Some of the elements in runoff from grazed pastures, e.g. P and dissolved organic carbon DOC are important contaminants of water bodies.  Both P and DOC occur in an anionic form in water.

Laboratory studies showed that amendment of bentonites with a synthetic liquid polymer (poly-DADMAC) - widely used as a coagulant in drinking water treatment - leads to their adoption of a positive charge and strong uptake of phosphate and other anions (including arsenate).  We have also shown that all types of clays and soils can be amended the same way.

 

Using a rainfall simulator to generate runoff, we show (Fig. 1) that most (on average > 70%) of both the P and DOC that is lost from untreated pastures can be retained in the soil. 

 

Figure 1. Mean values for concentration of P following treatments of soil with different amounts of poly-DADMAC (soil weight calculated to depth of 5mm)

Further work with a rainfall simulator shows that is only necessary to treat a buffer strip comprising 10% of the whole area in order to retain most of the P and DOC, so minimising costs. Whereas the effectiveness of the polymer treatment diminishes with time and was minimal after 85 days of treatment, the seasonality of rainfall in Southern Australia, where the research was conducted, means that most of the total year's losses of P and DOC can usually be restrained by one well-timed application.  Our current field-scale research is examining this possibility. We will also examine the effectiveness of the treatment to restrain added P.

This amendment may be useful for the restraint of P from other sources besides pastures and also for the restraint of anionic pollutants e.g As.

Acknowledgement: Dairy Australia (Grants UA11129 and UA12286 for pilot and field studies).