Soil with a steep slope, while having good surface run-off (often confused with good drainage), may have poor subsurface drainage if the texture is fine (high in clay) or if underlying soils create a barrier to water movement. Water is always held more tightly in fine soils than in coarse, sandy soils. A fine-textured soil underlaid with buried organic matter, sand and even gravel will not drain well. The water will not move through the coarse layer because it is held more tightly in the finer-textured soil above. The best soils for growing plants are uniform in texture throughout the root zone with a good balance of minerals, air and organic matter.
--Texture Test: Roll some slightly moistened soil between your thumb and forefinger. If it forms a firm ball, feels smooth and becomes sticky when moistened, it is too high in clay. If you cannot form a ball, the soil won't stay together and it feels somewhat grainy, the soil will be of a better texture. If, on the other hand, the soil feels very coarse, it may be too sandy and will not hold an adequate amount of water.
--Subsoil Drainage Test: Dig a hole in the Garden area about 12 inches deep and the diameter of a spade. Pour water in the hole to the rim. Refill the hole a day later and observe how long it takes for all the water to soak in. If the water soaks in within a few minutes, the subsoil drainage may be too good. Such soils may not hold enough water to sustain plant life and can loose valuable nutrients through leaching. If the water takes more than one hour to soak in, the subsoil drainage may be poor. Plants may suffer from oxygen starvation (drowning) under these conditions.
