Variety of soils Rogue Valley soils range from sandy loam to heavy clay. Most of the clay soils originate from rocks of ancient volcanic activity in the Cascade Mountains. Granite soils are from older, more durable and slowly weathered rocks of the Siskiyou Mountains. The granite rock materials formed sandy loam soils that drain well but are not as rich in nutrients as clay soils. Gardeners usually prefer loam or sandy soils.
Clay soils drain and dry slowly.
When wet, they are sticky and difficult to garden. If you work clay soil when it's too wet, it will make a poor seedbed of hard clods. Prepare garden soil when it's no longer sticky and when a handful squeezed together doesn't form a hard lump. Improve clay soils by adding organic matter, an excellent soil amendment regardless of soil type. In well-drained granite soils, organic matter improves water retention, while in heavy clay soils it improves soil drainage.