Water on contact with a soil’s surface may do one of three things. Depending on the geography and soil, water may continue downhill, or pool on the surface, or soak in. Water that soaks into a soil is said to infiltrate it, and it follows that it is this water that will ever reach plant roots.

But infiltration in itself isn’t enough — water also needs to enter at an infiltration rate sufficient enough to wet enough soil at enough depth that plant roots can access it. Just think of a very light rain spotting a dirt surface. You only need to gently scratch that surface to see how dry the soil is underneath. It may as well not have rained at all.

Infiltration and infiltration rate will vary depending on the amount of water applied to a soil as well as the type and structure of that soil. A gentle, lengthy rain on dry, well-structured soil will readily enter the surface cracks and channels between peds, soaking into the pores until they fill. If that rain continues long enough, the water will continue to percolate deeper and deeper down the profile, filling even more pores as it goes. Eventually it will reach a permanent zone of saturation known as the water table, from which bores, wells and springwater are sourced.

Rain falling on heavily compacted, poorly-structured soil may not soak in much at all, but rather pool on surfaces, leading to runoffs and soil erosion. But this will be the fate of any water regardless of the soil, if the intensity of its fall is greater than the ability of the soil to soak it up.

Very sandy soils, with their large minerals and pores, as a rule will enable more water to infiltrate and at a higher infiltration rate than that same water into high clay soils with their smaller particles and pores. Conversely, the deep cracks that form in cracking clay soils when dry can enable both a high infiltration and infiltration rate of water deep into the soil profile before the clay minerals swell and seal the cracks.

Infiltration rate is measured as mm (height) of water soaking in per hour, and can range from zero in water-repelling sands to several hundreds of mm in very coarse sands. Infiltration rate into soil decreases the wetter the soil becomes, but over time reaches a steady rate known as the saturated hydraulic conductivity of the soil. This is the rate at which water enters a soil’s surface and percolates through the topsoil to drain below. Any compacted sublayers or underlying bedrock will reduce this rate. Eventually water already in the soil will no longer be able to drain away and the water table will rise. In extreme cases too much infiltration will lead to waterlogging and eventual root death if that water cannot drain away quickly enough.

Even very gentle rain falling on wet soil will pool or lead to runoff if the steady infiltration rate is very low, at less than 5 mm an hour. Another thing to be aware of if using an irrigation system, is that if the system delivers at a rate higher than the infiltration rate, water will pool and be wasted. And water that doesn’t infiltrate soils in low-rainfall areas is just too valuable to allow wastage through surface pooling and runoff!

It’s best to avoid traffic on any soil on which water has pooled for days after rain stops, so as to minimise compaction and disruption of the soil’s structure.

Increasing infiltration rates can be an easy solution for excessive pooling, and with quick results. Coring or spiking the soil to create instant large holes and channels may be enough. Some instances may require digging to break up compaction, whether of the soil itself, or a sublayer below, or of any thick compacted mats of dead and dried organic matter on its surface. Digging into the soil may reveal the compacted sublayers or other obstructions such as shallow bedrock that were the problem all along, which can then be remedied or avoided by relocating plants.

Next week we’ll examine how water, once it has infiltrated soil, then moves through that soil.