When we look at a tree or shrub, we generally only consider the aspects above ground: shape and size, leaf color, flowers, and the like. We rarely stop to think that at least half the plant — its extensive root system — is underground. The underground portion, more than anything else, determines whether the plant will thrive or fail.
The Soil Factor
Before you bring home the proper trees, shrubs, or vines for your yard, you must analyze the soil, water, and drainage conditions in your yard. You then must determine whether any changes should be made. It is always easier to do this before you begin planting. It’s also less costly: You won’t find yourself having to replace dead or dying plants.
The soil your plants grow in serves many purposes.
Be sure to analyze your soil before purchasing any plant.
The soil in which your plant grows serves four basic purposes. It helps, through its structure, to hold the plant upright, and it supplies food, water, and air to the roots. Most soils are already capable of meeting these purposes and can be used with little amendment. Called loam soils, they contain a mixture of large and small soil particles plus abundant organic matter.
Some soils, however, may be extremely dense, with little air space between the particles. This type of soil, called clay, is made up of particles of rock so tiny and close together they allow little air circulation. Clay holds water well, sometimes perhaps too well. Sandy soil, on the other hand, contains larger particles of rock. Air is present in abundance in sandy soils, but water runs straight through. This creates dry growing conditions, even in a moist climate.
To determine which type of soil you have, squeeze some slightly moist soil in your hand. Clay soils will form a compact lump and retain their shape. Loam soils will form a ball but fall apart if poked at. Sandy soils won’t hold their shape at all.
Both sandy and clay soils can be improved in the same way: by adding organic amendments. Add about one-third compost, well-rotted manure, or other organic matter, and mix carefully. Clay soils also require the addition of sand to open up air spaces. Use coarse sand in combination with organic matter, and incorporate both amendments thoroughly. Use of sand alone will do little to open a clay soil-and may make the situation worse. If possible, do this for the entire lot or at least the entire planting area. You don’t want to create a single pocket of good soil surrounded by poor soil because your plant, as it grows, will want to send its roots further afield, beyond the original planting site.
Roots may be hesitant to leave the pocket of good soil to penetrate the surrounding inhospitable soil. It is also worthwhile before planting to test your soil’s pH level. This is a measure of acidity and alkalinity. The pH scale runs from 1 to 14, with 7 being neutral. Anything above 7 is increasingly alkaline, anything below, increasingly acid. Most soils in North America fall in a pH range of 6 to 7, from slightly acid to neutral. This is ideal for most plants.
Garden centers and local Cooperative Extension offices usually offer an analysis service and will explain how to collect a soil sample. You can also buy a pH kit and do your own testing, but the results will be less specific than with a professional test.
If your soil is on the alkaline side (7 or above), consider either planting plants that tolerate alkaline soils or amending it with peat moss or sulfur. If your soil is very acid (below 5.5), try either planting acid-loving plants or adding ground limestone. The exact quantities of amendment needed to change your soil’s pH to the one you want depends on a great many factors, notably its original pH and the type of amendment used. A professional soil analysis will indicate the exact amount.
Water makes up about 90 percent of the tissue in leafy plants. Woody plants have an advantage over perennial ones in that they generally have extensive root systems that reach down and out for great distances, often far beyond the circumference of the plant’s branches. Thus, woody plants can seek out moisture and continue to grow even as other plants suffer from lack of water. But there are limits to this ability. Most woody plants prefer soils that are evenly moist, meaning soils that may dry out on the surface but remain slightly moist underground.
If your area is subject to regular or prolonged droughts, you should consider planting trees, shrubs, and vines that are naturally drought-tolerant. But even drought-tolerant plants don’t appreciate extreme drought. You might want to consider installing an irrigation system to facilitate watering. Newly planted trees and shrubs will need extra care in watering since their root systems are quite limited, especially during the first year.
Some soils suffer from chronically poor drainage. They are spongy and moist at all times and may even be inundated for days on end. There are some woody plants that thrive in soggy soils, such as river birch, (Betula nigra) and sweetbay magnolia (Magnolia virginiana). To expand your choices, it will be necessary to improve the drainage of your yard before you begin to plant.
There are three levels of soil: the top and the
bottom are porous soil. The middle is hardpan soil.
Dig through these layers for planting.
There are several reasons why soils drain poorly. Often, their clay content is too high, a condition best improved by adding abundant organic matter and sand. Sometimes the soil is located in a depression or at the bottom of a slope. Other times the soil is simply too shallow. Shallow soils do not allow roots to grow downward and properly anchor the plant, and they may prevent excess water from draining away.
If your garden is underlaid with hardpan, a nearly impervious layer of clay about a foot beneath the soil’s surface (a common problem in dry climates), you can solve the problem by breaking through the hardpan in several places and filling the resulting holes with porous soil. In many cases, however, the best solution for poorly drained soils is to install drainage tiles. You can also consider planting woody plants in raised beds or mounds to give them a few extra feet of soil in which to grow. This will provide enough well-draining soil for good air circulation, and the moister soil beneath will ensure that roots never lack water.