SUMMARYThe capability of some plant species to mobilize phosphorus (P) from poorly available soil P fractions can improve P availability for P-inefficient plant species in intercropping. White lupin (Lupinus albus) has been investigated as a model P-mobilizing plant for its capability of enhancing the P acquisition of neighbouring species under P-limited conditions. To date, investigations have led to contrasting findings, where some reports have described a positive effect of intercropped lupins on companion plants, whereas others have revealed no effects. This review summarizes the literature related to lupin–cereal intercropping. It explores the underpinning mechanisms that influence interspecific facilitation of P acquisition. The P-mobilization-based facilitation by lupins to enhance P-acquisition of co-occurring plant species is determined by both available P concentration and P-sorption capacity of soil, and the root intermingling capacity among two plant partners enabling rhizosphere overlapping. In lupin–cereal intercropping, lupin enhances the below-ground concentration of labile P pools through mobilization of P from sparingly available P pools, which is accomplished through carboxylate exudation, where neighbouring species acquire part of the mobilized P. The non-P-mobilizing species benefit only under P-limited conditions when they immediately occupy the maximum soil volume influenced by P-mobilizing lupin. Positive effects of mixed cropping are apparent in alkaline, neutral and acidic soils. However, the facilitation of P acquisition by lupins to companion species is eliminated when soil becomes strongly P-sorbing. In such soils, the limitation of root growth can result in poorer root intermingling between two species. The P mobilized by lupins might not be acquired by neighbouring species because it is bound to P-sorbing compounds. We suggest that the lupins can be best used as P-mobilizing plant species to enhance P acquisition of P-inefficient species under P-limited conditions when plant species are grown with compatible crops and soil types that facilitate sharing of rhizosphere functions among intercropped partners.
Sri Lanka is an agricultural country totally depending on imported fertilisers for paddy cultivation. Importation of fertilisers including triple superphosphate (TSP) is a heavy burden on the country's national economy. Hence, optimum utilisation of the locally available Eppawala phosphate deposit (EPD) will save a signifi cant amount of foreign exchange. The objective of this study was to assess the eff ectiveness of locally produced Eppawala single superphosphate (ESSP) as a source of phosphate fertiliser for rice in comparison to TSP. Small tonnage of ESSP was produced for the purpose of experiments using fi nely ground Eppawala rock phosphate (ERP: 90 % < 150 μm) mixed with 70 % sulphuric acid stoichiometrically, allowing the resulting slurry to solidify. Resulting product granulated after curing for 2-3 weeks was characterised by X-ray fl uorescence (XRF), X-ray powder diff raction (XRD) and the available phosphorus was determined. The available phosphorus was found to be in the range of 20-22 %, which is far in excess of the stipulated requirement of 17 %. Agronomic eff ectiveness of ESSP over the imported TSP was tested for rice cultivation in diff erent soil conditions in the dry, wet and intermediate zones of Sri Lanka in fi ve consecutive seasons. The results showed that important parameters such as the grain yield and root dry weight in plots applied with ESSP as a source of phosphorus were comparable with that of TSP applied plots. Application of TSP and ESSP showed similar results in dry and intermediate zones of the country. Therefore, if ESSP is produced in suffi cient quantities the importation of TSP could be reduced without much eff ect on agronomic eff ectiveness for rice cultivation.
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