How water moves in soil

From low spots to damaged tile, nearly everyone has some sort of drainage dilemma. But do you know how your drainage system actually works?

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 high- intensity 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 non- compacted 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.

TILE CONSIDERATIONS

Luymes reiterates tile drainage does not remove needed moisture from soil; it just removes excess gravitational water.

“We’re just lowering the water table faster than it would naturally,” says Luymes. She adds tile also pays in dry years, as removing excess water early in the growing season drives the development of larger crop root systems—a benefit when hot, dry weather arrives later in summer.

Jefferies says water can enter tile from above through “macropores,” or large spaces formed by worms, roots, cracks, and so on. While this is a good thing, he cautions farmers need to be mindful when it comes to nutrient applications, as macropores offer a “direct conduit” to field tile.

On the whole, though, water enters tile from below as saturation raises the water table.

“We often say water runs downhill, but agriculture depends on water going up a lot…Water still has to go down, but then there’s that pressure difference, which is why it would be going down to go up again,” says Luymes.

Differences in pressure within the soil also create humps of moisture between tile drains. The larger that hump is, the less venly a field will drain. This can be remedied with the right tile spacing and depth. Ontario Ministry of Agriculture, Food, and Agribusiness Publication 29, Drainage Guide for Ontario, lists different spacing and depths for different soil types.

“Soil has its own way of draining,” says Luymes, reiterating farmers need to understand both natural drainage and how much water tile systems can drain—the latter being called the “drainage coefficient.” Regardless of tile spacing and depth, the size of the tile outlet is critical to a system’s ability to handle water. Trying to save money by opting for an outlet smaller than what’s recommended by the contractor is not a good idea.

“Imagine a bottleneck. If you want to have good drainage, you have to pay for a massive outlet…There is math involved. You can’t skimp on the outlet. Ever,” says Luymes in a later interview. “That system will need to be designed, or potentially overdesigned, for the future.”

Should a grower want the ability to retain more water, controlled drainage systems are an option. Luymes describes them as underground dams, where water is retained until it reaches above the intercepting panel housed within a tile main. Different types of tile, too, can move water more quickly or slowly, and terraced tile systems can provide good drainage in fields with topographical differences. Jefferies and Luymes add good soil health practices—avoiding compaction, keeping soil covered, adding organic amendments, responsible tillage, crop rotation—and improving drainage.

Some fields, though, have waterlogged areas that won’t go away. For Luymes, such cases “just could be a wetland. Let’s just admit that.” •