The influence of clay type on reduction of water repellency by applied clays: a review of some West Australian work

In western Australia water repellency mostly occurs in soils with sandy tex ture; the severity of water repellency is influenced by very small changes in clay content. Additions of 1-2% clay can prevent water repellency and fo r some time clay amendments have been used by farmers to overcome water rep ellency. Tne aim of this study was to assess the effectiveness of clays in ameliorating water repellency. Clays were assessed for effectiveness in reducing water repellency by mixin g with water repellent sands and measuring water drop penetration time (WDP T) on the resultant mixtures. WDPT was measured on the initial mixtures, a wetting and drying cycle was imposed and WDPT measured again. Two sets of c lays were assessed: four simple clays containing kaolinite (2) or smectite (2) group minerals and a group of clayey subsoil materials which had been c ollected by farmers. For the simple clays, clay mineral type was the most significant factor in determining response. Kaolin was much more effective than smectite, imposit ion of a wetting and drying cycle greatly reduced water repellency. The dom inant exchangeable cation of the clays (sodium or calcium) had little effec t on the ability of the clays to reduce water repellency. The factor that w as most predictive of the effectiveness of clayey subsoils materials in red ucing water repellency was texture: clay content (r(2) = 0.18) or clay + si lt content (r(2) = 0.23). These properties were more predictive of water re pellency values after the wetting and drying cycle treatment (r(2) = 0.36, r(2) = 0.44). The proportion of the clay fraction that consisted of kaolini te was next most predictive in determining effectiveness which is again ind icative of kaolin group minerals being more effective than smectite group m inerals. The exchangeable sodium percentage and clay dispersibility had no systematic effect on the ability of these clays to reduce water repellency. These results provide a basis for developing a practical field procedure to enable farmers to recognise suitable clays for application to their water- repellent soils. An information kit for farmers is currently being develope d which describes how to assess the suitability of clayey materials for tre atment of water repellency. (C) 2000 Elsevier Science B.V. All rights reser ved.