Mobilisation of rock phosphate by surface application of compost

The poor solubility of rock phosphate limits its usefulness for crop production, but it has been shown that combined application of rock phosphate with plant residues can increase its P availability compared to rock phosphate alone. To investigate if compost mulch has a similar effect as residues, the effect of two rock phosphates [Nutriphos Guano Powder (RPA) and Indian Potash Limited (RPB)] with or without two different composts C1 (from garden waste) and C2 (from wood chips and bark), on soil P pools and P uptake by wheat was evaluated in a 75-day glasshouse experiment in a loamy sand soil. The composts were applied as a 2.5-cm-thick layer on the soil surface and rock P was mixed in the soil at a rate of 0.250 g kg(-1) soil (equivalent to 35 mg P kg(-1) of RPA and 26 mg P kg(-1) soil as RPB). Further treatments included an unamended soil and a soil amended with soluble P (KH2PO4) at a rate of 50 mg P kg(-1) soil. Wheat was sown and harvested after 42 and 75 days. At sampling, the compost layer was removed and the underlying soil was sampled. Soil pH and total organic C concentration were not affected by the amendments. Soil respiration was significantly higher in soil amended with composts with or without rock P compared to unamended and inorganic fertiliser treatments. On day 75, addition of composts alone or with rock P increased plant growth and P uptake by 30-50 % compared to the unamended soil, which was greater increase than with soluble P. Compared with either rock P or compost applied alone, the combination of rock P with compost increased the concentration of NaHCO3-Pi and microbial P by about 30 %, suggesting that compost increased the mobilisation of rock P. The combination of compost and rock P also changed the concentrations of non-labile P pools compared to rock P alone indicating effects on P flux between the pools. This study showed that surface compost application can increase P mobilisation from rock P, but plant growth and P uptake were not increased compared to compost alone indicating that the composts supplied sufficient P to the plants.