Page 614 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

578 part IV Soils, Ecosystems, and Biomes
• A moist soil is filled to about half of field capacity (the usable water capacity of soil), and its consistence grades from loose (noncoherent) to friable (easily pul- verized) to firm (not crushable between the thumb and forefinger).
• A dry soil is typically brittle and rigid, with consistence ranging from loose to soft to hard to extremely hard.
Soil Porosity Porosity refers to the available air spaces within a material; soil porosity denotes the part of a volume of soil that is filled with air, gases, or water (as opposed to soil particles or organic matter). We dis- cussed soil porosity, permeability, and moisture storage in Chapter 9.
Pores in the soil horizon control the movement of water—its intake, flow, and drainage—and air ventila- tion. Important porosity factors are pore size, pore con- tinuity (whether pores are interconnected), pore shape (whether pores are spherical, irregular, or tubular), pore orientation (whether pore spaces are vertical, horizontal, or random), and pore location (whether pores are within or between soil peds).
Porosity is improved by the presence of plant roots; by animal activity such as the tunneling actions of gophers or worms (see Figure GIA 18); and by human actions such as supplementing the soil with humus or sand, or planting soil-building crops. Much of the soil- preparation work done by farmers before they plant— and by home gardeners as well—is done to improve soil porosity.
Soil Moisture As discussed in Chapter 9 and shown in Figure 9.9, plants operate most efficiently when the soil is at field capacity, which is the maximum water avail- ability for plant use after large pore spaces have drained of gravitational water. Soil type determines field capac- ity. The depth to which a plant sends its roots deter- mines the amount of soil moisture to which the plant has access. If soil moisture is below field capacity, plants must exert increased energy to obtain available water. This moisture-removal inefficiency worsens until the plant reaches its wilting point. Beyond this point, plants are unable to extract the water they need, and they die. More than any other factor, soil moisture regimes and their associated climate types shape the biotic and abi- otic properties of the soil.
Chemical Properties
Recall that soil pores may be filled with air, water, or a mixture of the two. Consequently, soil chemistry involves both air and water. The atmosphere within soil pores is mostly nitrogen, oxygen, and carbon dioxide. Nitrogen concentrations are about the same as in the atmosphere, but oxygen is less and carbon dioxide is greater because of ongoing respiration processes in the ground.
Water present in soil pores is the soil solution and is the medium for chemical reactions in soil. This solu- tion is a critical source of nutrients for plants, providing the foundation of soil fertility. Carbon dioxide combines with the water to produce carbonic acid, and various organic materials combine with the water to produce organic acids. These acids are then active participants in soil processes, as are dissolved alkalis and salts.
A brief review of chemistry basics helps us under- stand how the soil solution behaves. An ion is an atom or group of atoms that carries an electrical charge (examples: Na+, Cl−, HCO3−). An ion has either a positive charge or a negative charge. For example, when NaCl (sodium chloride) dissolves in solution, it separates into two ions: Na+, which is a cation (positively charged ion), and Cl−, which is an anion (negatively charged ion). Some ions in soil carry single charges, whereas others carry double or even triple charges (e.g., sulfate, SO42−; and aluminum, Al3+).
Soil Colloids and Mineral ions The tiny particles of clay or organic material (humus) suspended in the soil solu- tion are soil colloids. Because they carry a negative elec- trical charge, they attract any positively charged ions in the soil (Figure 18.6). The positive ions, many metallic, are critical to plant growth. If it were not for the nega- tively charged soil colloids, the positive ions would be leached away in the soil solution and thus would be una- vailable to plant roots.
Individual clay colloids are thin and platelike, with parallel surfaces that are negatively charged. They are more chemically active than silt and sand particles, but less active than organic colloids. Metallic cations attach to the surfaces of the colloids by adsorption (not absorp- tion, which means “to enter”). Colloid surfaces can exchange cations with the soil solution, an ability called cation-exchange capacity (CEC), which is the measure of soil fertility. A high CEC means that the soil colloids
Georeport 18.1 Soil Compaction—Causes and Effects
Soil compaction is the physical consolidation of the soil that destroys soil structure and reduces porosity. The increasing weight of today’s heavy agricultural machinery, in addition to earlier planting and the arrangement of row crops, tends to
increase soil compaction and can result in a 50% reduction in crop yields owing to restricted root growth, poor aeration of the root zone, and poor drainage. Scientists now suggest that no-till agricultural practices (in which ploughing does not occur) combined with maintaining a continuous cover of actively growing plants is the best way to reduce soil compaction, since roots increase porosity and water availability, preserve organic matter content, and reduce surface erosion.