Effects of Site Characteristics

Site characteristics that affect tree growth include soil depth, texture, moisture, fertility, pH, and topography.

Soil Depth, Texture, Moisture, Fertility, and pH

On the whole, deep soils are better for tree growth than shallow soils because they potentially have a greater nutrient supply and water-holding capacity. Rooting depth may be restricted by bedrock, coarse gravel, a hardpan layer, or excess soil moisture. Tree roots that absorb the most nutrients and water usually are found in the top two feet of the soil profile.

Soil particles are classified by size as sand (<0.002mm diameter), silt (0.002mm to 0.05mm), and clay (0.05mm to 2.0mm).

Soil texture refers to the relative proportions of sand, silt, and clay in a mass of soil. Soils with a high percentage of sand have large pore spaces between soil particles. Since they absorb and drain water quickly, they are droughty unless there is a shallow water table. Clay soils have a large water-holding capacity, but they absorb water slowly and water adheres so tightly to the soil particles that much of it is unavailable for plant use. Soils with a high percentage of silt have the most favorable texture for moisture absorption and drainage.

Soil fertility is based largely on the type of parent material from which the soil originated. Some of the most fertile soils originated from limestone, shale, and windblown deposits, whereas some of the least fertile soils originated from sandstone and granite. On the whole, fine-textured (clay) and medium-textured (silt) soils have a greater nutrient supply than coarse-textured (sandy) soils.

Most tree species grow well when the soil remains moist much of the year, but only a few species tolerate very dry or very wet conditions for long periods. Soil may be too dry for good tree growth where the soil is sandy, rocky, or shallow. Soil may be too wet where the soil is clay and the area has high rainfall or groundwater close to the surface. Soil pH is a measure of its acidity or alkalinity. Soil pH affects absorption of minerals by plant roots. A pH of 7 is neutral, neither acid nor alkaline. A pH below 7 is acidic; above 7 is alkaline. Most tree species grow best in a slightly acid soil, but the preferred pH varies by species.