and release from soils after addition of biochar at different rates. Hardwood biochar (HWB) and poultry litter biochar (PLB) were added to two soils (Candler and Apopka series) at three different rates of 1%, 2% and 5% (w/w) each, respectively. Soil and each mixture of soil and biochar at various rates were incubated at 25 ⁰C for 14 days after adding 10 mL of water. At the end of the incubation period, 8 levels of P in the form of KH₂PO₄ solution (ranging from 0 to 100 mg L^-1) were added to each of the treatments. Results showed that after certain limits of P addition, corresponding to a threshold P saturation ratio (PSR), P began to be released from the soils for all treatments irrespective of the nature of the biochar.  However, the maximum P retention capacity of the soils (S_max) increased with increase in biochar additions. The amount of P sorbed in the solid phase was higher for PLB as compared to HWB for all rates of biochar application. The absolute amount of additional P that can be added to a soil as fertilizer will depend on the P retention property of the soil and not on the biochar properties, and therefore would be site-specific. Increase in S_max with increase in biochar rates might be a result of additional moisture (and therefore additional P) being retained by biochar, but this hypothesis will have to be tested further.