FIGURE 1.19 Although this celery was purchased in a Virginia grocery store in early fall, the black, mucky soil clinging to the base of the stalk indicates that it was grown on organic soils, probably in New York state. (Photo courtesy of R. Weil)

FIGURE 2.22 A pond, typically formed by glacial action, receives nutrients and sediments running off the surrounding uplands. These encourage aquatic plant growth, especially in the shallow water around the pond edges. Organic debris fills the bottom of the pond as increasingly rooted, emergent vegetation invades. Eventually shrubs and trees take root in the peat and cover the area. Many such bogs have been cleared of trees and drained by ditches to remove some of the water, exposing an organic muck soil that is often highly productive for vegetable crops. The bog in the photo is in Central Michigan. (Photo courtesy of R. Weil)

FIGURE 2.37 A soil profile forming in recent alluvium in the Atlantic coastal plain. The upper 80 cm of this profile has developed in recent alluvium laid down by occasional floods during the past 300 to 400 years. The A horizon is the result of soil formation, but the various C horizons differ because of geologic rather than soil-formation processes. Several of these layers show unique properties associated with the history of the past three or four centuries. The dark layer at about 50 to 60 cm depth contains abundant bits of charcoal. This charcoal was probably washed off the watershed slopes by heavy rains shortly after European settlers extensively deforested the area using slash-and-burn techniques to open the land for farming in the early 1700s. The layer just above the charcoal was laid down somewhat later and contains such artifacts as bits of antique glass from the late 1700s or 1800s. The layer below the charcoal was laid down prior to widespread European settlement. It contains numerous oyster shells that probably washed in from Native American encampments along the stream. Still deeper in the profile lies a buried soil. Part of a buried A horizon (Ab) and a buried Bg horizon (Bgb) are visible. These layers probably formed over long period of time when the streambed was in a different location and the site was a poorly drained area with fine-textured materials. (Photo courtesy of R. Weil)

FIGURE 4.7a The "feel" method for determining soil textural class. A moist soil sample is rubbed between the thumb and forefingers and squeezed out to make a "ribbon." The gritty, non-cohesive appearance and short ribbon of a sandy loam with about 15% clay. (Photo courtesy of R. Weil)

FIGURE 4.7b The smooth, dull appearance and crumbly ribbon characteristic of a silt loam. (Photo courtesy of R. Weil)

FIGURE 4.7c The smooth, shiny appearance and long, flexible ribbon of a clay. (Photo courtesy of R. Weil)

FIGURE 4.8 Particle size analysis involves breaking the soil down into its primary particle fractions and then ascertaining the proportion of each fraction present. A sample of soil is first treated to remove organic matter and is then thoroughly dispersed in water. Using the pipette method, the soil suspension is poured onto a fine sieve with 0.05-mm openings to separate out the sand fractions; the silt and clay fractions are washed through the sieve into a sedimentation cylinder. This suspension of silt and clay is thoroughly stirred and allowed to settle. The clay content is determined by sampling the suspension with a pipette at a certain depth after allowing a settling time determined by Stokes’ law. The washed sand that remains on the sieve is dried and then shaken through a series of sieves with openings corresponding to very coarse, coarse, medium, fine, and very fine sand, and the weight of each sand fraction is determined. (Photos courtesy of R. Weil)

FIGURE 4.41 A walk along a beach such as this one in Oregon illustrates the concept of soil strength for sandy materials. The dry sand (lower right) has little strength and your feet easily mire into it as you walk along. There is nothing to hold the individual sand particles together to permit them to serve as a firm base. As you move toward the ocean where the soil has been thoroughly wetted by the incoming waves, but where there is no standing water (lower center), you find firm footing, indicating considerably higher soil strength. Thin water films act as bridges between sand particles, holding them together and thereby resisting penetration by the feet. If you stand in shallow water along the edge of the ocean (lower left), once again your feet penetrate the surface sand, indicating that soil strength has been reduced. Each sand particle is completely surrounded by water, which acts more as a lubricant than as a binding force. If you were to drive an automobile over the same areas, only the wetted but not submerged sand would provide a firm base. Soil strength is a property of great concern to engineers, but also must be reckoned with by plant roots as they try to penetrate soils with high bulk densities. (Photos courtesy of R. Weil)

FIGURE 5.15 Instrumental measurement of soil water content using time domain reflectrometry (TDR). The electronic instrument sends a pulse of electromagnetic energy down the three parallel metal rods of a waveguide that the soil scientist is pushing into the soil. The TDR instrument makes precise picosecond measurements of the speed at which the pulse travels down the rods, a speed influenced by the nature of the surrounding soil. Microprocessors in the instrument analyze the wave patterns generated and calculate the apparent dielectric constant of the soil. Since the dielectric constant of a soil is mainly influenced by its water content, the instrument can accurately convert its measurements into volumetric water content of the soil. (Photos courtesy of R. Weil)

CHAPTER OPENER 6 Water cycles in the Teton Mountains. (R. Weil)

FIGURE 9.21 Influence of pH on the growth of shoots (top) and roots (bottom) from two wheat varieties, one sensitive and one tolerant to aluminum. Note the stunted shoot growth and extremely stunted, stubby root system of the sensitive variety in the low-pH treatment. [Photos courtesy of C. D. Foy, USDA/ARS, Beltsville, MD]

FIGURE 9.26 Bulk application of ground limestone by specially equipped trucks is the most widespread method of applying liming materials. The scene pictured occurred on a particularly windy day. Such dramatic dispersion by wind is not common, but illustrates the finely ground nature of the agricultural limestone applied. To avoid problems with heavy trucks bogging down in soft, recently tilled soils, it is often preferred to make lime applications to land that is in sod, under no-till management, or frozen hard. In many cases, similar trucks are used to spread commercial fertilizers (see Section 16.8), which are much coarser-grained and not subject to such wind dispersion. (Photo courtesy of R. Weil)

FIGURE 9.31 Young leaves that stick together and fail to properly unfold are a typical symptom of calcium deficiency in monocots such as this sorghum-sudangrass, a hybrid forage species. Foliar symptoms of calcium deficiency are not commonly seen. However, they may occur in very acid soils where plants are likely to also suffer from aluminum toxicity and other problems. (Photo courtesy of R. Weil)

FIGURE 9.32 Root growth was almost completely inhibited by low calcium in the nutrient solution (left) compared to healthy roots in the same nutrient solution but with calcium added (right). If the ratio of calcium to all other cations in solution drops below 5:1, the integrity of root membranes is lost, causing many other elements to become toxic to the plants. (Photo courtesy of R. Weil)

BOX 10.1 The impacts of irrigation on wildlife remind us of the interconnectedness of all parts of the environment and the need to manage soil and water resources with a holistic view of their roles in the larger ecosystem.

FIGURE 10.5 The sparse vegetation and exposed soil surface of a degraded rangeland in Arizona. Overgrazing by cattle has caused some of the bunch grass to be replaced by shrubs. Only a few scattered fragments remain of the biological crust (see Section 11.15) that once covered the soil between clumps of vegetation. Much of the soil is now bare and its degraded structure seals the surface, reducing its capacity to absorb water from the infrequent but high-intensity rainstorms that occur in this region. Ecosystems in arid and semiarid regions are often quite sensitive to such disturbances as overgrazing and off-road vehicle traffic. Partly because of the precarious water balance in these systems, recovery of rangeland vegetation may be very slow. Knife handle is 12 cm long. (Photo courtesy of R. Weil)

FIGURE 10.7a Saline seep formation in a semiarid area where the salt-rich substrata are underlain by an impermeable layer. Under deep-rooted perennial vegetation, transpiration is high and the water table is kept low. After conversion to agriculture, shallow-rooted annual crops take up much less water, especially if fallow is practiced, allowing more water to percolate through the salt-bearing substrata. Consequently, in lower-elevation landscape positions, the wet-season water table rises close to the soil surface. This allows the salt-laden groundwater to rise by capillary flow to the surface, from which it evaporates, leaving behind an increasing accumulation of salts. Note that the diagrams greatly exaggerate vertical distances. The photo shows a spreading saline seep area in eastern Montana where wheat fallow cropping has replaced natural deep-rooted prairie vegetation. (Photos courtesy of R. Weil)

FIGURE 10.7b Close-up of salt crust over moist soil. (Photos courtesy of R. Weil)

FIGURE 10.8a Salinization, the accumulation of soluble salts in soils, can be observed in the potting medium of house plants. The salt accumulates because of evaporative water loss from soil that is repeatedly supplied with water that contains dissolved salts, even if in low concentrations. Only pure water evaporates; the salts dissolved in the water do not. Note that the salt tends to concentrate at the highest points of the soil surface, from which evaporation loss is greatest and to which the soil solution flows by capillary action. (Photos courtesy of R. Weil)

FIGURE 10.8b Salinization, the accumulation of soluble salts in soils, can be observed in irrigated fields. The salt accumulates because of evaporative water loss from soil that is repeatedly supplied with water that contains dissolved salts, even if in low concentrations. Only pure water evaporates; the salts dissolved in the water do not. Note that the salt tends to concentrate at the highest points of the soil surface, from which evaporation loss is greatest and to which the soil solution flows by capillary action. (Photos courtesy of R. Weil)

FIGURE 10.9 Measuring the electrical conductivity (EC) of a soil sample in a field of wheatgrass to determine the level of salinity. A sample of the soil is stirred with pure water until a saturated paste is made. The paste is then transferred into a special conductivity cup that has a flat, circular electrode on either side (inset). This is then inserted into a stand that connects the electrodes to a conductivity meter. Note readout of 7.78 dS/m on the conductivity meter. This level of EC_p indicates a highly saline soil that would inhibit the growth of many crops. (Photos courtesy of R. Weil)

FIGURE 10.11 A portable electromagnetic (EM) soil conductivity sensor used to estimate the electrical conductivity in the soil profile. When placed on the soil surface in the horizontal position (lower left), this instrument senses electrical conductivity of the soil down to about 1 m depth. When placed in the vertical position (as in the inset photo), the effective depth is about 2 m. This type of EM sensor (model EM-38, made by Geonics, Ltd., Ontario, Canada) was mounted on a special vehicle for the mobile soil salinity mapping that produced the data points in Figure 11.10. (Photos courtesy of R. Weil)

FIGURE 10.16 The upper profile of a sodic soil (a Natrustalf) in a semiarid region of western Canada. Note the thin A horizon (knife handle is about 12 cm long) underlain by columnar structure in the natric (Btn) horizon. The white, rounded "caps" of the columns are comprised of soil dispersed because of the high sodium saturation. The dispersed clays give the soil an almost rubbery consistency when wet. [Photo courtesy of Agriculture Canada, Canadian Soils Information System (CANSIS)]

FIGURE 11.2 A predatory mite (an Astigmatid, m) dining on its prey, a microscopic roundworm (a nematode, n). Predation of this type keeps the populations of various groups of organisms in balance and releases nutrients previously tied up in the bodies of the prey. (Photo courtesy of Marie Newman, North Carolina State University)

FIGURE 11.5 Two dung beetles (Scarabaedae), one on top and the other underneath, roll a ball that they have fashioned out of buffalo dung over the leaf-strewn surface of a sandy soil. The female will lay her eggs in the ball of dung and bury it in the soil (some coarse sand grains from which are seen sticking to the surface of the dung ball). Burying the dung is very important for making the nutrients therein available to the soil food web. Dung burial also prevents the reproduction of carnivorous flies and other pests of dung-producing mammals. Different dung beetles have evolved to specialize in the burial of dung from particular species of animals. (Photo courtesy of R. Weil)

FIGURE 11.20 Fruiting bodies of a Bird’s Nest Fungus (Cyathus olla) contain disc-shaped spores ("eggs") that are scattered forcefully when the cone is hit by a raindrop. Like other members of the Basidiomycota, this fungus produces enzymes that break down cellulose, hemicellulose, and lignin in materials such as woody forest litter or corncobs left on agricultural fields after harvest. See tip of ballpoint pen for scale. (Photo courtesy of R. Weil)

FIGURE 11.31 Tiny pinnacles of a microbiotic crust in Arches National Monument, Utah, seem to reflect the larger pinnacles of an arid landscape. These crusts consist of algae, cyanobacteria, fungi, and other organisms living together in a mutualistic relationship. The inset shows a scanning electron micrograph of cyanobacteria filaments that make up the backbone of many crusts. Microbiotic crusts typically cover the soil surface in the unvegetated patches between clumps of desert shrubs and grasses. The crusts provide considerable protection against erosion by wind and water. They also help conserve and cycle nutrients, add nitrogen, enhance water supplies, and improve desert productivity. However, the fragile crusts can be easily destroyed by wheels, feet, and hooves. [Large photo courtesy of Ben Waterman; inset courtesy of Jayne Belnap (U.S. Geologic Service, Moab, Utah) and John Gardner (Brigham Young University, Provo, Utah)]

FIGURE 12.11 A special machine turns large scale compost windrows (direction of travel is away from the reader) to mix the material and maintain well-aerated conditions at a facility in North Carolina where university dining hall food scraps are processed into compost for campus landscaping. [Photo courtesy of R. Weil]

FIGURE 12.29 Atmospheric composition in these open-top field chambers altered plant growth and physiology, and thereby also affected the amounts and forms of soil organic carbon. Increasing atmospheric CO₂ from low (260 mg/L, the ambient level) to high (500 mg/L, the level expected by 2050) enhanced photosynthesis in the plant, and thus increased the amount of fixed carbon available for translocation to the roots and eventually to the soil. [Photo courtesy of R. Weil]

FIGURE 12.31 Mining Histosols for fuel. Blocks of peat cut from the Histosol profile (see trench in background) are stacked to dry so that they can be burned to heat rural homes in northwestern Scotland. Similar nonsustainable exploitation operations on a much larger scale provide peat fuel for electricity generating plants in other cold, wet regions with Histosol-dominated landscapes. (Photo courtesy of Josh Weil)

FIGURE 16.12 Clear-cut harvest of a red alder stand on steep Andisols in Oregon. The slash and root mass from this nitrogen-fixing species has a narrow C/N ratio, so mineralization is rapid and nitrogen losses after harvest may be high. In this particular case, the risk of nitrogen loss was increased further because the operator wanted to replant with Douglas fir and therefore planned to use herbicides to suppress weeds and alder regrowth. Note the crawler tractor skidding a log and the truck taking on a load. (Photo courtesy of R. Weil)

BOX 16.1 Not only do animals seek out the more fertile soils, but their activities also actually enhance the cycling of nutrients, making the soils they frequent more fertile.

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