| In a soil column experiment, we studied the effect of four soil amendents (PAM,β–cyclodextrin, water super absorbent polymer, and humic acid on the structure, water holding capacity and water conductivity of three soils from the Loess Plateau. The three soil types were Aeolian sandy soil, Huangmian soil, and Heilu soil.The results showed that all four amendents increased the > 0.25mm water-stable aggregate content and mean weight diameter (MWD) in the loess-derived soils and reduced the aggregate fractal dimension. Soil amendments played a role in improving the structure and enhancing the function of the soils.The 0.05% PAM treatment significantly increased the > 0.25mm water stable- aggregate content, MWD, and fractal dimension of Aeolian sandy soil and Huang- mian soil after treatment for three weeks. However, treatment time had no significant effect on soil structure. There was significant improvement in the structure of Heilu soil in the 0.1% PAM treatment. In the 0.2% PAM treatment, the effect of PAM on soil structure increased as the treatment time increased.There was a significant improvement in soil structure after three weeks in the 0.1% water super absorbent polymer treatment. We suggest that lower concentrations of water super absorbent polymer may also improve soil structure if the treatment time is increased. For example, there was a significant change in soil structure in the 0.05% water super absorbent polymer treatment after four months. The structure of the Heilu soil was significantly improved only when water super absorbent polymer concentrations were >0.2%. The improvement increased as the treatment time increased. The effect of water super absorbent polymer on the Aeolian sandy soil was similar to the effect of PAM.There was a significant improvement in the structure of the Aeolian sandy soil after three weeks in the 0.05% treatment, but overall, treatment time had no signifi- cant effect on the structure of Aeolian sandy soil. The structure of the Aeolian sandy soil improved significantly after two months when humic acid concentrations were> 0.2%. The structure of the Heilu soil improved only when humic acid concentrations were 0.4% and the treatment time was two months.There was a significant improvement in the structure of Aeolian sandy soil and Huangmian soil after treatment with 0.05%β–cyclodextrin for three weeks. An increase in the treatment time had no further effect on soil structure. The structure of the Heilu soil improved significantly only after incubation for four months in the 0.4%β–cyclodextrin treatment.There are several reasons for the different effects of the four amendments. First, the mechanism of the amendments'action on soil structure was different. PAM andβ–cyclodextrin increased the > 1mm aggregate content in the soil. The effect of PAM andβ–cyclodextrin was especially great in the > 5mm aggregate size class. In contrast, water super absorbent polymer and humic acid mainly increased the 1 ~ 2 mm aggregate content. Secondly, the soil physical and chemical characteristics were different. Specifically, the organic matter and clay content of the Huangmian soil and Aeolian sandy soil were lower than the Heilu soil. Therefore, the structure of the Huangmian and Aeolian sandy soils improved at lower amendment concentrations compared to Heilu soil.The effects of the amendents on the Huangmian soil all conform to the equation between soil moisture content and soil suction:θ=aS-b. The soil water holding capacity of the Huangmian soil increased as treatment time increased. When the soil amendment and suction were the same, the soil water holding capacity of the Huangmian soil increased as the amendment concentration increased. When the amendment concentration and suction were the same, PAM had the greatest effect on soil water holding capacity followed byβ–cyclodextrin, water super absorbent polymer, and then humic acid.The four soil amendments had significantly different effects on soil water conductivity. PAM and humic acid had a significant effect on Huangmian soil. The optimum concentration of these amendments was 0.05%. PAM also significantly improved the surface structure of the Heilu soil. Humic acid and water super absorbent polymer had greater effects on the subsoil. The optimum concentration was 0.05%～0.1%. Humic acid and water super absorbent polymer had a significant effect on the surface structure of Aeolian sandy soil. The effect ofβ–cyclodextrin and water super absorbent polymer was greater on the subsoil of the Aeolian sandy soil. The optimum concentration was 0.05%.
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