54 which it dissolves more easily. Another option is to produce K-struvite instead of the more common NH4-struvite, e.g. by an ammoniastripping process followed by K-struvite precipitation, which is a technique that is already available. The agricultural value of K-struvite should be further investigated because its direct application as a fertiliser seems more logical than its current use as a secondary phosphate ore. 6.4.3. Alternative sources of phosphate 1. Enhanced release of the accumulated phosphate fraction from soils Over time, agricultural topsoils have accumulated a large amount of phosphate. As estimated in Chapter 5, phosphate accumulated in agricultural topsoils represents at least 40-90 years of phosphate fertiliser application. It seems logical to see if, and how, this phosphate ‘resource’ can be exploited in the future. Average phosphate offtake is currently 95 kg P2O5/ha on mowed grassland and 50 kg P2O5/ha on arable land (Ehlert et al., 200945). Accumulated residual phosphates from fertiliser and manure are available to crops (Ehlert et al., 200646). How these soil phosphate pools can be used as an available buffer at reduced phosphate use is uncertain. Micorrhizae might possibly be helpful, but its significance to disclose soil phosphate fraction through its extensive network of hyphae and its biochemical alteration of the rhizosphere is still unclear. 45 Ehlert, P.A.I., P.H.M. Dekker, J.R. van der Schoot, R. Visschers, J.C. van Middelkoop, M.P. van der Maas, A.A. Pronk, A.M. van Dam, 2009. Fosforgehalten en fosfaatafvoercijfers van landbouwgewassen : eindrapportage Alterrarapport nr. 1773. Ehlert, P.A.I., Salm, C. van der, & Schoumans, O.F., 2006. Long-term effect of soil of restricted use of phosphate fertilisers. The International Fertiliser Society. Proceedings No: 593. 46 2. Use of phosphate from natural areas In former eras, litter and sods from forests and heathland were applied to fertilise the land. Analogous to this approach, nutrient-rich topsoil that has been removed to speed up nature restoration, could be used to improve the fertility of agricultural land. In principle, any organic waste from nature restoration projects could be applied (e.g. litter, woodchips etc.). This is currently hampered by existing regulations for soil transportation and/or requirements for the application of organic material on agricultural land. 3. Use of aquatic plants as animal feed or as biomass for energy production Eutrophication in temperate climates is usually accompanied by the growth of Lemna species (duckweed) and other plants. Lemna accumulates dissolved phosphate from surface water and the phosphate returns to the water once the plants decay. Harvesting Lemna will therefore decrease eutrophication. After harvesting, these fast-growing plants can be used as relatively cheap animal feed. Aquatic plants can also be used to produce bio-energy, and phosphate from the plants can be recovered as struvite after fermentation (Schuiling, 200647). The economics, including factors such as preventing eutrophication, the price of biogas, and phosphate recovery need to be worked out. The far greater ecological problem of the water hyacinth in tropical countries (Eichhornia crassipes) could be tackled 47 Schuiling, R.D. 2006. Co-vergisting van varkensmest met andere residuen. Spil 223 – 224, nummer 2, pp 17-19. Pagina 63

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