Fertiliser Phosphorus Diffusion and Availability: Recent Work on Movement in Soil

Keywords: phosphorus (P) use efficiency, phosphorus (P) mobility, phosphorus (P) fractionation, phosphorus (P) pools.

This paper presents and discuss current research a) regarding phosphorus (P) mobility after the application of mineral fertilisers, b) importance of soil volume fertilised on yield and c) results on movement and availability of P after the application. The P fertiliser sources under investigation were an indigenous phosphate rock (PR), triple superphosphate (TSP, calcium [Ca]-P), di-ammonium or mono-ammonium phosphate (DAP or MAP, NH₄-P), potassium pyrophosphate (poly-P) and two nitrogen-phosphorus-potassium compound fertilisers (NPKs) produced from different routes: a nitrophosphate NPK and an agglomeration-type NPK (wet-blend NPK). The products were applied to two tropical acid soils from Brazil (contrasting soils in its texture) and one from Colombia. Strategies to improve P use efficiency by influencing distribution of P in soil and amount and nature of the applied P forms by means of combining P sources are presented and discussed.

The results shows that the potential movement of P (soil volume affected) and its distribution in the soil profile (concentrated areas) vary between different P fertilisers. In agreement with previous studies, only P from water-soluble P fertilisers have the potential to move into the soil profile. For the soils investigated the P movement follows the order of poly-P > NH₄-P > Ca-P = nitrophosphate-NPK = wet-blend NPK >> PR.

Regarding the characterisation of P availability in the affected soil volume after two weeks of application, with the exception of PR all P fertilisers contributed mainly and in the same quantity (ca. 60%) to the readily plant available P pool. Phosphorus recovered in the moderately plant available pools related to Ca- (medium-term supply of P at the root surface) and aluminium (Al)- and iron (Fe)-associated P (long-term supply of P) differed considerably depending of the P fertiliser used. Regarding the Ca-associated P pool, a pool of especial importance in acid soils, only P from PR and the nitrophosphate-NPK were recovered. This fraction was localised in the first mm close to the point of application and represent unreacted product. Contribution to the Al- and Fe-associated P pool, a pool considered more as a sink than a source of P in acid soils, was in the order of Ca-P > wet-blend NPK > NH₄-P ? nitrophosphate-NPK > poly-P > PR. The contribution of P to this pool represent a reaction with the soil constituents and in agreement with other studies was affected by the cation (i.e. Ca, NH₄, K) present in the formulation of the fertiliser. All P fertilisers contributed between 5 and 10% of the total—P applied to the non-available P fraction (extracted with strong acids), a fraction representing precipitates and secondary minerals.

Considering that all different P-pools play an important role to maintain a continuous supply of P (short-, medium- and long-term availability of P) to feed the plants during the crop season. The previous data indicate that this might not be attained by using a single straight P fertilisers. The presence of different P forms in the same fertiliser granule, similarly to the one attained in the nitrophosphate-NPK, contributes differentially to each soil P pool. It supplies P to the readily plant available pool for crop establishment, it reduces the complexation with Al and Fe hydro oxides and it allocates rich Ca-P sources with the potential to constantly supply P, especially at the root surface, not only at early crop stages but during the entire crop season. All together it enhances the P use efficiency from mineral fertilisers.

Andrés Felipe Rangel Becerra, Luis Omar Torres Dorante, Michael Basten and Joachim Lammel, Yara International ASA, Research Centre Hanninghof, Hanninghof 35, D-48249 Dülmen, Germany

28 pages, 13 figures, 1 table, 76 references.