Forest soil structure
and relative fertility
Curiosity about the apparent response of several woodland species to rich garden soil has prompted an modest and informal investigation into soil structure and soil fertility as an element of the total habitat of varied species.
Background: Initial Landscaping
The inhospitable soil that prompted direction into using native species in the first place was disregarded for a while. To develop encouraging beds for plants, some rich humus, amended with decaying organic matter, had been piled deep behind low stone walls.
These retaining walls, rarely more than sixteen inches high, complemented some of the natural variance of the property and bordered new 84-inch paths, created following light excavation and grading, then filling and rolling with high-grade imported topsoil seeded with common shade to part-shade commercial grass mixes.
Another angle of a new quarter-acre area about to be developed, showing a large page of naturally-occurring Epigaea repens in the foreground. A number of arbitus patches will have to be considered in the design.
Early planting beds were established as much for novice-level learning about what might and might not grow in the environment and to what degree of success. Design primarily focused on larger stock to the rear, lower species to the front. Everything was regularly blanketed with freely available wood mulch from a community recycling depot.
As might be expected, results were mixed. Some species did extremely well, others did not. No measure of carbon-nitrogen impact, general moisture, soil pH or NPK ratios were available to provide useful conclusions.
Inquiry Foundation
Excessive growth, as much as three times normal size, resulted in many plants with tall but weak stems susceptible to collapse, particularly during heavy rains and required staking. Some root systems were so shallow that they could not sustain stems and leaves. Far too much nitrogen in the soil was suspected, but available data proved anecdotal: "grows in rich humusy soil."
Contemporary published information offered very little serious discussion of soils and fertility. However, chance discovery of two out-of-print books at a used book sale fueled further investigation, leading to bibliographies and titles of even more. This merits a separate report on conducting research and building a good library, perhaps for another time.
Initial suggestions counseled use of far higher percentages of sands and introductions of gravels and some clays to improve drainage and provide better habitat for root systems, along with using small quantities of organic matter to provide nutrients and retain some moisture. Rock gardening books, in particular, presented the case for harsher habitat.
Transition from lawn to moss into a quarter-acre area to be developed. At right is a river-stone border of an experimental bed needing reconstruction; at right, large stone is already gathered for a sweeping line of ostrich ferns.
The pace of investigation quickened on two fronts. Again, newly acquired "old" books discussed woodland soils and how to synthesize them, adding the factor of soil pH and charting out species favoring particular ranges of soil acidity, a dominant factor in woodland habitats primarily due to hardwood leaves and fallen conifer needles. The larger suggestion was that the soil itself might be a dynamic in habitat for specific species of plants; by listing naturally-occurring natives on an impromptu chart, for example, a general pH of 4.8 was obvious within moments. Casual texts about the region describe the ridges as having "thin, nearly sterile soil" with the surface packed with a dense network of roots, harvesting what they can from the meager ground.
In fact, however, this is ideal ground for the hardwood forests that dominate the region because these trees require very little in nutrients. Further, it prompted the question that if species indigenous to UPL, FACU and perhaps FAC environments (USGS designations for probability of a plant occurring in a wetland) fit into the soils under an oak canopy, then perhaps they, too, would be best suited for soils that were relatively poor.
The second front proved to be more challenging until interactive maps relating to geology and the requirements of civil engineering were located. They offered a specific summary of the soil for the property: 17 percent clay, 42 percent sand, 38 percent silt, and only three percent organic matter. The soil type is mapped as "OxB," named "Oquaga-Lackawanna" and described as a very stony loam on a slight grade with good drainage, poor for lawns, too thin for conventional septic systems, and a challenge for those wishing to build homes with basements.
The soil structure is essentially glacial till from the Wisconsin glacier, deposited to a general depth of 24 inches on sandstone bedrock that was subjected to 5,000 years of intense frost and subsequent fracturing of the bedrock itself. The soils consequently contain a high volume of channers (small flat sedimentary stone, usually shales and sandstone) that slowly work their way to the surface during freeze-thaw cycles.
A view of the transition from a current service path constructed from compacted wood chip mulch into an undisturbed moss-covered region planned for new beds
Preciptation averages 35 to 37 inches per year, although recent years have been wetter and warmer than normal.
Neighboring soil data for the ridged portion of the county is similarly available, including streambanks and marshy areas around some of the glacial ponds. Neighboring soils are similar, with variances primarily in steepness of grade and shifts of organic matter, particularly on the surface, by just a couple of percentage points. It raised the question about what woodland species might be particular about the exact composition versus those that adapted to grow over a wide range. Why do some species occur in, for example, only one or two counties in a distant section of the state? What is unique about the habitat in relationship to the plant?
A briefly-held goal of mapping plant distribution statewide against soil structure statewide was abandoned as a much too complicated project for the resources at hand. However, evaluating upland habitats in a single county offered both the opportunity to explore manageable variances and an informal plausible theme (the natives of a specific region). The latter supports the theory that an individual can grow anything in the garden if they are willing to go to the expense to maintain an ecosystem. Such an ecosystem would be increasingly expensive and labor-intensive in proportion to its artificiality.
Working in a tighter range (essentially shade, dry to mesic and perhaps some small areas that border on wetland) is more realistic with limited resources.
Status of the project
Existing beds have been reidentified as "holding beds" to keep existing stock in reasonably good care while new beds are built in the normal expansion of the garden. However, these are being developed with a considerable amount of sands, compost, and shredded leaves and wood chips to more accurately suggest normal forest litter in decomposition.
Focus will be placed on soil pH, localized for logical groups of specimens, compressing a wide range of woodland into an area that is reasonable to manage. Ammonium sulfate and superphosphate are recommended as soil amendments, but it is uncertain how these will impact N-P-K. Concurrently, the application of urea in stocks of hardwood mulch and shredded leaves to encourage decomposition may also impact particularly nitrogen levels to an artificial degree. It therefore seems logical to test indigenous ground to determine baseline N-P-K levels and evaluate them on species health.
More distant projects are being roughed out, especially in the context of drainage and stormwater runoff, to present logical opportunities for shade species in the FACW-FAC range. Other sections will be established as FACU-UPL, primarily to examine species that prefer a drier habitat and in recognition that widespread irrigation is not feasible.
As stock is transplanted and propagated, existing beds will be reconstructed to gather some of the organic matter and replace it with sand, gravel and clay, then replanted with logical collections.
Initial conclusions
Success with unusual garden habitats appears to be predicated on a willingness to work with the ecosystem rather than against it, and it offers opportunities for avocational growth unique to an isolated habitat. Hundreds of species are plausible for this type of ground, and many are readily available. However, realistic landscapes in the general setting appears to require careful construction of artificial soils to enhance, rather than redefine, the original terrain.