I sometimes find it hard to imagine myself as a new gardener. After all, I’ve been gardening all my life, so when I hear a term like “moist, well-drained soil”, I know exactly what that means. Yet, new gardeners find the term confusing. It is indeed an oxymoron, a figure of speech that juxtaposes two elements that appear to be contradictory, yet they really aren’t.
So, how soil can it be both moist (rich in water) and well drained?
The term simply means a soil that retains some moisture, but not too much. Even after a good rain, there will be no puddles (puddles are a sure sign of poor drainage) because the excess water drains down out of the root zone. Enough of the moisture adheres to the soil particles so the soil remains humid (moist but not wet) for a fairly long period of time. How long depends on a lot of circumstances (temperature, exposure to sun, the avidity of plants growing there, how dry the soil was to begin with, etc.), but a good “moist but well-drained soil” will probably still hold enough humidity to keep plants from wilting for a week after a decent rain.
And the vast majority of plants prefer soil where some moisture is present, but where there is still good air circulation (because the roots need to breathe too). That’s why gardeners are so interested in making sure their soil is “moist but well drained.”
To be sure, there are plenty of exceptions, plants that will grow in soil that is always soggy (such as bog plants and semi-aquatic plants) and others that are extremely drought tolerant (cactus and succulents, notably), but the majority of the plants you will want to grow, and that includes not only perennials and annuals, but herbs, vegetables and fruit trees, do best in moist, well-drained soil.
Making It So
Some soils are naturally moist and well drained, but most garden soils have been modified to that state by the gardener, usually by adding a lot of organic matter. Most purchased soils (3 in 1 mix, top soil, container mix, etc.) will also be “moist and well drained”, at least if you’ve bought a quality soil.
How to Improve Sandy Soil
Very sandy soil drains quickly… too quickly: it holds almost no moisture. Mere hours after a heavy rain or a thorough watering, plants may already be wilting from a lack of water. By adding a good portion of organic matter (compost, shredded leaves, manure, etc.) to a sandy soil, it will be able to retain considerably more water, seriously reducing watering needs, since organic matter absorbs and retains moisture. However, organic matter also decomposes quickly in sandy soil, so you have to add more regularly.
How to Improve Clay Soil
Clay soils are just the opposite of sandy soils. Their very fine texture (clay particles are almost microscopic!) make them so compact and dense that water has difficulty flowing through, creating an environment that not only tends to be moist after a good rain, but out-and-out wet, hence the appearance of puddles. The richer the soil is in clay, the soggier it it tends to be. You can even create a pond by covering the bottom of a depression a good layer of clay, making it essentially waterproof! But air does not circulate well in heavy, wet soils and plant roots then tend to rot.
Curiously, when a clay soil finally dries out, which usually happens only in during a drought, it cracks and hardens and even repels water, so the poor plants that grow there go from a soil so dense and wet their roots can barely breathe to one that is so dry water has a hard time penetrating. Gardening in clay soil is far from an easy task!
Still, you improve a clay soil by adding organic material: compost, shredded leaves, manure, etc. And certain products sold primarily as mulch (forestry mulch, for example) also have a secondary use: they can be mixed with a clay soil to aerate it and improve its drainage. Mixing organic matter into clay soil is however very difficult: it forms heavy clods that are hard to break up.
One thing you should not do to improve a clay soil is to add sand. Although it may seem logical that adding sand, with its open texture, to clay soil, with a very fine one, ought to aerate the latter, and even though this bit of false advice is often bandied about about in horticultural circles, this treatment doesn’t work. Instead, it gives a rock-hard soil more like cement than good garden soil. (More info here).
However, there is one advantage to clay soil: once you have succeeded after much effort in working organic material into clay and turning it into a decent garden soil, at least the effect is long-lasting (the opposite of what happens when you add organic matter to a sandy soil).
To improve the water retention of sandy soil or the drainage of clay soil, it is often easier to simply cover them with a good layer of top quality garden soil (about 8 inches/20 cm will do). This is, in fact, what Mother Nature strives to do, gradually adding a thick layer rich in organic matter above the sand or clay soils by regularly depositing dead leaves and other organic materials. But it can take her hundreds of years to reach her goal, while you can create the same effect in an afternoon by covering your soil of questionable quality with a decent layer of top quality garden soil.
Yet again, the lazier you are, the easier it is to garden!