Effect of anionic polyacrylamide on aggregation and dispersible clay percentage in soils treated with plant residues

Anionic polyacrylamide application as an organic conditioner alone or in combination with organic material, can improve soil properties. Improment of soil arregation and increase of aggregate stability by addition of organic matter are the most important ways for improving soil physical properits, which can increase carbon retention and sequestration in soil, water infiltration, porosity, aeration and seed germination and decrease soil compaction, water runoff and erosion of soil surface layers. The objective of this study was to investigate the influence some organic conditioners on mean weight diameter (MWD) and dispersible clay percentage (DCP).
Soil samples collected from the top 30-cm layer of two loamy sand and clay soils were treated with alfalfa and wheat residues ( The addition of plant residues and polyacrylamide decreased the percentage of dispersible clay in the both clay and loamy sand soils. The effect of these organic soil amendments onthe decrease of DCP in soils was synergistic and the lowest percentage was measured in concomitant application of both amendments in soil. The mean weighted diameter in the both soils increased significantly with the addition of plant residues. This beneficial effect in wheat straw treatment was higher. In both soils, we found that in the soil without plant residue, the application of polyacrylamide caused an increase in MWD. But in heavy soil treated with wheat straw and alfalfa straw, the application of polyacrylamide had detrimental effects on MWD. However, this detrimental effect was not significant in the light soil. The mean weighted diameter of the clay soil in all treatments increased during soil incubation up to that measured in the interval time of 10 to 20 day and then it decreased. The percentage of dispersible clay had a reverse change. It had the lowest level in this interval time. The temporal changes of soil MWD and DCP in loamy sand soil were similar to those in clay soil. But the highest levels of MWD and the lowest level of DCP were measured in a later interval time (30 to 45 day).
Although separate application of plant residues and polyacrylamide in soil can increase MWD but in concomitant application they had an antagonistic effect on soil MWD. The application of polyacrylamide decreased soil MWD in heavy soil treated with wheat straw and alfalfa straw. Plant residues by increasing biological activity and polyacrylamide especially in clay soil by physico-chemical reactions can improve soil aggregation and aggregate stability. Thus they do not have a synergistic effect on soil MWD. This occurrence may be related to the reaction of polyacrylamide with clays in soil that inhibits the beneficial effect of biological interaction between plant residue and soil clay particles in aggregation.