Evaluating Other Soil Properties

Introduction

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:

• Bulk density
• Horizonation
• Texture
• Drainage
• Infiltration
• Structure
• Consistence
• Color
• Cation Exchange Capacity

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

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.

Figure is from NRCS Soil Survey Manual, Chapter 3, Figure 3-16

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.

Drainage

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:

• Surface flow. Erosion can be a serious problem on sites that have been recently logged or otherwise disturbed. The ability of water to infiltrate soils inversely affects the amount of surface flow that contributes to erosion.
• Infiltration. This is defined as water flow from the surface into the soil profile. It depends on the soil hydraulic conductivity, which is unique to each soil type, and the wetness of the soil at the time of measurement. This relationship is shown graphically in the figure below. Measurement of infiltration will not be included in the procedures recommended for the HWWFF Site Assessment Worksheet, since we regard it as less informative and more difficult to interpret than estimates of subsurface flow of water (described further below). If you would like to try the "infiltrometer" method of estimating infiltration. You can go to the NRCS Soil Quality Test Kit Guide [] and download the "Infiltration Test" pdf file.
• Subsurface Flow. Below ground flow of water depends on the size of pores within the soil, which varies in soils of different textures (sand/silt/clay ratio) and soil structures (aggregation). Furthermore, rate of drainage depends on whether the soil is already saturated with water or is unsaturated (various stages of dryness).

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.

Soil mottling shows as red patches scattered within the predominantly gray soil. Red patches caused by oxidation of iron indicates a history of repeated wetting and drying of the soil.

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 Classes based on soil color and the depth of mottling (Anelli, 2005).

References