The Dirt Issue 35 – Surface Drainage

Posted on January 30, 2019
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Surface Drainage Explained
Anyone driving past Skagit Valley farmland has seen them, V-shaped ditches snaking through a field to empty into a field-bordering, usually roadside ditch. Their purpose is to slowly, with low velocity, drain the field of standing water ponded in a slight depression.  But do you know why they’re here and rarely seen elsewhere?

Once again, it’s the soil.
As with so much in the story of Skagit farming, the reason lies in the soil. Remember, soil is a complex mixture made up of mineral content, organic matter, air and water. The mineral makeup is the result of geologic history, climate, the lay of the land, organisms present at various times, and time itself. Mineral decomposition creates sand, silt and clay. The amount of each and their percentages relative to each other are what determine soil texture. The composition of soil in the Skagit Valley covers a wide range, sometimes varying considerably in a single field and many times over on a farm. The soil exhibits properties based on its composition.

What separates sand from silt and from clay is particle size. Sand is the largest, easily seen with the naked eye. Clay is the smallest, so small, in fact, that a single particle cannot be seen with the naked eye. A frequent comparison is that if a clay particle were as large as a BB, the corresponding grain of sand would be the size of a chair!  What doesn’t work in that analogy is that sand is round (like the BB) but clay particles are thin, flat, and prone to stick together.

Expanding clay in Skagit farmland
Certain soils in the Skagit Valley have a higher percentage of clay. There are many different kinds of clay, but those that lead to V-ditching are known to expand when wet and shrink back when dry. The fall-to-spring rains cause soil expansion and the result is a nearly impermeable barrier that closes off access to the soil’s structure of pores. It’s the pores that allow infiltration of water and air into the soil. If a crop is in the ground—such as tulips, for instance—expansion shrinks the pores and cuts off oxygen to the root zone and the soil temporarily becomes anaerobic (without oxygen), preventing the uptake of nutrients and water that the plant needs to grow and thrive.

A long-standing cultural practice
Surface drainage has been a remedy in the Skagit Valley for decades. Because a farmer is intimately acquainted with the land, knowing the soil profiles and natural contours, the ditches are dug to compensate for the topography of the land and the propensity of water to pool. It is a temporary necessity brought on by seasonal weather. When the surface ditches are no longer needed they are filled in and normal cultivation practices resume.

Other benefits of V-ditching
Even if a field is fallow over the rainy season, surface drainage is important to soil health. Waterlogged soil is not workable soil. Water saturation over time weakens soil structure making it prone to compaction. And, no matter how fertile soil is, if the structure is not strong, if it collapses due to compaction, the channels by which a plant obtains nutrition are inadequate and the crop suffers or is lost. Draining excess water protects the soil structure. It allows heavy farming implements to be used on the land and prepare it for planting.

Another negative effect of standing water is its cooling effect on soil temperature. If soil isn’t warm enough, seeds don’t germinate. The planting season has to be delayed until conditions improve, naturally and ever so slowly.

Simple and effective
Surface drainage is an easy fix for low or wet spots in a field, targeting those specific pockets that do not drain well due to the expanding clay. Their simple V-shape keeps the sides from collapsing in a heavy downpour and allows those isolated areas to be well managed throughout the rainy season. So, whether to help overwinter a crop susceptible to drowning, maintain soil structure, or optimize planting conditions, V-ditching is a proven, effective tool for successful farming in the Skagit Valley.