Re: when people say "good drainage" what do they mean?
Whether your soil has good drainage or poor drainage is a statement of how fast surface water seeps away into the subsoil. A simple test to determine this is to dig a hole about a foot deep and about spade width. Fill this hole up to the rim with water and allow the water to drain out completely. Do this on a day when it's not rainy. Return the following day and fill it back up to the rim with water. With your watch or stop clock, observe the time it takes for all the water to disappear.
If the water is draining so fast that you can't even fill up the hole in the first place, then your soil is too well drained. Even if the water soaks in within the first few minutes, the subsoil drainage is still too fast. These types of soil usually has a high content of sand and grit. It will not be able to retain water or plant nutrients for healthy plant growth.
If the water takes more than one hour to disappear, then the drainage is poor. You will probably have noticed this already by the persistence of puddling after a shower. These soils are usually clay, which has very fine particles that adhere to each other extremely well and firmly, making the particles pack themselves tightly together. Not only is there poor drainage, but aeration is also very poor. Plants don't grow well in these conditions, unless bog or aquatic, because of the lack of oxygen supply to the roots - i.e., they drown. The soil, starved of oxygen, encourages the growth of anaerobic organisms, giving rise to the typical foul smell, and damaging plant roots by direct rotting, or effect of the usually acid pH or other chemical by products of anaerobic activities.
Whether your soil is too fast draining or poorly drained, the best fix is the same - add as much organic material as is necessary. Organic ammendments include backyard compost, composted manure, leaf mold, etc. I find peat moss not a good alternative, as the organic material in peat moss is short lived and in very clayey soil, the small organic particles mix with the clay to make what looks like the stuff you make hypertufa containers out of when dried! At the extremes of soil conditions, you may have to add a lot of organic material. If the soil is extremely clayey, you may need to add coarse sand or grit (not fine sand - because that's like making concrete) or treat the clay with gypsum.
Last edited by Weekend Gardener; February 5th, 2006 at 12:10 AM.