The basic idea: The texture of soils can be divided into three parts according to particle size and structure: sand, silt, and clay. The various concentrations and organization of these three types of particles is what gives a soil its physical structure, and it is the physical structure of your soil that has a lot to do with how well your plants will fair in your garden.
The details: The texture of your soil is fairly easy to determine and will tell you a lot about you’re your growing capabilities. Texture is divided into three categories according to particle size: sand, silt, and clay.
Sand has the largest particles of the three basic types of soil. When you feel a soil with a lot of sand in it, it will feel gritty and coarse. Because of its coarse nature and round particles sand will naturally have more air because of larger space between the particles. Sand drains water better for this reason, but that can also lead to a washing-out of the soil. Sandy soils can’t hold nutrients very well, so they’re pretty poor soils as far as nutrient content is concerned. Plants that need good soil drainage do better in soil that is largely made up of sand. To improve a sandy soil so it can hold more nutrients and still keep its drainage quality, the best thing you can do is add organic matter, compost, and well-decomposed manure.
Clay is the smallest of the three particles. The particles are actually smaller than dust particles; they are microscopic so they feel smooth to the touch. Unlike sand, clay particles are angular in shape and are thus able to pack together very tightly. Imagine the difference between putting a bunch of tennis balls in a bucket and a bunch of blocks. The edges of the blocks fit smoothly against each other. This is why clay soil does not drain nearly as well as sand, and when it dries it can create rock-hard lumps. The picture to the right can give you a pretty good idea of the nature of sand vs. clay soils. Because of its microscopic structure, it can pack together tightly. Clay holds water very well, and thus it also holds nutrients a lot better than sand. To improve drainage in clay soil, without losing its capacity to hold nutrients, the best thing you can do is add organic matter, compost, and well-decomposed manure.
Silt soil is somewhere in between. It is smaller than sand so it will also feel quite smooth to the touch. However, its particles are larger than clay and its structure is different, so it doesn’t pack as well.
Soils with smaller particles, such as clay, have a greater capacity for holding water and nutrients because of their increased surface area. However, with too much clay, the soil can hold a lot of water and become very boggy. Most plant roots don’t like that -they need air to breath. In fact I remember one of my professors saying that you can effectively seal in water to create a pond by laying down a thick layer of clay, a thin layer of sodium chloride (the sodium ion scatters clay), and another layer of clay on top of that. The ground will be virtually impenetrable by water and it will nearly be as though you put in cement. I don’t think I would ever want to do that in my landscape, but it was an interesting side note.
Sand has the advantage of draining better. It doesn’t hold water or nutrients as well as clay, but it does allow air to get in and around the roots. Also, it doesn’t become as compacted. This is an especially important concept for places like playgrounds or sports fields, where the ground is getting tromped on all the time. Mixing in a good dose of sand will help prevent the soil from being packed so tightly that it crushes the roots, as can happen with clay soils. The two pictures above are a very good example of this. You’ve probably seen ‘cow paths’ similar to this. In this case, walking off that corner of the sidewalk over to the parking area was more convenient than taking the roundabout way of the sidewalk.
I just want to add, as a last note, that clay, silt, and sandy soils can all be improved with organic matter (see this article here on organic matter – it’s one of the most important ones we’ve produced!). Sandy soils aren’t devoid of nutrients as some people think, but if you want plenty of nutrients for plants to grow, add some organic matter. Composting, especially making your own, is becoming more popular, and it is a great way to add nutrients and organic matter to any kind of soil.
Determine your Soil’s Texture. If you want to get a rough idea for your soil’s texture there’s a way to do it with your hands. Pick up a good handful of soil and feel it carefully between your fingers. If the soil sample feels gritty then you probably mostly have sand in your soil. If the sample feels smooth then you probably have mostly silt or clay or both in your soil. The way to tell between a predominantly silty or clay-ey soil, since they both feel smooth to the touch, is to pinch the soil between your thumb and index finger and push the soil out over your index finger, using your thumb, into a “soil ribbon”. If the soil ribbon stays together and extends out into the air without crumbling then the soil is predominantly clay. If the soil ribbon readily falls apart when pressed out over the index finger then the smoothness was mostly silt.
The picture above right is a good example of a decent-sized soil ribbon, indicating a large amount of clay.
Of course, most soils are rarely pure sand, silt, or clay, but a combination of the three. You can get a ‘feel’ (literally) for soil combinations with practice. A soil that isn’t gritty at all but produces a small soil ribbon is probably a good mix of silt and clay. A soil that is gritty and produces a decent-sized ribbon is probably a combination of sand and clay. Get the idea? Just remember the rules: sand feels gritty, silt feels smooth and won’t make a soil ribbon, and clay feels smooth and will create a soil ribbon.