How, When, and Why of Forest Farming

Unit 2 : Site Assessment and Non-Timber Forest Crop Selection
Section: 4.4

Evaluating Other Soil Properties


Soil fertility, pH and organic matter are especially important attributes of soils for forest farming. Other soil properties that make a difference in the performance of crops include:

Information about each of these factors and how they are evaluated is available in the Additional Soil Analysis Resources box, below. Among them, soil texture and drainage are particularly useful indicators of plant performance

The Soils section of the Site Assessment Workbook provides a table for; pH, Organic Mater, Mineral Analysis, Depth, Texture, Moisture, Drainage and Compaction for several of your zones. Look at the table and take notes as you work through this section. Also note examples in the MNG Case Study Site Assessment Worksheet, Soils nut icon

Soil Texture

Soil texture refers to the relative portions of different particle sizes that make up soil, including sand (large), silt (medium) and clay (fine). The mixture and distribution of these 3 size classes determines important properties like drainage and water holding capacity, and affects fertility. Soil texture classes include: sand, loamy sands, sandy loams, loam, silt loam, silt, sandy clay loam, clay loam, silty clay loam, sandy clay, silty clay, and clay. These categories are named based on % content of silt, sand and clay as shown below in the “classic” soil texture triangle.

Importance of Soil Texture.

As clay (and/or organic matter) content increases, soils have higher cation-exchange capacity (they retard the loss of cationic nutrients including nitrogen in the ammonium (NH3+) form), and higher levels of calcium, (Ca+2), magnesium (Mg+2), and potassium (K+). An extremely important consequence of soil texture is its effect on the amount of water that is available to plants. The range of soil drying is from full saturation (field capacity) to the “permanent wilting point” (death due to drought stress). Soils that are sandy have less water available for plants than silty soils, and silty soils have less available water than clayey soils. This relationship is shown in the figure below where the greater the distance between the lines for field capacity and permanent wilting point, the greater the amount of water available to plants. Clearly a silt loam, and to a lesser extent more clayey soils, have more available water than sandy or loamy soils with lower clay content.

Use the Soil Texture Resources in the box below to determine soil texture for your sampling points:

Soil Texture Resources


Water is one of the most import soil-related resources necessary for plant growth and development. Soils affect and are affected by the movement of water in the following three ways:

A percolation test described below is one way to assess drainage. For the most reproducible and meaningful results it should be performed after soil has been pre-wetted. A drawback to the percolation test is that it usually takes many hours to perform, and the results depend on how deep the hole is dug, as subsurface water flow (percolation) may differ among different soil horizons in the same soil profile. Consequently, interpreting the results may be complicated.

Alternatively soil color, especially mottling may be used as an indirect indicator of soil drainage. This is because the upper portion of a soil profile that is well drained will rarely be saturated for long, and will remain well oxygenated most of the time. On the other hand, a poorly drained soil will go through repeated cycles of wetting to the point of saturation for prolonged periods of time followed by periods of drying, as the saturated water table moves up and down. This repeated cycle of raising and lowering of the upper margin of a perched subsurface water table, over a long period, will cause localized zones of red oxidized iron (insoluable) and/or blue/green reduced (soluable) iron (“leopard spots”) described as mottling, as shown in the photograph below.

If the zone of mottling is found near the soil surface it can be assumed that the surface layer is poorly drained and frequently saturated with water. If the zone of mottling is deeper, this indicates a well drained surface layer, which is rarely if ever completely saturated. Soil scientists have used depth-of-mottling zone as a means of dividing soils into different drainage classes as shown in the figure below.

Soil Drainage Resources

Additional Soil Assessment Resources


Anelli, J.R., 2005. A Short Course on Soils, MacDaniels Nut Grove, Department of Crop & Soil Sciences, Cornell University, Ithaca, NY.