Soil hydraulic conductivity (K) is one of the most important physical properties of soil, indicating the soil's ability to transmit water under a hydraulic gradient. Knowledge of hydraulic conductivity plays a key role in water resources management, the design of irrigation and drainage systems, prediction of water infiltration, and control of groundwater pollution. Studies indicate that soil texture, porosity, bulk density, organic matter, biological activity, agricultural management and salinity can significantly affect soil hydraulic conductivity; understanding these factors is essential for optimizing soil use and preserving natural ecosystems.

Factors Affecting Hydraulic Conductivity: Soil texture, which includes the percentage of sand, silt, and clay particles, has a direct impact on hydraulic conductivity. Sandy soils often have high hydraulic conductivity due to their coarse and continuous pores but have low water retention capacity. In contrast, clay soils with fine and irregular pores have very low hydraulic conductivity but a high water-holding capacity. Loam soils generally have hydraulic properties that are intermediate between sandy and clay soils. Because the hydraulic properties of soil are influenced by the combined effects of its physical, chemical, and biological characteristics. Nevertheless, in the surface layers of soil, hydraulic conductivity is influenced more by bulk density and organic matter than by soil texture. Bulk density and soil porosity are key factors determining hydraulic conductivity. Homogeneous soils with higher bulk density, compared to loosely packed soils, exhibit a significant reduction in saturated hydraulic conductivity due to the compaction of large pores. Increasing bulk density (resulting from soil compaction) reduces porosity and pore connectivity, which leads to a decrease in hydraulic conductivity. Amendments such as biochar can improve hydraulic conductivity by reducing bulk density and increasing porosity. The activity of soil organisms such as earthworms, plant roots, and microorganisms has a significant impact on hydraulic conductivity. Earthworms increase water permeability by creating subsurface channels. Plant roots also increase hydraulic conductivity by creating macropores and improving soil structure. However, biological clogging caused by excessive microbial growth can block soil pores and reduce hydraulic conductivity. Studies show that undisturbed forest soils have higher hydraulic conductivity compared to agricultural soils due to greater biological activity. These soils have a higher water retention capacity, which leads to an increase in the median pore radius and a decrease in water diffusivity, consequently improving soil permeability. Soil-dwelling animals also have a significant impact on the physical processes of soil. Organisms such as earthworms, termites, ants, and enchytraeids influence soil structure through activities like burrowing, casting, and the transport of organic and mineral materials; they create structural pores within the soil. These activities lead to increased porosity, water permeability, aeration, and aggregate stability, resulting in a significant enhancement of the soil's hydraulic conductivity. Soil organic matter increases hydraulic conductivity by improving soil structure and promoting the formation of soil aggregates. However, at very high concentrations, organic matter can clog soil pores and reduce hydraulic conductivity. Biochar, as an organic soil amendment, can have varying effects depending on soil type and environmental conditions. In sandy soils, biochar particles may fill the pores and reduce hydraulic conductivity, whereas in clay soils, biochar improves hydraulic conductivity by increasing porosity. The pyrolysis temperature of biochar also affects its effectiveness, with biochar produced at higher temperatures exhibiting greater stability and porosity. Soil salinity and sodicity can reduce hydraulic conductivity through the dispersion of clay particles and the blockage of pores. Studies indicate that in clay soils, increasing sodium concentration initially causes an increase in hydraulic conductivity; however, after reaching a critical threshold, a significant decrease in hydraulic conductivity occurs. In sandy soils, the effect of sodium is less pronounced and may even increase hydraulic conductivity. In other words, sodium increases the thickness of the diffuse double layer surrounding clay particles and disrupts the soil structure. These conditions lead to a decrease in saturated hydraulic conductivity, an increase in soil bulk density, the formation of a hard surface crust, poor aeration, runoff, erosion, and ultimately reduced plant productivity. It is also worth noting that contradictory results have been observed regarding the effect of sodium in soil.

Conclusion: The hydraulic conductivity of soil is affected by several factors, including texture, bulk density, living organisms, organic matter, soil management, salinity, and chemicals. In other words, any factor that alters the characteristics of soil pores and the fluid (water) can also change the hydraulic conductivity. To maintain and improve hydraulic conductivity, sustainable soil management, reduced compaction, increased organic matter, and salinity control are essential.