Soil water retention and hydraulic conductivity dynamics following tillage

Authors: KOOL, DILIA - Iowa State University; TONG, BING - Nanjing University Of Information Science And Technology (NUIST); TIAN, ZHENGCHAO - North Carolina State University; HEITMAN, JOSHUA - North Carolina State University; Sauer, Thomas; HORTON, ROBERT - Iowa State University

Submitted to: Soil and Tillage Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/4/2019
Publication Date: 5/22/2019
Citation: Kool, D., Tong, B., Tian, Z., Heitman, J.L., Sauer, T.J., Horton, R. 2019. Soil water retention and hydraulic conductivity dynamics following tillage. Soil and Tillage Research. 193:95-100. https://doi.org/10.1016/j.still.2019.05.020.
DOI: https://doi.org/10.1016/j.still.2019.05.020

Interpretive Summary: Tillage doesn't change the soil particles so much as it changes their arrangement. Tillage is done for several reasons including mixing in chemicals, crop residues, and manures and the preparation of the soil for planting of seeds. The soil is loosened by most tillage practices and increases the air space in the soil. With time and rainfall the soil settles and becomes more dense. The objective of this study was to measure the changes in soil density and water storage with time. The measurements showed that soil density and water retention both changed over time and with depth in the soil. These results show that large changes in soil properties occur after tillage and it is important to be able to accurately quantify those changes. This information will assist researchers, simulation modelers, and growers with improved understanding of the changes in soil physical properties following tillage.

Technical Abstract: To improve hydraulic properties, soil bulk density (BD) may be purposely reduced in agricultural fields using tillage. However, tillage alters the soil structure, resulting in unstable soils. As the soil stabilizes, BD increases over time. While this is known, studies on soil hydraulic properties in tilled soils, including comparisons between tilled and non-tilled soils, commonly assume a rigid soil structure. This study presents changes in soil water retention and saturated hydraulic conductivity (Ksat) as BD increases dynamically with time following tillage. Over the summer of 2015, soil cores were collected from a loam-textured field site at several depths below the surface following precipitation events after a tillage operation. In the laboratory, the soil cores were saturated and placed in pressure cells to determine soil water retention curves. Saturated hydraulic conductivity was determined using the constant head method. Initial BD was 0.94 g cm-3 for the 0-30 cm layer. The 0-5 cm layer eventually reached a BD of 1.11 g cm-3, while the 10-15 cm layer BD increased to 1.31 g cm-3. Changes in BD indicated that the increases in BD resulted in a 25% reduction in the thickness of the tilled layer. Soil water retention curves were markedly steeper for samples with higher BD. Water retained at a soil matric potential of -33 kPa varied from 0.2 cm3 cm-3, about one third of the initially high pore volume immediately after tillage, to 0.36 cm3 cm-3, about two thirds of the pore volume towards the end of the season. Evaluation of two modeling approaches for water retention as a function BD indicated that reasonable values could be obtained when a matching point was used. No clear relationship between Ksat and BD was obvious for BD < 1.06, but for BD > 1.06, Ksat decreased markedly (order of magnitude) as BD increased. Hydraulic properties varied strongly depending on time since tillage and soil depth, and results have implications for models of tilled soils, as well as for studies comparing tilled and non-tilled soils.