Pore size distribution derived from soil-water retention characteristic curve as affected by tillage intensity

Authors: Jabro, Jalal - Jay; Stevens, William - Bart

Submitted to: Water
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/30/2022
Publication Date: 11/3/2022
Citation: Jabro, J.D., Stevens, W.B. 2022. Pore size distribution derived from soil-water retention characteristic curve as affected by tillage intensity. Water. Special Issue 14(21). Article 3517. https://doi.org/10.3390/w14213517.
DOI: https://doi.org/10.3390/w14213517

Interpretive Summary: As increasing periods of drought are limiting the amount of water available for crop production, it is even more important than ever that crop water use efficiency is maximized. One way to achieve this goal is to ensure that cropland soils are able to optimally absorb and retain water. Soil pore size distribution is considered one of the most important physical characteristics that affects water movement, aeration, and water storage in the soil and, ultimately, water availability to plants. This soil characteristic can also directly influence soil chemical and biological properties, functions and processes. Soil pores come in different shapes and sizes and are inter-connected to various degrees. Soil pore sizes are often classified as small, medium, and large, depending on soil conditions and land use practices. Pore size is affected by soil structure, soil texture, organic matter content and tillage practices. Good soils have about equal amounts of small and large pores. Results from this study showed that intensively-tilled soil has a higher amount of large and medium pores and a lower amount of small pores compared to untilled soils, at least in the short-term. Tillage often increases the number of larger pores which are mainly responsible for water movement, soil drainage and aeration. A higher number of small pore spaces were observed in untilled soils which resulted in greater available water capacity compared to tilled soils. However, the long-term impact of converting to no-tillage practices could improve soil pore size distribution, pore connectivity and other physical, chemical and biological properties compared to intensive tillage as a result of increased soil organic matter levels in the top soil layer over time. This work improves our understanding of how various farming management practices affect water use efficiency, environmental quality and soil health.

Technical Abstract: Tillage practices can influence the pore size distribution (PSD) of the soil, affecting soil physical and hydraulic properties as well as processes that are essential for plant growth, soil hydrology, environmental studies and modeling. A study was conducted to assess the effect of no-tillage (NT) and conventional tillage (CT) on PSD derived from soil-water retention curves using the van Genuchten’s equation (vG) at 0-15 cm and 15-30 cm depths in a sandy loam soil. Values of PSD or slopes (C(h)) were calculated from the soil-water retention curves (SWRC) by differentiating the vG equation. Soil water retention curves under both tillage systems and within two depths were determined using the evaporation HYPROP method. The vG equation was well fitted to measured soil water retention data. The diameter (D) of soil pores retaining water at various matric suctions (h) of water in soils was calculated by the capillary equation. Results indicated that the effect of tillage on soil PSD was significantly observed in the macro-pore (D>1000 µm, at h<3 hPa) and meso-pore (D between10 and 1000 µm, at h between 300 and 3 hPa) classes, while the micro-pores (D<10 µm, at >300 hPa) were unaffected at the 0-15 and 15-30 cm depths. Larger values of C(h) or PSD in CT were associated with greater soil loosening induced by the CT operations and forming greater proportion of large pores (structural porosity) in soils under CT compared to soils under NT. Macro-pore and meso-pore proportions were significantly greater in soils under CT than in soils under NT within both depths. The hydraulic parameters of the vG equation and its derivative function can essentially be used to compare soil-water retention curves and pore size distributions between soils under untilled and tilled conditions.