Rapeseed dreg additive reducing soil infiltration and improving water retention

Abstract: Plant dreg is a type of organic matter and a byproduct of vegetable oil extraction. Plant dreg as a fertilizer can be added to soils and it may also improve soil physical properties. An experiment based on the indoor vertical one-dimensional infiltration soil column was conducted to investigate the impact of rapeseed dreg additive on soil-water infiltration, movement, re-distribution and water retention. The soil in the experiment was collected from the 30-cm depth in a cultivated field in the district of Yangling in Shaanxi Province on the Loess Plateau of China (34°17′28″ N, 108°04′30″ E). The particle size of selected soil was measured by Mastersizer-2000 (made in Malvern Instrument Co. Ltd., Britain), and the soil was sandy loam with a particle size distribution of 3.75% for 0-0.002 mm, 21.73% for 0.002-0.02 mm and 74.52% for 0.02-2 mm. Samples were air dried, sieved through a 2 mm mesh, and compacted into plexiglass soil columns with a height and inner-diameter of 40 and 15 cm, respectively. The total soil depth in the column was 34 cm and soil bulk density was 1.45 g/cm3. The rapeseed dreg was air dried, pulverized, and uniformly mixed with soil samples. The plant dreg accounted for 2% of soil weight. The depth of mixed layer was set at 14, 24, and 34 cm. Pure soil samples without additives were used as a control (CK) treatment. A Mariotte bottle was used to provide a free water supply with about 1.5 cm in depth on the surface. The experiment started when the Mariotte bottle opened. The filter paper was laid at the soil surface to make the water head stable. The characteristics of soil water infiltration, distribution and water holding capacity were comparatively analyzed. The results showed that both Philip and Kostiakov models could well describe the relationship between cumulative infiltration and infiltration duration (R2>0.99). Compared with the CK, the soils mixed with plant dreg helped to decrease cumulative infiltration and infiltration rate, both of which decreased as the depth of mixed layer increased. The cumulative infiltration for the soils mixed with 14, 24, and 34 cm was 3.9%, 7.8%, and 11.0% lower than the CK, respectively. The infiltration rate for the soils mixed with 14, 24, and 34 cm was 25.0%, 33.3%, and 41.7% lower than the CK, respectively. From the final water distribution in soil profiles, the soils mixed with plant dreg helped to increase saturated soil moisture and soil water content in soil layers, which were increased by 14.3% and 11.3%, respectively, compared with the CK. This indicated that plant dreg additive could increase soil water retention and water storage in root zone. Plant dreg could increase clay and silt contents from 3.75% to 9.97% and from 21.73% to 55.15%, respectively, and reduce sand content from 74.52% to 34.88%, and the experimental soil changed to silt loam. This indicated that the ratio of medium and small particle-size increased, and the ratio of large particle-size decreased, demonstrating that plant dreg had the potential in improving desertification soils. From the above, mixing plant dreg powder with soils is of significant practical meaning for cultivated soils because of the enhancement of water retention and water storage. This study may provide valuable information for the promotion of plant dreg to cropland and the application and popularity of plant dreg in soil improvement and water-saving agriculture.