Knowing how water moves and how different conditions enable or hinder drainage can help remedy recurring problems in the field, according to Mel Luymes, executive director of Land Improvement Contractors of Ontario, and Danny Jefferies, agronomy lead with Honeyland Ag Services in Ailsa Craig. Speaking at the 2025 Southwest Agriculture Conference, the pair detailed the interaction between the physics of water, soil type, and drainage systems. The subject is particularly timely, says Jefferies, considering the previous year saw significantly higher than average rainfall across much of Ontario. Weather trends also highlight more highintensity rainfall events, delivering substantial volumes of moisture over short periods of time. Having effective drainage is, consequently, of growing importance. PORE SPACE Soil structure is a key factor in how water moves, says Jefferies, “but as with everything in the world, it happens for a reason—and that reason is usually physics.” The interaction between pore space and water changes in soil depending on pore size. Water in wet soils flows primarily through large pores, the same way water flows in a river. This is referred to as the gravitational flow of water. In unsaturated conditions, water moves horizontally and upwards due to what Jefferies calls “matric capillary forces,” a result of the adhesive properties of hydrogen ions in water molecules, which bind them to soil particles, and cohesive properties binding water molecules to each other. “This is what gives water tension,” says Jefferies. The smaller the radius of the soil pore, the higher the water will rise. However, smaller pores also mean more adhesion. Water will often move horizontally across soil with similar pore sizes before moving into layers with larger pores. “Capillary action is quite slow, especially in these loamy texture soils, so we do get capillary movement, but water moves much more slowly than in the sand.” INFILTRATION AND CONNECTIVITY Water has to first get into the soil, of course, and good infiltration can stave off ponding and runoff during periods of significant rainfall. Pores on the soil’s surface are critical for infiltration, and damaging surface pores— through compaction or the battering effect of rain on bare ground, for example—lead to silting and crusting. Compaction also creates smaller pores and compresses the connective pathways between soil pores, meaning water has to take a slower, more meandering path to reach depth. Infiltration can take time, even in noncompacted soil. In sandy loam, for example, gravitational flow water moves at eight feet per day. In clay loam, it’s 1.5 metres per day. Jefferies says this significant difference in movement has direct implications on the number of field work days available and should be accounted for when considering drainage systems. From low spots to damaged tile, nearly everyone has some sort of drainage dilemma. But do you know how your drainage system actually works? Matt McIntosh How water moves in soil Considerations for better drainage ONTARIO GRAIN FARMER AGRONOMY
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