Influence of green manuring and phosphate rock inputs on soil phosphorus cycling and availability

Abstract

In organic farming systems continued application of phosphorus (P) is necessary to maintain crop and animal production, especially in the long-term, if economically viable returns are to be sustained. The most commonly adopted organic farming systems permit the use of sparingly soluble P fertiliser such as phosphate rock (PR). One possible strategy for improving the availability and utilisation of PR in organic cropping is via the inclusion of a legume green manure. The major objective of this study was to examine the influence of green manuring on soil P cycling and availability in relation to the addition of PR. A two-year field trial together with a rhizosphere process study and laboratory incubation experiments were carried out using a Templeton soil (Immature Pallic). In addition, a glasshouse experiment was conducted using three soils with different levels of P and P retention capacity (Templeton, Gammack (Immature Pallic soil) and Craigieburn (Allophanic Brown)). Lupin (Lupinus angustifolius, cv. Fest) was used as the green manure and maize (Zea mays, cv. Elita) was used as a test crop. Results from the glasshouse study showed that the application of PR increased lupin P uptake by 35-38 % compared with the corresponding control treatments. In the Gammack and Templeton soils the combination of green manure and PR significantly increased maize P uptake by 21 % and 48 %, respectively. In the Craigieburn soil, the formation of organo-mineral complexes from lupin incorporation resulted in a 71 % decrease in maize P uptake compared with the fallow treatment. Data obtained from the field experiment confirmed that the combination of green manure and PR significantly increased maize P uptake by 21-27 % compared with the fallow treatments. Results from the rhizosphere experiment showed that PR dissolution in the rhizosphere of lupin was 72 % greater than the adjacent non-rhizosphere soil. This was further confirmed by the isotopic exchange kinetics data that the utilisation of slowly exchangeable inorganic P by lupin was markedly greater than that by maize. In addition to increased dissolution of PR, the inclusion of a green manure also resulted in a 5-fold increase in the rate of gross organic P mineralisation in the Templeton soil as determined by an isotopic dilution technique. The findings of this study clearly demonstrated that the combination of a legume green manure with PR application is an affective strategy for increasing P availability in organic cropping systems.