Environmental Engineering Reference
In-Depth Information
Sand
Silt
Clay
Fig. 3.2. Relative sizes of three major types of soil particles. (Drawing by R. Castro.)
Soil Structure
Development of a stable soil structure often helps to ease the challenges
resulting from imperfect soil texture. Soil structure is the way in which
individual soil particles are bound together into larger units called
aggregates
. With some variation around the world, soils are usually
classified as structural, moderately structural or weakly structural. Ag-
gregates are generally described as blocky, granular, platy or prismatic
(Fig. 3.4). Organic matter is the most important factor contributing to
stable soil aggregation. Ideal soils are composed of approximately one-
half solid material and one-half pore space. Organic matter helps to
achieve this ideal composition. Organic matter in the soil undergoes
quite complex biological and chemical transformations, glues soil par-
ticles together, helps to create small pores within each aggregate and
larger pores between aggregates, and in this way improves aeration,
moisture retention and air movement, and makes the entire soil more
fertile. In addition, soil organic matter helps to supply available plant
nutrients, decrease loss of soil to erosion, and improve soil's cation
exchange capacity.
Soil Porosity
Soil texture and structure directly affect the volume of pore space in the
soil. For example, as shown in Fig. 3.5, clayey soil contains considerably
more total pore space than a sandy or loamy soil. Because pore spaces are
occupied by water and air, both needed by plants, having the highest
possible pore volume might seem beneficial. In fact, it often is beneficial,
