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 forma tion. Seal formation reduces infiltration rate and generates high leve ls of runoff. Chemical soil amendments (e.g., phosphogypsum, organic p olymers) can improve aggregate stability and limit clay dispersion and thus prevent seal formation. This review discusses the effects of org anic polymers on controlling seal formation as characterized by change s in soil infiltration rate and dependence on soil properties. Soil su sceptibility to sealing depends on a number of soil properties, includ ing soil texture and mineraology, composition of the exchangeable cati ons, and water quality. Addition of small amounts of polymers (10-20 k g 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 i nfiltration rate of a polymer-treated soil subjected to distilled wate r 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 phosphog ypsum addition) in the soil solution at the soil surface. Combined app lication of anionic polymers with electrolytes results in final infilt ration values of approximately 25 mm h-1, which are 10 times higher th an the control. Polymer effectiveness in controlling seal formation de pends also on charge type and density and on the molecular weight of t he polymer. The effect of polymers and water quality on seal formation is in good agreement with the effect of the polymers on the flocculat ion patterns of soil clays. Of the polymers currently available and un der study, anionic polyacrylamide has been found to be the most effect ive in controlling seal formation, and soil erosion and has the longes t residual effect.