ORGANIC POLYMERS AND SOIL SEALING IN CULTIVATED SOILS

The beating action of rain or sprinkler drops causes the breakdown of aggregates and clay dispersion, which subsequently leads to seal formation. Seal formation reduces infiltration rate and generates high levels of runoff. Chemical soil amendments (e.g., phosphogypsum, organic polymers) can improve aggregate stability and limit clay dispersion and thus prevent seal formation. This review discusses the effects of organic polymers on controlling seal formation as characterized by changes in soil infiltration rate and dependence on soil properties. Soil susceptibility to sealing depends on a number of soil properties, including soil texture and mineraology, composition of the exchangeable cations, and water quality. Addition of small amounts of polymers (10-20 kg ha-1), either sprayed directly onto the soil surface or added to the applied water, stabilizes and cements together aggregates at the soil surface, thereby increasing their resistance to seal formation. The infiltration rate of a polymer-treated soil subjected to distilled water rain is two or three times that of a non-treated soil. The efficacy of anionic polymers in preventing seal formation is enhanced when the soil clay is maintained in a flocculated state. The latter is achieved by addition of electrolytes (either in the ''rain'' water or phosphogypsum addition) in the soil solution at the soil surface. Combined application of anionic polymers with electrolytes results in final infiltration values of approximately 25 mm h-1, which are 10 times higher than the control. Polymer effectiveness in controlling seal formation depends also on charge type and density and on the molecular weight of the polymer. The effect of polymers and water quality on seal formation is in good agreement with the effect of the polymers on the flocculation patterns of soil clays. Of the polymers currently available and under study, anionic polyacrylamide has been found to be the most effective in controlling seal formation, and soil erosion and has the longest residual effect.