Soil isn't just dirt and it isn't just an anchor for plants. While plant shoots reach up to the sun above ground for light to convert to food, the roots of plants travel down into the dark, collecting nourishment from soil, which is made of minerals, organic material, water and oxygen. Soil texture refers to the size of soil particles, which varies from coarse to fine; and the types of soil are described by texture. Though the broad categories of soil are sand, clay, silt and loam, a lot of overlap exists between the types.
In sandy soil, clay (the finest kind of soil particles) and silt (the next finest kind of soil) make up the minority of particles. Fine sand is about .02 to .2 mm, while coarse sand is .2 to 2 mm. Sandy soils, especially those with coarse particles, allow for a lot of drainage. They also hold a lot of air. The roots of plants need that air for oxygen. Sandy soil also warms up well, which aids in germination.
Why isn't sandy soil ideal for all plants? For one thing, the drainage of sand means that sand might not hold enough moisture for some plants. Another problem is that draining water takes minerals--nutrients--away. This leaching action makes sandy soil more acidic than other soils.
Clay soils are made up of about one-third clay. The texture is extremely fine compared to sand--less than .002 mm. Because the particles are so fine, there isn't a lot of space and water does not drain as well as in sandy soils, so clay retains nutrients and moisture--often too much moisture. The fine soil also makes the soil prone to compacting, so that clay can be difficult to till. The retentive property of clay makes it tend toward alkalinity.
The size of silt particles is about .02 to .002 mm. Silty soils have more sand than clay (otherwise the soil would be too clay-like) and when it gets wet and then dries, it becomes crusty, which makes it hard for roots to spread and for seeds to push sprouts through the surface of the soil.
Of all the soil types, loam is considered ideal. The majority of loam is sand, with silt the next highest component, with clay being the smallest percentage of the soil. The sand lets the water drain and keeps soil aerated, while the clay and silt don't let the water drain away too much. Loam is the most important kind of soil, for it most easily allows plants to thrive. Gardeners with clay-ridden, silty or sandy soil can amend those soils by adding some amount of underrepresented soil type and by adding organic matter.