If you read planting guides and gardening magazines, you'll come across information stating this plant does well in loamy soils or that plant tolerates clay. But what are the different types of soil, and how do you know which one you have?

Soils are often typed or classified by the proportions of three particles: sand, silt, and clay. Our soil determines much about how our garden will perform, what plants will do well, and any needed improvements. We'll cover the basics and show you how to test your soil at home to get a better idea of what you're working with.

Where does soil come from?

The substance we refer to as soil has two main solid components: tiny bits of geologic material and organic material. There are also uncountable numbers of microorganisms, air and water, and maybe some human-deposited items.

When the earth was new, there wasn't any soil. Physical and chemical weathering processes broke down rocky material over long periods of time. The slowness of the process is why soil scientists warn that it takes centuries or even millennia to replace an inch of soil lost. 

The original rocks and minerals a soil is formed from are called the parent material. For example, some soils may be formed from igneous rocks and others from sedimentary rocks. Parent material can be bedrock, leftovers from glaciers, or even flood deposits. 

Physical, chemical, and biological forces wear down the parent material into soil particles as time passes. Big rocks became little rocks, pebbles, and eventually sand and dust. If you garden in an area where glaciers visited, your soil is likely highly influenced by the glacial till, or jumble of rocky stuff, which the glacier left behind when it melted.

When we discuss soil particles smaller than gravel and pebbles, we separate them into three size classes: sand, silt, and clay. Together, they form the geologic portion of the soil. Rarely is a soil only one of the three, but a few soils are so heavily weighted toward one particle that the others seem absent, like sugar sand soils or very heavy clay soils. When we add microbial life and organic material to the geologic portion, we have soil.

Sand

The largest of the three, sand particles are easily seen with the naked eye. We know they feel gritty and coarse. Sand particles range widely in size from 2mm to 0.05mm. Their large size creates a lot of pore space between soil particles for air and water flow. To imagine the pore space, imagine a wire cage full of basketballs. There's a lot of air space between the basketballs because they don't stack very tightly together. 

Sandy soils drain very rapidly, as you've experienced if you've ever watched waves come ashore at the beach. A soil heavy in sand will feel gritty, and you will likely even see bits of sand in a handful.

Silt

Silt particles are the middle-sized component in our list of fine soil particles. They're smaller than sand but larger than clay and range from .05mm to .002mm. That's still a large variation, but to our eyes, they are getting small enough to be difficult to discern without magnification, especially at the smaller end. 

Silty soils have some pore space but not as much as sand. Instead of basketballs in a cage, imagine baseballs. The empty, non-occupied spaces are much smaller, which slows percolation and drainage of water, as well as air movement. Silty soils drain well depending on their location in the landscape, feel like flour when dry, and are smoothly slippery when wet but not sticky. 

Clay

Clay is a soil particle formed from rocks and minerals like the others but weathered into much smaller pieces. Clay is the smallest fine soil particle, less than .002mm in size. They're so fine they're indistinguishable to our eyes and are 1000 times smaller than a small sand particle.

If sand particles are basketballs and silt particles are baseballs, clay particles could be frozen peas or even the period at the end of this sentence. They pack together pretty efficiently without much pore space, which is why they are famously slow to drain. It’s hard for the water to find a path. We call clay soils heavy or poorly drained, and in fact, water moves through heavy clay so slowly that it is used as an impervious liner for ponds and earthen dams.

Clay soils feel sticky and easily form ribbons when wet. The particles are too small to see with the naked eye, and clay soils are hard and blocky when dry.

Testing Soil Types at Home

For the curious gardener, there are ways to narrow down what sort of soil you're working with. Of course, we can visually see if the soil is quite sandy, and using our sense of touch is another. While classifying soil by feel sounds a bit imprecise, you can actually get a pretty good idea by following this flowchart from the NRCS. 

Squeezing wet soil between your fingers and seeing how it reacts is a fun project to keep in mind when getting kids involved in gardening, too. 

The jar test

Here's a fun one to do at home. Since sand, silt, and clay are different sizes and masses per particle, they will settle out of a solution in a stratified pattern. While you won't always get a clear boundary between each soil type, you can get a pretty cool picture of the amount of sand, silt, and clay in your soil. 

• Grab a clear jar and some liquid dish soap.

• Obtain a sample of soil from your garden and remove any rocks, sticks, worms, or mulch. You'll need about half a jar full.

• Place the soil in the jar, add 2-3 TBSP of liquid dish soap, and fill the rest with water to an inch from the top.

• Put the lid on tightly and shake the jar—really shake it! Try to shake for about five minutes to ensure everything is well-mixed.

• Allow the jar to sit undisturbed for 24 hours.

After the jar sits for a day, measure the height of settled soil, for example, 3.0 inches. Reshake the jar for five minutes and then set it down, but only wait 40 seconds. Measure the height again. That's the amount of sand in the sample, for example, 1.5 inches. 

Measure again after 30 minutes (no more shaking), and that's the amount of sand and silt in the sample. In our example, this measurement is 2.0 inches. Now, it's just a bit of math. 

Percent Sand = (sand height / total height) x 100   

Ex:  (1.5/3.0) x 100 = 50% sand

Percent Silt = (sand and silt layer - sand layer) / total height x 100    

Ex:   (2.0-1.5)/3.0 x 100 = 17% silt

Percent Clay = the rest, or 100 - (% sand + % silt)    

Ex:  100 - (50 + 17) = 33% clay

A result of 50% sand, 17% silt, and 33% clay classifies as a sandy clay loam on the soil triangle. Pretty decent stuff! Search online for the soil triangle, and you'll find tons of images. Take a moment to understand the directions for each line when using it and enter your own numbers.

Soil Particles’ Role in Drainage

Gardeners dream of loamy soil, the perfect mix of sand, silt, and clay, because most plants thrive in it. A really good loam soil might have 40% sand, 40% silt, and 20% clay. However, most of us aren't that lucky and have to work with what we've got. 

Soil drainage, or the movement of excess water through the soil profile, is important for many plants. While a few plants have adapted to wetter conditions, most garden crops don't like what we call wet feet or soil that drains slowly. 

All soils hold some amount of water that is available for plant roots and some water that adheres too tightly to soil particles and is unavailable for plants. The rest of the water is free to drain away and open the pore spaces up again. Without adequate drainage, excess water displaces the air in the soil, and the roots can't get oxygen. Eventually, without oxygen at the roots, the plant drowns just like we would. 

Considering the pore space and our analogy of basketballs, we can visualize drainage patterns. A soil that leans more toward sand will drain quickly (large pores and paths) but also has less capacity to hold plant-available water for use between rain or irrigation events. Surprisingly, loamy soil holds more plant-available water than clay soil, even though the clay soil drains slower. Clay soils often hold water very tightly, and thus have a lot of unavailable water but not as much for use by plants. A good loam is the best of all three, for drainage and fertility.

An important note: Don't add sand to help with drainage in clay soils. While it seems like a good idea, what you normally end up with is concrete. Instead, add lots of organic matter like compost, chopped-up leaves, and mulch to increase pore space and help with clay soil drainage.