Woodland soils: an introduction
All soils are built from three basic grades of rock: sand, silt and clay. They are respectively finer in size:
- Sand - 0.05 millimeters to 2 millimeters
- Silt - 0.002 to 0.05 millimeters in diameter
- Clay - less than 0.002 millimeter in diameter
While sand can be almost any kind of rock sufficiently ground and weathered to be considered small enough to no longer qualify as gravel, silt and clay are chemically different and created in different ways (physical versus chemical processes). What silt and clay have in common is the ability to hold water, which is why soils dominated by silt and clay tend to be categorized as "poor drainage" and soils dominated by sand are often described as "well drained."
In a forest environment, that's only the beginning.
The relationship of sand, silt and clay, its sources and alterations over time, form the foundation of an ongoing chemical process involving minerals that is so complex that entire books have been written about it. Chemical reactions make mineral compounds conveniently available for plant life, which in turn engages in chemical processes that alter the soil. These processes are additionally variable. For example, the chemical composition of the leaves of a particular species of tree will be different depending on the age of the tree and the time of year. Again, conveniently, when trees shed leaves, the fallen waste enriches the soil for the next season's growth of that particular species.
Another mitigating factor is soil fertility - the commonly known balance of nitrogen, phosphorus and potash as well as a host of trace elements and minerals essential to the health of the plants on the surface. Soils categorized as prime farmland not only enjoy a nearly perfect balance of sand, silt and clay but also higher amounts of organic matter traditionally caused by rapid decomposition of the previous year's stems and leaves.
Woodland soils, however, are much more stingy. Trees and some understory shrubs don't mind because their nutritional requirements are very lean. They produce all they need from annual leaf fall that decomposes within only an inch or so of the active soil surface, protected by a carpet of natural mulch: the leaves that form a future storehouse of food. The chemistry of forest litter itself is designed to prevent competitors from taking root, and a forest only grudgingly allows a few vascular plants, mosses and lichens to share the limited buffet. An entire legion of species of living creatures ranging from microbes to burrowing mammals assists in the process, creating pathways for oxygen, causing chemical reactions, and providing organic matter themselves. This combination of factors forms a balanced ecosystem
Finally the entire soil system of a forest, meadow or swamp may span many hundreds of acres, over centuries of time, and be affected by a host of climactic variables. Soil constantly moves by wind and water (liquid or frozen) and even plant life itself, the power of which should not be underestimated. Freezing water can exert pressure of up to 30,000 pounds per square inch (no wonder that bottle broke in the freezer), and a tree's roots can move several hundred tons of buried rock. The atmosphere is continually filled with particles of silt from distant corners of the globe, as evidenced by golden sunsets and acid rain. The entire city of Philadelphia is built upon glacial outwash that came from a hundred miles north, borne by the placid-looking Delaware River, and the mountains of Pennsylvania used to be the compressed sediments of a vast shallow sea.
For a time, moving soil collects in pools of earth to form habitats, and it is into these habitats that the earnest gardener steps with shovel and ambition.
Fortunately for intrepid gardeners such as us, a large number of native plant species are sufficiently pliable to adapt to circumstances that are perhaps not ideal but are nonetheless manageable and we can clump them into workable categories:
Sun to Part Sun
- Dry meadow or field
- Moist meadow or field
- Swamps and wetlands
Part Shade to Shade
- Dry upland woods and thickets
- Moist, rich forest
- Wet bogs, fens and glades
An additional factor critical to many species of plants is soil acidity, especially among woodland plants that may be, for example, more comfortable in the needles of pine and hemlock than under beech and birch. While some soils, like deserts, can be alkaline, most commonly found soils range from a pH of 4.0 to 7.0 and create two categories:
- Strongly Acid - pH of 4.0 to 5.5
- Weakly Acid - pH of 5.5 to 7.0
Some researchers prefer to divide it three ways:
- Strongly acid (4.0 to 5.0)
- Moderately acid (5.0 to 6.0),
- Slightly acid or neutral (6.0 to 7.0)
The ideal soil pH for most lawn grasses and crop garden is around 6.8. Most forest soils hover at 6.0 and less, which is why conventional lawns and forests don't mix well; to keep the lawn alive, a lot of lime is required, which in turn is not healthy for the trees.
Another factor impacting lawns in the woods is the stony nature of soils usually deposited as glacial till. As mentioned above, the Appalachian and Allegheny mountain chains were formed when Devonian sea beds were uplifted to form the ridges characteristic of the northeastern United States. From ancient vast river deltas, which underwent dramatic changes every few million years, sheets of sandstone and shale were laid down in ribbons of gray and red that we see when the bedrock is cut away for such enterprises as modern highways. The predecessor to the Atlantic Ocean closed to nearly the width of a river and as it did so it wrinkled the surface of the continent. Some of it, along the current eastern seaboard, was smoothed out when the Atlantic Ocean formed (the process is still ongoing, with a tear in the surface of the bedrock in mid-ocean as evidence).
Now came all the expected weathering, but none of it as dramatic as successive ice ages. Where the glaciers ran parallel to the ridges, they eventually deposited about two feet of till; where they ran opposite the ridges, they filled entire valleys and altered the course of rivers. The deposit of fractured sandstone and shale mixed with clays, sands and silts, is mostly what comprises current soils. The additional factor was bedrock shattered by intense permafrost for several thousand years after the glaciers withdrew.
All of this loose rock in varying sizes slowly works upward by frost and erosion activity but makes a splendid habitat for relatively shallow tree root systems: well-drained channels for surface water, cool, and mineral rich. But with a workable surface layer that's thin, relatively sterile and filled with countless flat rocks called "channers," it's awful soil for a lawn that needs eight to ten inches of friendly loam.
It's the stony nature of the loam that separates forest from farmland in this region. A clay content of 17 to 20 percent by weight is a typical range for Monroe County, Pennsylvania, where there are nearly 140 identified soil types, but sand and silt ratios vary widely. Clymer, Wellsboro and Lackawanna soils are roughly 40 to 42 percent sand and 38 to 40 percent silt and considered ideal farmland. Forest soils are similar: roughly 43 percent sand and 40 percent silt. But in this area, the best farmland is on glacial outwash where embedded stone, if any, resembles rounded river gravel and deposited soil is much deeper. Forest soils, beside being shallow, are thick with sharp rock in the six to 12 inch range and therefore almost impossible to plow. Evidence of an almost heroic struggle by long-gone farmers is in the many coarse stone rows that meander through the ridges defining the borders of pastures now reverted to mostly hardwood forest.
Meanwhile, streams and glacial ponds have served as gathering points for soils that are very low in sand content, but high in silt. These are in a variety of combinations, but Norwich and Chippewa turns up often: 26 to 27 percent sand and 54 to 55 percent silt. All of these are described as having very poor drainage, exceeded only by bogs that are entirely decayed organic matter many feet thick. These are swamps, wetlands or floodplains, all creating habitat for hundreds of species of native plants, some of which are found nowhere else in the world.
Finally we have a fickle climate. Precipitation can whisk overhead in brief passing storms from the west or saturate everything when drawn up the Atlantic coast - the traditional nor'easter. As often as not, a drenching day or two of rain, perhaps several inches, is followed by long periods of dry weather, all of which challenge the survival of even the most hardy species.
All of these create very specific habitats and, over time, whole families of plant life have organized themselves to take the fullest possible advantage of it. Some are flexible and can be found over vast ranges. Others are particular, and will thrive only when conditions - soil content, nutrition, acidity, light, precipitation - are perfect.
And it's at this very point that the gauntlet is laid down at the feet of the eager gardener, trowel in one hand and a packet of seed in the other. Those to the south and west will find much of the land hospitable to traditional vegetables as well as classic prairie wildflowers. Those to the north and east may become more easily discouraged; this is mixed hardwood forest for a reason. Yet it need not be so; by making proper choices, the understory of a Pennsylvania forest can flourish with many species and create truly unique and individualized landscapes.
Gardeners are prone to cheating nature as much as they can. That patch of sunlight is believed to be so much partial sun as to qualify as full sun; it might be possible to squeeze a Zone 6 plant into Zone 5b soil. And there might be enough water in the well to keep a favorite specimen satisfied during the driest of Augusts. But in the end the harsh environment will win out. The shade is dappled, perhaps leaning to full. There will be a week or two of bone-chilling winter, sometimes late in the season when plants are stirring from dormancy. And only so much water can be drawn from the family well to pamper a plant.
Yet properly chosen plants will not only endure, but flourish. This makes gardening in this environment not so much management but stewardship, and the rewards are savored when strolling along a serene path in a healthy woodland.