CORVALLIS, Ore. — Healthy plants start with healthy soil, and that means understanding soil texture, said James Cassidy, a soils instructor at Oregon State University.
“The texture of a soil is its proportion of sand, silt and clay. Texture determines drainage, aeration, water-holding capacity, erosion potential and even nutrient storage.”
“The texture of a soil is its proportion of sand, silt and clay,” Cassidy said. “Texture determines drainage, aeration, water-holding capacity, erosion potential and even nutrient storage.”
Cassidy recommends the “hand method” to get acquainted with your soil. Dig beneath the organic matter to the mineral layer, about 6 to 8 inches deep, and scoop out a moist handful. Knead it into a ball, adding water if necessary.
If it can be rolled into a ribbon, the soil has clay content. Each inch of ribbon equals roughly 10% clay, so a 4-inch ribbon is about 40% clay. If wet soil feels gritty, it’s sandy. The remainder is silt.
When gardeners praise “nice loam,” they’re referring to a balance of sand, silt and clay. Loam holds enough water for plants but drains well enough for roots to breathe.
Clay’s benefits and challenges
Clay can be slow to drain and hard to work, but it’s also where soil nutrients are stored.
“Sand and silt don’t store nutrients; they’re just rocks,” Cassidy said.
Many Willamette Valley soils are clay-heavy. The solution is adding organic matter. Cassidy recommends planting cover crops from mid-August to mid-September, such as annual rye grass or winter wheat with Austrian field peas or fava beans. Cut them before seed set in spring, let the residue rot, then incorporate it into the soil.
When starting a new plot, Cassidy advises using a garden fork instead of a rototiller, which can destroy soil pores that help water infiltration and drainage.
“Think minimum tillage,” he said.
If tilling is necessary, do it when the soil has the right moisture. If a squeezed handful is too hard to break, it’s too dry. If it breaks apart easily, it’s ready.
Add organic matter yearly, and in about five years even clay-heavy soils will have better drainage and more life.
“There are literally a billion microorganisms in a single pinch of soil and they all need to eat,” Cassidy said.
They get energy from organic matter, not photosynthesis, and thrive best in well-drained soils with organic content.
Previously titled Sand, silt or clay? Texture says a lot about soil
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