Long-Term Planning for Vegetation Management

Soil is matter in a layer above the underlying rocks of the Earth's crust. Soil consists of solid, liquid and gaseous matter. The solid component of soil includes mineral and organic matter. Organic in this context means "of life" and includes living organisms and the remains of dead organisms. The liquid matter, or "soil solution", is critical for plants - this is where they obtain their nutrients. The gaseous component is essentially air at the surface and is also important for plants as a source of oxygen for their roots. Soil classification is a complex subject. Fortunately, we only need general information about the soil in order to assist in identifying existing or especially replacement vegetation types.

Soil texture: this is dependent on the size and shape of the mineral particules. Particles may range in size from visible (gravel and sand) to the microscopic (silt and clay). The exercises describe a set of simple field tests for soil texture.

Soil Depth: In some case the depth to bedrock can be probed with a steel bar driven into the ground, but soil depth may be difficult to determine in this way if the soil is rocky or has a hardpan layer. Roadcuts or unvegetated streambanks can provide a window into the soil layers.

Soil Origin: In some circumstances the soil has been transported into the site by water, wind or perhaps humans, and the origin may be difficult or impossible to determine. However, if soil is mainly derived from the weathering of local rocks, then these rocks may be identified. North Coast vegetation series descriptions list the following rock types: granite, sandstone, shale, serpentine, volcanic and schist.

Soil pH: Some vegetation types require acidic or alkaline soil. The determination of pH requires a water-based solution derived from the soil.

Soil Drainage: Some vegetation types occur on well-drained soil, while wetland types occur on poorly-drained soil. Sandy soils drain very quickly while soils with high clay content tend to drain slowly. Gardners know that the ratio of organic to mineral matter is a key determining factor in drainage. The topography of the site and the presence of hardpan, an impermeable layer below the topsoil, can affect the drainage. A field observation of the presence or absence of standing water during the winter can provide the evidence needed of the overall site drainage for wetland vegetation types. Otherwise, a percolation test is recommended; there are a variety of proceduers, from simple to more complex. The basic steps are to dig a hole, fill it once and let it drain completely. Then fill it again and measure the rate of drop of the water level. If the rate is:

• less than 1/2 " per hour, the soil is poorly-drained;
• more than 1 " per hour, the soi is well-drained.

Soil can be highly variable from one location to another. For example, weathering of serpentine outcrops gives rise to pockets of serpentine soil which are quite different from the surrounding soil. Soil has considerable influence over the vegetation types that may thrive in a particular location, but there are many other determining factors, including microclimate, topography, groundwater availability and disturbance. Check your conclusions about a suitable vegetation type by looking for that type in the vicinity of your site. If such nearby sites can be found, they may serve as a valuable source of local seeds and cuttings for propagation.