A four-year experiment was conducted to investigate the effects of polyacrylamide (PAM) and potassium polyacrylate (PAM-K) application on the growth characteristics of oat (Avena nuda L.) and soil at different soil depths and in different years (2011, 2012, 2013, and 2014). The correlations between the growth characteristics of oat and soil microbial biomass, nutrients, moisture content, electrical conductivity, and bulk density at the respective soil layers in a dry land area of Inner Mongolia were also examined. PAM and PAM-K were applied at a rate of 75 kg/hm² per year. The experiment consisted of a total of 8 treatments and a control. The application groups were as follows:conventional tillage (CK), (1) M1:PAM 2011, (2) M2:PAM 2011 and 2012, (3) M3:PAM 2011, 2012, and 2013, (4) M4:PAM 2011, 2012, 2013, and 2014, (5) A1:PAM-K 2011, (6) A2:PAM-K 2011 and 2012, (7) A3:PAM-K 2011, 2012, and 2013, and (8) A4:PAM-K 2011, 2012, 2013, and 2014. This was a two-factor experiment involving a randomized complete block design with three replications. Each treatment occupied a plot area of 4×5 m. Water-retaining materials were spread equally on the surface of the land and then incorporated into the soil by tilling. Oat seeds were sown on May 25 and harvested in late September during the four experimental years (2011, 2012, 2013, and 2014) at a rate of 3750000 plants per hectare with a row spacing of 25 cm and seeding depth of 3-5 cm. With increasing PAM and PAM-K application, differences in micro-ecological effects were observed at different times for the rain-fed farmland. The results showed the following superiority order:4 years > 3 years > 2 years > 1 year. At a depth of 0-60 cm soil, compared with the control, for the treatments administered continuously for four years (A4 and M4), soil moisture increased by an average of 27.18% in A4 and 34.40% in M4. A decreasing trend was observed for soil bulk density and soil electrical conductivity; soil bulk density decreased by 2.33% and 6.64%, while soil conductivity reduced by an average of 29.50% and 22.70% for A4 and M4, respectively. Soil nutrient content (available P, N, and K, organic matter) increased in both A4 and M4. Microbial biomass carbon (MBC), nitrogen (MBN), and phosphorus (MBP) showed an increasing trend; MBC increased by an average of 24.11% in A4 and 31.89% in M4, MBN increased by 46.52% in A4 and 69.96% in M4, and SMBP increased by 35.21% in A4 and 52.70% in M4. Interestingly, at a depth of 10-20 cm, MBN increased by 98.95% in M4, while at 0-10 cm, MBC increased significantly by 31.13% and MBP increased by 74.49% in M4, respectively. At a depth of 20-40 cm, MBC increased significantly by 62.27%, while MBP increased by 49.91% in M4, respectively. Additionally, the oat seedling aboveground fresh biomass increased for A4 and M4 by 90.53% and 146.91%; aboveground dry biomass increased by 101.56% and 128.13%, plant height increased by 33.67% and 76.39%, and grain yield increased by 19.27% and 22.40%. Significant improvement was observed with continuous application of PAM and PAM-K in the plowing layer (0-20 cm) and near the application layer (20-40 cm). We recommend PAM treatment, which is more suitable for yellow loamy soil, than PAM-K treatments, to improve soil quality and increase crop yield in dry farming areas.