PAM Moisture Infiltration and Soil Erosion

Flood and or furrow irrigation is still a major main mean by which water is applied to irrigated crops in the world. In 1993 it was still the main method of irrigation in California, USA (Carter et al, 1993) In Australia furrow and flood irrigation dominate rice and cotton production. A consequence of flood /furrow irrigation is water runoff at the end of the row and erosion of suspended soil particles. Soil sediments lost in return flows along, with pesticides and nutrients absorbed on the sediment, are regarded as major contributors to non-point pollution of surface water (Sojka and Lentz, 1993). Ross et al. (1996) reported topsoil losses of 5- 50 tonnes per hectare per year on erodible soils in the USA Pacific Northwest. Some of the major impediments to changing irrigation practices from flood to pressurised systems have been the low cost of water, the high cost of developing pressurised systems and general resistance to change. As a result of these concerns the use of organic polymers to modify soil properties has been investigated (Azzam, 1980). The use of PAM has resulted in changes in reduced rates of soil erosion and increased water infiltration.

Sojka and Lentz (1994) documented the role that PAM soil application played in reducing irrigation-induced erosion. The most impressive results in erosion control were obtained with the use of high molecular weight PAM. Soil erosion was controlled with the application of 10mg/L PAM (18% anionic) in the advancing furrow stream. PAM was also seen to be more effective when applied with the irrigation water rather than added as dry blended product onto the soil. Sojka and Lentz (1994) documented how applications of PAM in furrow irrigation eliminated up to 94% of runoff. Increases in water infiltration of up to 50% at the furrow outlets were reported from the use of PAM in this system. Aggregate stability was increased from 54 to 80% in 1993 and 63 to 84% in 1994.

Trials conducted across Northern Australia from 1995 - 97 (Schiller, 1997, unpublished) investigated the role of PAM in reducing soil runoff and improving infiltration. This work revealed reductions in the advancement of water when treated with PAM at rates of 1.2, 2 and 3.6 ppm. Measurement of water infiltration using a neutron probe indicated that infiltration was improved on coarser soils, whereas on the heavier Darling Downs soils, very small infiltration increases were recorded. Sediment runoff and turbidity were reduced in tailwater (the water at the end of the furrow irrigation) with the use of PAM. Measured levels of sediment in tailwater decreased from 0.4 - 1.8 g/L in control areas to O.lg/L -(undetectable) where 3.6 ppm PAM was applied per volume of irrigation water. Further studies of tailwater indicated that with the use of PAM, pesticide and herbicide levels could be reduced. In the case of Oxyfluorfen residues in tailwater, reductions of 100% were achieved through the use of PAM.

Ross et al. (1996) wrote on improvements in infiltration and reduced sediment runoff through the use of PAM. High molecular weight PAM were observed to reduce sediment runoff by 96% in furrow irrigated highly erodible Portneuf silt loam soils of Idaho. Net infiltration over the furrow length in a 12 hour period increased by an average of 10%. Studies in this trial revealed that uptake of acrylamide by the plant was undetectable. The results of this trial indicated that there was no adverse environmental effects or potential health risks if recommended procedures were followed.

Studies by Levy et al. (1991) showed that an inverse relationship exists between vegetative growth of cotton on the one hand and the level of runoff on a commercial field scale on the other. The use of PAM (at 20 kg/ha) was seen to significantly reduce runoff from both vertisol and loess soils. A trend was observed where treatment with PAM increased yields compared to the control. In modelled sprinkler irrigation systems (Levy et al. 1992) the effects of 5, 10 and 20ppm concentrations of 2 PAM. These were applied during the first three consecutive irrigations, and thereafter the soils received two subsequent irrigations with water only. Both PAM treatments stabilised soil aggregates and increased their resistance to erosion. It was suggested that PAM is an effective agent in limiting runoff and erosion from soils sensitive to sealing and therefore could be beneficial in increasing yields.

Stern et al. (1992) documented runoff from sprinkler-irrigated wheat crops at 36% of total irrigation. When water was treated with PAM plus phosphogypsum, these had runoff of only 1.4% of total irrigation volume. Phosphogypsum treated water alone had runoff of 13% irrigation volume. The PAM treatments also gave significantly higher grain yields and greater crop water use efficiency than the control plots.

Trout et al. (1995) reported that 0.7 kg /ha of a high molecular weight PAM reduced furrow induced erosion by 85-99%. PAM was also seen to increase net infiltration by 30%. It was postulated that this was the result of reduced sediment movement and furrow surface sealing. Infiltration was also inversely related to the maximum sediment concentration in the flowing water. The Murray and Darling Rivers are regarded internationally as being high in colloidal material. Growers using PAM in drip irrigation systems have recorded increased infiltration and lateral spread of moisture via moisture monitoring equipment and observing changes to flow patterns as a result of increased flocculation of this colloidal material. The flocculating nature of PAM used in drip irrigation systems appears to remove colloidal material from within the drip line. This may therefore increase flows (increasing pipe size or friction losses). Alternatively as sediment is reduced in the water flow, infiltration and subsequent lateral spread of moisture is increased. The number of flushing valves in drip-irrigated enterprises highlights the amounts of impurities that still get past the primary filtration systems whether they be sand or disc filter systems.

Vallant (1997) describes how the use of PAM helped to reduce erosion and increase yields of furrow irrigated tomatoes and peppers. Soil loss was reduced by 63% and yield increases of 30% were recorded. Pryor (1988) also highlighted the role PAMs play in increasing the yield of processing tomatoes. Application of up to 15 kg/ha of a cross linked PAM resulted in yield increases of 30% when sidedressed into canning tomato beds. The paper further highlights the potential of PAM in drought prone or sandy soils, or where water is highly priced or in short supply. Sousa and Osterli (1998) used PAM at a concentration of 10 ppm to irrigate processing tomatoes. In one section PAM was used in the first 6 of 9 irrigations and in the other section a further PAM treatment was applied in the eighth irrigation. Yield increases at the 90% level of statistical significance were recorded. Results from this trial showed that treatment with PAM resulted in more water being available to the plant.

Gardiner and Shainburg (1996) described how the use of PAM at 10, 25 and 40 ppm improved hydraulic conductivity of soils significantly. The effects being most apparent in the first few weeks then tapering off. This paper reported that the use of PAM had a greater impact on soil hydraulic conductivity than gypsum. PAM may also have a role in reducing wind drift. Chamberlain (1988) suggests that polymer treatment of the ground will stabilise soils against wind drift. In crops such as carrots and onions where wind damage at emergence can seriously reduce yields, polymers may play a stabilising role when applied after seeding.

Lentz et al. (1993) showed that PAM charge type or charge density had a major influence on furrow erosion. PAM was applied at 10ppm during the initial 30 minutes of irrigation. The nature of the charge on the PAM influenced the efficacy of erosion control. With anionic and cationic charge PAM efficacy was increased with increasing charge density. The effectiveness with respect to charge was anionic>neutral>cationic. The toxicity problem associated with cationic PAM seems to be combined with a reduced effectiveness at equivalent concentrations in solution.

Orts et al. (2000) used biopolymers to reduce shear induced erosion in laboratory studies. The use of biopolymers at rates up to 120ppm reduced suspended solids by more than 80%. The use of PAM reduced runoff by >90% at rates as low as 5ppm. This work indicated the effectiveness of PAM in reducing suspended solids in runoff at low application rates.

Many other studies have produced similar information with regards to reduction in irrigation induced soil erosion. Becker (1997) reports that PAM applied at 5-10 ppm almost completely eliminated rill erosion. Combining gypsum and soil applied PAM also improved infiltration and reduced soil erosion. The low operating costs of furrow irrigation make it a likely system to remain in place until either environmental or economic opportunities allow change to proceed. In these systems the use of PAM offers growers a solution to minimise issues involving irrigation induced soil erosion. In a world where economic and environmental issues are increasingly important the cost of using PAM in furrow based systems may offer irrigators a low cost option in reducing erosion and increasing water infiltration over installing pressurised systems.

In terms of nutrient retention in soil and reduced runoff polyacrylamides play a significant role in controlling potential pollution issues by controlling runoff from fields.