All soils are composed of roughly equal quantities of weathered rock particles and the spaces between them, along with a small but absolutely crucial amount of organic matter. The different types of soil are defined by the particle size of the rocks — sand, clay, or silt — and how much of each you have.
Sand particles are large enough to be clearly visible, and you can feel them as grit when you rub the soil between your fingers. The spaces between them are big, so they keep soil open, providing good drainage, and there is lots of room for oxygen. When there is too much sand in the mix (the problem I have at our lake cottage!), soil cannot hold water, and nutrients wash away.
Clay particles are tiny. You can’t see them, but you can feel them as sort of slippery when the soil is wet. The spaces between them are very small, so they hold water and nutrients for a long time. But that very smallness can make it hard for plant roots to get at the goodies. Soils that contain too much clay water-log when wet, and tend to turn rock hard when dry.
Silt falls between sand and clay. Particles are still quite small, but the spaces between them aren’t as tight as in clay, so drainage is much better.
Loam is the garden ideal, the rock part composed of roughly 40 percent each silt and sand, with 20 percent clay, which is just enough to hold water and nutrients without causing problems. Most gardens fall short of this Shangri-La, tending more toward clay or sand, based largely on the geological history of the area where they are located.
Never Too Much Compost
Unless your soil falls into the ideal, you’ll want to consider amending your soil. Compost, compost, and more compost can only do good — assuming there’s nothing damaging in it. You know the homemade sort is fine, but don’t make hasty assumptions about the stuff you buy.
In its backyard incarnation, compost is the glorious refutation of “garbage in, garbage out.” You put in banana peels, egg shells, and dead flowers; out comes the basic building block of healthy soil. But not everything nasty is capable of transformation. Some commercial compost is made from sewage sludge, which often contains heavy metals that composting does not remove. Other large-scale producers might use paper bleached with dioxin. And some sellers may offer ‘compost’ that is actually nothing more than partially composted bark or sawdust.
These products are almost all carbon, though they can look very much like compost made from a proper mixture of carbon and nitrogen. They take nitrogen from the soil in order to finish decomposing, and when they’re done they don’t give much back except improvements in texture.
On the other hand, many commercial compost makers are environmental heroes, entrepreneurs who saw the diamonds hidden in rough stuff like fish waste. When in doubt, just ask — and don’t buy from anybody who won’t give you an ingredient list.