40 As a result of the tendency of phosphate to precipitate and adsorb in the topsoil layer, only some 60% of the agricultural phosphate consumption in the Netherlands is eï¬ectively used for the production of crops, animal products and animal feed (123 700 t P2O5). The remaining 40% of the agricultural phosphate consumption accumulates in agricultural soils (77 900 t P2O5), or gets lost through surface run-oï¬ (6 900 t P2O5). These ï¬gures represent average values for the Netherlands for 2005, and actual values vary from place to place depending on the type of agricultural practice and the local soil properties and topography. Nevertheless, these values reveal that substantial phosphate accumulation in the Dutch agricultural soils is still ongoing. Although a small part of the phosphate will have moved into the deeper soil layers, the bulk of this phosphate is still present in the upper soil layer (roughly the top 0 â 40 cm). To give an idea of the total amount of phosphate present in Dutch agricultural soils, we used the density corrected relative enrichments from Van der Veer, 2006. These values represent the excess or surplus of P2O5 in the topsoil compared with the concentrations in the deeper, undisturbed soil layer (see Table 5.4). From this it is estimated that the top 20 cm of agricultural soils contain 1.9 Mt of P2O5 per ha, which excludes the phosphate that is naturally present in the diï¬erent soil types. This phosphate is present in the ï¬xed and occluded, adsorbed and organic phosphate pool (see Table 5.4). In a study of Reijnveld et al. (to be published) it is estimated that about 4500 kg P2O5/ha has been accumulated. The calculation shows that the accumulated amount of phosphate in the topsoil layer is equal to ~40 years of the current inorganic phosphate fertiliser consumption (46 000 t P2O5/y in 2005). More data on the P surpluses from manure and fertilisers are available on: http://www.cbs. nl/nl-NL/menu/cijfers/statline/zelf-tabellen-maken/default.htm and http://www.pbl.nl/nl/dossiers/Mest_en_Ammoniak/feitenencijfers/ index.html. The phosphate that is present below the plough layer, albeit a small amount, will however not be available for crops. Although a part of the accumulated phosphate fraction can become available for plants, the relevance of the accumulated phosphate fraction for agriculture greatly depends, among other things, on the local soil properties and the relative size of the diï¬erent phosphate pools. Nevertheless, this oï¬ers opportunities to decrease application of phosphate fertilisers in the Netherlands without a substantial loss of crop yield. Table 5.4: Estimated total amount of accumulated P2O5 in the Dutch soils. Calculations based on a median P2O5 concentration in the subsoil of 0.27 wt% and a soil density of 1600 kg/m3 as used by Van der Veer (2006). The corrected enrichment is the relative density corrected enrichment as determined in diï¬erent soil types (for details see Van der Veer, 2006). Soil type Sand Silt/clay Peat Total EnrichExcess P2O5 ment (%) (mg/kg) 253 198 207 0.070 0.054 0.057 Area (103 ha) 984.0 1,003.6 172.2 2,159.8 Volume (109 m3) 2.00 2.00 0.34 4.30 Mass (1000 Mt) 3.15 3.21 0.55 6.91 Accumulated P2 O5 (1000 t) 865 883 151 1900 The application of nutrients such as phosphorus (P) and nitrogen (N) has signiï¬cantly aï¬ected the quality of the environment through Pagina 49
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