Gypsiferous Amendments: Sequestering Phosphorus in Soils.

Many studies have linked soil phosphorus (P) content (e.g. water extractable P) in grassland and arable fields with surface water quality. Internationally, programmes of measures to mitigate critical losses of P from agricultural fields have focused on soil testing and retro-fitting P applications accordingly. Results from recent modelling studies, show that soils with high P status can take many years (soil specific) to fall to acceptable levels. Therefore implementation of a mechanism that makes the P insoluble in the interim will reduce dissolved P concentrations in surface runoff, subsurface drainage and indeed recharge to groundwater and thereby could limit restrictions in such areas. Indeed a mechanism that can prevent incidental losses of P after manure application attempts also to prevent such losses. Gypsiferous amendments have been used effectively in this respect. Phosphorus sorptive processes vary with soil type and can be explained by differences in soil attributes such as pH, extractable Al and Fe. These have implications for P uptake into the solid phase that can replenish more labile and soluble P fractions when P is depleted. The processes of P supply in soils is to a limited degree represented by agronomic soil testing however, differences in P buffering and sorption capacities that exist amongst soil types can significantly influence P supply and availability in a water soluble form. For these reasons, P buffering capacity in soil has been incorporated with standard agronomic soil testing to derive soil type specific fertilizer application policy, and depends largely on differences in soil pH and extractable Al. In addition, P buffering processes can vary between calcareous and non-calcareous soils where the dominant mechanism can be sorption or precipitation of P. The variation in P buffering across soil types has implications for amendment use on different soil types specifically if changes in soil pH are observed. Gypsiferous amendment of soils that alter soil pH may also alter the buffering or sorptive mechanisms in soil. Herein we suggest that such amendments may enhance the process of precipitation of Ca-P in instances where the soil pH is altered significantly. Whilst manipulating the soluble phase P in soil is the rationale for using them on high P status soils, this review will examine how this will depend on the nature and chemical attributes of the soil type.