Evaluation of the potential of a superabsorbent polymer for attenuating spring soil water stress in Alentejo.

In Mediterranean agricultural areas, superabsorbent polymers (SAP) can be beneficial in minimizing water stress in the hot seasons. SAPs can absorb and release significant quantities of water (100 to 1000 g per gram of polymer) promoting root and shoot growth (HUTTERMAN et al., 2009) and improve water and nutrients assimilation by plants (EL-REHIM et al., 2004). Field results also showed that biomass productivity can increase up to 38% in grassland in Mediterranean conditions (CASQUILHO et al., 2013) or up to more than 200% in roots, branches or leaves in poplar plants in semiarid regions (HUTTERMAN et al, 2009). In this work a characterization of a sodium polyacrylate SAP swelling capacity was done by absorption-desorption tests in laboratory in distilled and tap water and in aqueous solutions of typical soil salts, sodium chloride, ammonium sulfate or iron sulfate, which counter the effect of the SAP, lessening its water absorbency. Laboratory tests were also done with acidic solutions of nitric acid. The ionic concentrations and acidity on aqueous solutions were higher than the conditions in soil solutions. So, the results given by using laboratory data as input to an empirical deterministic relational model, for predicting SAP water release on soil in spring, were conservative. The model, also based on physical data on soil moisture and precipitation obtained with automatic weather stations, was implemented through a geographical information system software to evaluate the impact of soil application of SAP in spring in a transect of five municipalities of Southern Portugal. The model indicated that the polymer can increase soil moisture by about fifteen times and that SAP was more efficient in retaining soil water under conditions of lower precipitation. This simulation also indicated that about 37 000 ha or 9% of the total area of the five municipalities could benefit from the polymer application, in terms of biomass productivity of annual cultivations under unirrigated agricultural regime.