Know Your Soil—Grow Great Trees
All soils are not alike. While some soils are light and fast draining, others are heavy and difficult to work. Some make nutrients readily available to plant roots, while others are more stingy. In one type of soil, a certain Tree may thrive, while another species may struggle to stay alive. Getting to know your soil will help you make wise choices about which Trees to plant in your yard, and it will help you to be a good caregiver to the Trees you already have. Here’s the dirt on dirt.
What kind of soil do you have?
You should be able to get some idea of what kind of soil you have by handling it. Sand is gritty, and you can see the grains. Clay is sticky, and your back will tell you when you have been shoveling it! Silt is crumbly and a pleasure to deal with.
If you want to get a better idea of your soil type, you can do this simple DIY soil test. Gather two cups of soil from your garden, pick out the roots and stones, and put it in a straight-sided glass jar. Mark the soil line with a piece of tape. Add a squirt of dish soap (this helps the particles to separate), and then fill the jar with water, leaving one inch of space at the top. Screw the lid on tightly. Shake the jar vigorously for ten minutes and set it down. After two minutes, you’ll see how much sand has settled—mark that line with another piece of tape. After two hours, you’ll see how much silt has settled—mark it again. The remaining portion will be clay. Calculate the fractions.
Soil is made up of the tiny particles that result from the weathering of rocks that takes place over thousands or even millions of years. The size of the particles determines the soil’s texture, which in turn shapes the environment in the root zone. Soil particles fall into three distinct classifications based on size: sand, silt, and clay.
Sand particles are classified in the .05 mm to 2 mm range and can be seen with the naked eye. Sandy soil feels gritty to the touch. If you mold this type of soil into a ball, it will disintegrate readily when you press on it. Sand does a poor job of holding onto water and nutrients in the root zone of plants—water flows right on through to the deeper layers. On the plus side, sandy soils are well aerated, even in the lower layers. Roots move easily through this light, loose soil and they delve deeply, unhindered by the lack of oxygen they would encounter in heavier soils. Drought-tolerant Trees flourish here, such as Black Gum, Ginkgo, Golden Rain Tree, Juniper, Smoke Tree, and Zelkova.
Silt particles are much tinier than sand and range between .002 and .05 mm in size. Silty soil has a silky, flour-like feel when dry and is rather greasy when wet. If you mold it into a ball, it will hold its shape, but it will crumble apart with some handling. Silt tends to be rich in nutrients. This is the sediment that collects at the mouths of rivers. Silt also makes up a good deal of the fertile Great Plains prairie soil. Silt retains water well, holding onto it so that plants can make use of moisture stored between rain showers, but it also allows for drainage and for oxygen to permeate the soil. This type of soil does tend to form a crust when it dries, but it is easily broken up and rehydrated. Most Trees love silty soil.
Clay is composed of flat, microscopic particles that are less than .002 mm wide. Clay is sticky and dense and is backbreaking to work. You can form it into a rope, and it will hold its shape even when you handle it. This is literally the stuff used to make clay pots! Clay retains water, and plants susceptible to root rots may have a short life when growing in it. When clay dries, the ground becomes quite hard and large cracks form. The soil is difficult to rewet in that state—the water runs right off. On the bright side, clay soil tends to be rich in nutrients. Trees for heavy clay include Autumn Blaze® Maple, Crabapple, Honeylocust, Linden, River Birch, and Swamp White Oak.
Nearly all soils are made up of some combination of the three types of particles, and if they are found in the right proportions, the soil is considered loam. Loam is made up of 7 to 25 percent clay and roughly equal parts silt and sand. Water retentive but well drained, generally fertile and easy to work, loam is the Holy Grail of soils. It will grow a wide range of plants, and vegetable gardeners in particular sing its praises. However, if you don’t have loam and your aim is to grow Trees, you have nothing to worry about. Trees are much tougher than wimpy little vegetable starts, and there are Trees that will thrive in all types of soils.
How’s your soil’s drainage?
You may already have some idea of what drainage is like on your property. Does your yard stay soggy for a few days after a hard rain or does the water drain away quickly? The heavier your soil, the poorer your drainage is likely to be. To test your drainage, dig a 12 × 12 × 12-inch hole and fill it with water. Let it drain and fill it again. The water should drain within 24 hours. If it doesn’t empty completely, you may need to put in drainage tiles or regrade to redirect the flow of water. If that’s not feasible, choose Trees for planting that prefer wet soil, such as Bald Cypress or Willow.
Drainage can be affected by soil compaction, which often comes from pedestrian or vehicle traffic. Clay and silt soils are especially vulnerable to soil compaction, particularly when they are wet. Don’t work these soils when they’re wet, and always create a care bed around Trees to prevent compaction in their root zones. Protect Trees during construction and make sure that equipment operators give them a wide berth.