Minerals in soils determine its texture
What's the soil like in your garden? You'd probably describe it as sand, clay, loam, or something in between. To soil experts, these are all soil textures, determined by the percentage a soil contains of mineral particles of various sizes. Texture, in turn, influences drainage, a very important aspect of soil quality. Water applied to the soil surface percolates down through the pore spaces between soil particles. At first, it completely fills the pores. In time, however, it's drawn away--carried downward by gravity, absorbed by plant roots, drawn up from the soil surface and out through foliage. As water leaves the pores, air returns to them, until just a film of water remains on the soil particles.
Clay soil--whether you call it adobe, gumbo, or just plain "heavy" soil--is composed of many flattened, tiny particles packed tightly together to form a dense mass with microscopic pore spaces. (Clay soil can be so dense, in fact, that in some parts of the country it's used to make bricks!) Drainage is slow, since water and nutrients move through the pore spaces slowly. On the plus side, clay's slow drainage lets you water less often. And such soil is better able than others to attract, hold, and release certain nutrients. On the minus side, clay is difficult for plant roots to penetrate, and during rainy periods it can remain saturated and airless to the point of suffocating roots (which need air as well as water to live).
Claylike soils are produced by a number of factors. In parts of California, for instance, the particular series of geological events and climatic changes eventually broke a high proportion of mineral particles down to microscopic size; due to the lack of summer rain, vegetation was sparse, and what there was tended to dry up and blow away rather than decaying to help loosen the soil.
At the opposite end of the spectrum from clay is sand, typically found in areas near oceans and rivers and in places where these once existed. Its large, irregularly rounded particles fit loosely together, with large pore spaces between them. Water and nutrients drain through sand quickly--so fast that it can be difficult to keep plants well watered in hot weather. Sand is less fertile than clay, but roots penetrate it easily, growing deeply and rapidly, and it's far easier than clay to work.
A third soil texture, midway between the extremes of sand and clay, is loam. This is the excellent soil often found in climates with ample summer rain and cold winters: the rainy summers encourage lush vegetation, while the harsh, freezing winters make for the slow decay of dead plants directly into the soil. This combination of conditions tends to break down mineral particles into a variety of sizes, ranging from quite large (as in sand) to almost microscopic (as in pure clay), with particles of an intermediate size (known as silt) in between. Loam typically contains about 40% sand, 40% silt, and 20% clay. Soil of this texture that also contains a relatively high proportion of organic matter (5% or more of the total volume) is the ideal that gardeners strive to achieve.
To identify the soil texture in your garden, thoroughly wet a patch of soil, then let it dry out for a day. Now pick up a handful of soil and squeeze it firmly in your fist. If it forms a tight ball and has a slightly slippery feel, it's predominantly claylike. If it doesn't hold its shape at all but simply crumbles apart when you open your hand, it's sandy. If it is slightly crumbly but still holds a loose ball, it's close to loam.