Page 612 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

576 part IV Soils, Ecosystems, and Biomes
Crumb or granular
Platy
Blocky
Prismatic or columnar
corner of the triangle represents a soil consisting solely of the particle size noted (although rarely are true soils com- posed of a single separate). Every soil on Earth is defined somewhere in this triangle.
Loam is the common designation for the balanced mixture of sand, silt, and clay that is beneficial to plant growth (Figure 18.4). Farmers consider a sandy loam with clay content below 30% (lower left) as the ideal soil because of its water-holding characteristics and ease of cultivation. Soil texture is important in determining water-retention and water-transmission traits.
To see how the soil texture triangle works, consider Miami silt loam, a soil type that is common in the United States. Samples from this soil type are plotted on the soil texture triangle in Figure 18.4 as points 1, 2, and 3. Point 1 describes a sample taken near the surface in the A hori- zon; point 2 describes a sample taken from the B hori- zon; and point 3 describes a sample from the C horizon. Textural analyses of these samples are summarized in the table and in the three pie diagrams to the right of the triangle. Note that silt dominates the surface, clay the B horizon, and sand the C horizon. The Soil Survey Manual presents guidelines for estimating soil texture by feel, a relatively accurate method when used by an experienced person. However, laboratory methods using graduated sieves and separation by mechanical analysis in water allow more precise measurements.
Soil Structure Soil texture describes the size of soil par- ticles, but soil structure refers to the size and shape of the aggregates of particles in the soil. Structure can partially
◀Figure 18.5 Types of soil structure. Structure is important because it controls drainage, root- ing of plants, and how well the soil delivers nutrients to plants. The shape of individual peds, shown here, controls
a soil’s structure. [USDA–NRCS, National Soil Survey Center.]
modify the effects of soil texture. The smallest natural lump or cluster of particles is a ped. The shape of soil peds determines which of the structural types the soil exhibits: crumb or granular, platy, blocky, or prismatic or columnar (Figure 18.5).
Peds separate from each other along zones of weak- ness, creating voids, or pores, that are important for moisture storage and drainage. Rounded peds have more pore space between them and greater permeability than do other shapes. They are therefore better for plant growth than are blocky, prismatic, or platy peds, despite comparable fertility. Terms used to describe soil struc- ture include fine, medium, and coarse. Adhesion among peds ranges from weak to strong. The work of soil organ- isms, illustrated in Figure GIA 18, affects soil structure and increases soil fertility.
Soil Consistence In soil science, the term consistence is used to describe the consistency of a soil or cohesion of its particles. Consistence is a product of texture (par- ticle size) and structure (ped shape). Consistence reflects a soil’s resistance to breaking and manipulation under varying moisture conditions:
• A wet soil is sticky between the thumb and forefin- ger, ranging from a little adherence to either finger, to sticking to both fingers, to stretching when the fin- gers are moved apart. Plasticity, the quality of being moldable, is roughly measured by rolling a piece of soil between your fingers and thumb to see whether it rolls into a thin strand.
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