As garden educators, we often hear, “I can’t grow anything” or “I need to get rid of my bad soil and replace it.” Yes, sometimes serious problems can arise with the soil in our yards, but it is often not so black and white. The earth under our feet is a complex world made up of many constituents: Microbes, decomposing organic matter, insects, air, water, metals, and rocks and minerals in scores of shapes and sizes. If there is too much or too little of even just one these elements, many plants may grow well, but perhaps not the plants you want to grow. Getting to know the makeup of your soil will guide you in future decisions about your garden. It can also save you money if you realize that you don’t have to throw out all the soil in your yard and replace it with something else!
Soil Types and Structure
Soil is made up of inorganic and organic matter. Glaciers, volcanoes and floods throughout history have deposited the source inorganic matter in the Puget Sound area: Glacial till (hardpan), glacial outwash (clean sands and gravels) and glacial lake sediments (silts and clays). The soil in our yards is a unique combination of these slowly evolving parent materials. Topography, climate, soil swelling organisms and time cause these parent materials to weather and become what we recognize as soil.
Inorganic matter is mineral by nature. Sand is the largest type of particle, which can be seen without a microscope. Silt particles are smaller than sand and require a microscope to see individual particles. Clay particles are the smallest. The proportion of these particles in soil constitutes soil texture.
Each of these soil types have different textures, moisture-holding capacities and fertility. Soil with a fairly even mix of sand, silt and clay is called “loam,”considered ideal for growing plants.
Minerals are an important part of soil. Clay particles are electromagnetically charged so they can attract positive ions as well as water. This is important for plants’ ability to get the nutrients they need. There are very small air pockets or pore spaces between the tiny clay particles. It can be difficult for roots to penetrate these tightly packed soil particles. When soils with a lot of clay become saturated during winter and spring rains, they take a long time to drain. They hold water so well that plant roots can rot or become susceptible to disease. When clay soils dry out, it is also difficult to rehydrate them.
Sandy soils have much bigger pore spaces. They drain much more freely, but nutrients will also leach out as well. They will need more water during dry seasons. Sandy soils are also not as fertile as clay soils. Silt is a sediment that is carried by water, ice and wind a deposited in soil and bodies of water. Silt particles are smaller than sand, but larger than clay. Loam has a good mixture of larger and smaller particles so that air and water can move freely through the pore spaces but retain enough to hold and provide nutrients to the plants making it the ideal soil that gardeners aspire to.
Soil organic matter refers to all living or dead plant and animal material found in the soil. Some organic matter is depleted by soil management practices and erosion so it must be replenished. Believe it or not, it also makes up a very small part of the total mass of soil. Healthy soil may contain only 3 to 5 percent organic matter by weight! Even though it’s a small player, organic matter has important duties: It binds the mineral particles, giving the soil a structure; it increases the amount of water soil can hold and how much is available for plant growth; and it is a major source of nutrients critical for plant growth, supplying soil organisms with energy for growth and carbon for new cell formation. The go-to remedy for soil that is too sandy or too clayey is to add compost because it will improve the soil texture and increase the water holding or draining capacity of the soil.
How Does It Look?
Now that you understand the basics of what soil is, there are easy ways to get a better idea of what you find in your garden or yard. You can learn a lot about the composition of your garden soil by look and feel. Use a spade or trowel to take a vertical slice of soil. If the soil isn’t too hard, you should be able to go down 6-8 inches deep. Lift up the slice and take a look. The color of the soil will give you some clues as to what the soil is made up of:
- Light-colored, loose, visible sand and/or gravel: Low in soil organic matter
- Blue or gray, sticky when wet: High in clay content
- Yellow, gray, blue or black and may smell bad: Poor drainage and lack of air
- Dark, moist and crumbly and smells fresh and clean, not rotten: A balanced mix of minerals and organic matter – healthy soil!
While digging, did you notice many worms or other insects? Using pesticides or synthetic fertilizers can kill off critters that are important to soil health. These tiny creatures are responsible for making nutrients more available to plants and improving soil aeration. Spreading organic matter as a mulch on top of the soil is an easy way to build a healthier habitat for these important decomposers. When the deciduous leaves drop from the trees this fall, pile them up on your garden beds for the winter or spread a blanket of finished compost or unsprayed straw. Even laying down cardboard or burlap will build soil health by providing soil dwellers with food, shelter and protection from above-ground predators.
How Does It Feel?
After looking your soil, scoop a tablespoon into your hand and add water a drop at a time until it is moist. Rub between your fingers and start to pay attention to the texture. Sand feels gritty, silt feels smooth but not sticky and clay feels sticky. Then, squeeze the moistened soil into a ball. If it mostly sand or loamy sand, it will break easily when you apply light pressure. If it is mostly clay or clay-loam, it will be harder to break apart. Sandy loams and silt loams will stay together, but will be more pliable and change shape more easily.
Now use your thumb and fingers to squeeze the ball so that it forms a ribbon. A sandy ribbon will break off before it is an inch long and feel gritty. If it breaks but doesn’t feel gritty, it contains silt. The longer the ribbon grows before breaking, the finer the texture and the higher the clay content is. It is important to note that the result does not mean that this is the only mineral in the soil; it can behave like a clayey soil if it contains only 20 percent clay.
Many other at-home tests exist to reveal even more about your soil make-up. One easy favorite is the jar test. Put some soil in a jar with a lid, add water and a little bit of dish soap and shake vigorously. Sand particles are heavy so they will settle down to the bottom as soon as you stop shaking. It takes 3-4 hours for silt and can take a few days for clay to settle. Organic matter will float to the top. Measure how deep each layer is and use a simple formula to determine what percentage each one is. Use the Soil Textural Triangle to determine what texture your soil has. You may think it is nothing but clay, but it could be any combination of clay, silt and sand and be closer to the “loam ideal” than you originally thought. For more in-depth instructions on performing or interpreting this simple soil texture analysis, we suggest an Internet search for “Jar test for soil texture.”
The jar, ribbon and ball tests will give you an idea of soil texture and how well your soil holds water and drains, but they will not be very specific about soil fertility, pH, organic matter percentage or contamination. For this kind of information, you need to send a soil sample to a lab, such as King Conservation District’s Soil Testing Service for residents of King County. Regardless, these hands-on tests will give you a basic understanding of what is living under your feet and some clues as to why the plants in your garden might be failing or thriving.
To learn more about growing healthy soil and soil analysis, visit the Garden Hotline online at www.gardenhotline.org or call us at 206-633-0224. You can also find us on Facebook, Twitter, Pinterest and YouTube. Seattle Tilth also offers the Master Composter / Soil Builder certification program as well as classes on soil health: www.seattletilth.org.