3 Types of Soil Particles Sized From Biggest to Smallest
Though we might spend a lot of time trying to make sure our plants grow in nutrient rich soil, we may not think much about how soil size can affect our gardens' growth. After all, we might think, besides the nutrients, dirt is dirt, right? Well, no. The size of soil grains--its texture--changes how the particles fit together and that, in turn, affects how much air and moisture exists between grains, how well the soil drains and the amount of nutrients the soil can retain. It's a good idea, then, to know your soil's texture, and choose plants accordingly.
The USDA classifies sand particles as having a diameter of .05 to 2 mm. The grains are coarse. Consequently, they don't fit together tightly and have big spaces, or pores, between them compared to the other kinds of soil particles. With such porosity, sand drains quickly, so plants that like wet conditions won't enjoy the texture. Additionally, fast drainage means nutrients leach down and away from shallow roots.
On the plus side, though, is sand's aeration. Roots do need air to thrive--it lets the roots absorb oxygen--and sand amply provides that. Meanwhile, sand warms up quickly, which means a quicker start to seeds that wait for soil heat to signal sprouting time.
Silt particles range in size, according to the USDA scale, from .002 to .05 mm in diameter. It's a difficult soil to grow plants in. Water makes a crust on silt soil, and this hard layer disallows more water to drain through. It also blocks air. Less coarse than sand, silt can compact more easily and then is difficult to work. Yards with such soil need amendments to be added, for instance, sand.
It's difficult to see the individual particles of silt without a microscope. Lacking visual clues, if you're in doubt about whether a soil is silt or not, you can feel it. Silt is soft and smooth like flour.
While sand feels like grit and silt feels like flour, clay feels sticky. This kind of soil particle is the smallest of the three types, with a UDSA declared diameter of less than .002 mm. Since it is so small, clay particles don't drain the way sand does. Plants that don't like "wet feet" will have difficulty in such soil. There's also the danger of smothering plants, since clay's moisture-retaining properties block out air. A soil with a lot of clay will have to be amended with materials to make it lighter.
On the other hand, clay is the most fertile of the three types of soil.
- "Botany: A Functional Approach"; Walter H. Muller; 1970
- NASA: Soil Education Page