Soil Resources Chapter 11

Soil ResourcesChapter 11

You will learn • What soil resources are • The key functions of soil • How soils form • How soils are characterized and how they vary • The physical, compositional, and biological properties of soils • How soils are degraded or lost • How soil resources can be sustained

Soil Definitions and Uses • Most soil is a mixture of mineral particles of varying size and organic matter of varying composition. • Together, these components make up about 50% of a soil’s volume. • The remainder is air and/or water that fills voids (pores) in the soil. • Soil plays many roles in the environment: • Supporting the plants that provide us with food and fiber • Cleaning and storing water • Recycling waste • Providing habitat for diverse forms of life • Transferring matter among Earth systems

Figure 11-5 Wetlands Store and Clean Water The thick organic material in wetland soils has high porosity and filters contaminants from water that migrates through (blue arrows).

Earth Systems Interactions Soil intimately mixes components from all four Earth systems. Geosphere, atmosphere, hydrosphere, and biosphere. Facilitates their interactions. Soil contains about twice as much carbon as does the atmosphere. Carbon resides in soil for only 9 years on average. Continually in flux to the atmosphere, biosphere, hydrosphere, geosphere. Photosynthesis by plants provides carbon to soils. When plants die, their remains add carbon compounds to soils. Decomposition of organic material in soil produces CO2. Escapes to the atmosphere (soil respiration). Dissolves in groundwater. Soil has a significant role in determining the atmosphere’s content of greenhouse gases. All of the 16 essential nutrients that plants need to grow (except O, H, N, C) come from the geosphere through soil.

Soil Cleans Water Figure 11-6 Soil Cleans Water Water passing through soil is cleaned by a combination of processes: absorption, filtration, and the biological consumption of organic matter.

Soil as Habitat Figure 11-7 Soil as Habitat A complex food web in soil supports abundant life.

How Soils Form Types of soils are affected by: Climate, bedrock, surface topography, and duration of weathering Soil-forming Processes Physical processes (e.g. frost wedging) break rocks into smaller pieces Chemical processes break down and change the parent materials Hydrolysis, oxidation, and dissolution Produce material containing much clay, quartz, and oxide minerals Warmer, wetter conditions facilitate chemical reactions Organisms churn soil, metabolize organics, add their waste, emit CO2 Plant nutrients (e.g., P and N)—from decomposing organic matter Porosity, permeability—influenced by organic content Humus—accumulated organic matter Result = Mature, well-developed soil with abundant pores, organisms, decaying organic debris, clays, and grains of various minerals, including oxides and quartz

Soil and the Carbon Cycle Figure 11-8 Soil and the Carbon Cycle The global soil reservoir contains about 1580 Gt (billion tonnes) of carbon. Ongoing carbon exchanges between soil and other reservoirs (shown in Gt) are a part of the carbon cycle that people influence by their land use practices.

Typical Soil Profile • The O horizon is recognized where abundant organic material accumulates on the surface. • Soil components are leached or otherwise removed from the organic-bearing A horizon (topsoil) which is: • Nutrient-rich • Fertile medium for growing plants • Material leached from A accumulates in the B horizon from water: • Clay minerals • Yellow, red, orange from iron oxides • The C horizon contains remnants of the parent material. Figure 11-10

Soil Variations • Composition of the bedrock directly influences: • Abundance of nutrients • Mineralogical composition • Climate directly influences: • Physical and chemical weathering processes that form soils • Warm, wet climates—hydrolysis, dissolution of minerals • Increase leaching of nutrients • Arid climates—develop soils with little organic matter • Precipitated carbonate minerals (not dissolved and removed) • Topography—leads to soil variations where slopes are present • Steepness of slopes • Orientation with respect to the Sun—called aspect • Aspect—affects soil temperature • Influences rates of physical, chemical, and biological weathering

Soil Variations (cont.) • Soil scientists use a systematic classification system to categorize and compare soils • There are 12 major soil orders • Four examples—spodosols, aridisols, mollisols, and oxisols—show how soils vary regionally • Spodosols develop in cool, moist coniferous forest regions (Pacific Northwest, Great Lakes region, Northeast states) • Spodosols: • acidic—need to add neutralizing material (lime) for growing • subsurface accumulation of humus • combined with aluminum and iron oxides or hydroxides • oxidized = splotchy brown to reddish colors

Aridosol Profile • Aridisols form in arid climates and lack abundant organic matter. • Soluble minerals such as calcite commonly accumulate in the B horizon (the variably thick, lighter colored material between 2 and 4 on the scale). • Scale is in 10 cm (3.3 in) intervals. Figure 11-12

Soil Variations (cont.) • Aridisols—develop in arid regions • Calcite and other minerals remain in the soil or are deposited in it • Soluble minerals accumulate in the B horizon through redeposition by infiltrating waters from the surface • Adhesionpromotes the upward movement of water—capillary action • Soluble minerals deposited in the upper levels • Organic matter is not abundant • Have biological soil crust composed of cyanobacteria, mosses, lichen

Soil Variations (cont.) • Mollisols—develop in the grasslands (prairies) of the world • Thick, dark A horizon—from the accumulation of organic material • Very fertile and excellent for agricultural purposes • Produce most of the wheat, corn, soybeans, and other crops • Oxisols—also called laterites—develop in warm, wet tropical forests • Deeply weathered soils—leached of much of their original mineral content • Al- and Fe-oxide rich soil without many plant nutrients—light to reddish colors

Mollisol Profile • Mollisols are characteristic of temperate climate grasslands. • They have a nutrient-rich A horizon • (the dark brown layer from the surface down to about 20 cm depth). Figure 11-13

Oxisol Profile • Oxisols develop in tropical climates and characteristically lack well-developed horizons. • They are essentially thick, highly leached A horizons with abundant iron and aluminum oxide or hydroxide minerals. • The scale is 1 meter (3.3 ft) long. Figure 11-14

Soil Properties: Physical Properties • Texture is determined by grain size • Soil structure = how soil particles aggregate, how soil breaks apart: • Granular soils break into small pieces, approx same size; permeable • Blocky soils break into larger pieces • Platy soils break into small platy pieces • Prismatic soils break into elongate blocks • Density = mass of a soil per unit volume. Density depends on: 1) type of solid material and 2) proportion of voids (porosity) in the soil • Shear strength—how well soil resists forces before fracturing • Compressibility—how soil compacts under applied forces • In general, the higher the density of a soil, the greater its shear strength and the more suitable it is as foundation material • Soil is heavy—1 m3 weighs ca. 1900 kilograms (3200 lb/yd3)

Soil Texture Diagram The proportions of clay, silt, and sand define soil textures. Figure 11-16

Soil Structure Figure 11-17 Soil Structures Soil structure determines how the soil breaks apart when it is disturbed.

Compositional Properties Moisture content—amount of water in a soil Pores vary from completely filled with water (saturated) to completely filled with air Soil ph —measure of its acidity Acidic <7 Neutral = 7 >7 Alkaline Influences chemical weathering reactions and the stabilities of minerals Nutrient content—most are derived from the geosphere Removed from soil by plants Dissolved (and carried away) as water passes through Nutrient levels may be maintained naturally if plant material is added back to the soil when plants die Mollisols—have maintained nutrient levels over long periods of time Oxisols—low nutrient levels from extensive leaching Nutrients from decaying vegetation—used by the growing forest Oxisols need fertilizer to be productive farmlands

Figure 11-19 Soil pH Soils range in pH from 3 (as acidic as vinegar) to 11 (as alkaline as ammonia).

Biological Properties • Bacteria are primary producers of food by: • Photosynthesis • Nitrogen fixation (convert N from the air into useful forms) • Decompose organic matter • Proterozoa—mobile organisms—waste is a source of plant nutrients • Fungi—stationary organisms that absorb food from their surroundings • Mushrooms and yeast—>100,000 known kinds • Secrete enzymes—help decompose organic matter; make nutrients

Biological Properties (cont.) • Worms—roundworms (nematodes) and earthworms—recycle nutrients • Churn the soil as they eat their way along • Promote internal soil drainage, aeration, mixing of nutrients • Arthropods—churn soil—improve soil structure—make nutrients available • Thrive on decaying organic matter; control protozoa populations • Soil quality: Capacity to sustain plant growth and animal productivity, maintain or enhance water and air quality, and support human health and habitation

Life Is Abundant in Soil Figure 11-21 Life Is Abundant in Soil Healthy soils are ecosystems with tremendous numbers and diversity of soil organisms. Agricultural practices tend to decrease the abundance of soil organisms, especially fungi, nematodes, arthropods, and earthworms.

Soil Degradation and Loss Poor agricultural practices have degraded about one-third of the 1.5 billion hectares (3.7 billion acres) of cropland around the world. FIGURE 11-24 Soil Degradation Worldwide: The Scope of the Problem Today, soil degradation is a problem almost everywhere.

Wind and Water Erosionof Cropland Soils FIGURE 11-26 Wind and Water Erosion of Cropland Soils in the U.S. Although soil conservation has been a national goal since the 1930s, erosion continues to be a significant problem in many places.

Erosion Soil is susceptible to erosion wherever its vegetation cover is removed Tilling, construction-site clearing, overgrazing, deforestation, roads, trails The Dust Bowl experience—widespread awareness of the destructive consequences of poor farming practices on lands susceptible to erosion. W. C. Loudermilk—1938–1939 investigations to learn about soil use Creation of soil conservation organizations

Erosion (cont.) • Wind erosion—responsible for ≈ 45% of the eroded soil each year. • Wind erosion is more likely in arid regions • Increases the amount of fertilizer needed • More difficult for seedlings to survive—damage crops • Water erosion—soil is exposed to rain and surface runoff of water • Sheet flow—thin nonchannelized overland flow and small streamlets, responsible for most water erosion on cropland • Farming practices leave bare soils—eroded by runoff of surface water • Grazing and deforestation, destroy the vegetation cover on soil, make it susceptible to erosion

Rill and Sheet Erosion FIGURE 11-30 Rill and Sheet Erosion (a) Extensive rill erosion has washed away young plants in this Iowa field. (b) Sheet erosion commonly accompanies rill erosion in fields.

Soil Contamination • Salination: contamination by salt—occurs naturally in arid regions • In areas of low rainfall, water in soil contains dissolved mineral salts • The water is drawn by capillary action to the surface • Evaporation leaves mineral salts in the soil and on the soil’s surface • This process degrades soils, making them toxic to vegetation • Irrigation can also lead to salination, especially in dry regions

Soil Contamination (cont.) • Fertilizers: a source both of pathogens and of toxic elements in soil • Natural organic fertilizers (fresh manure)—potential source of pathogens • Methods to treat it (remove pathogens) before applied to fields • Contamination with pathogens from nearby animal production facilities • Manufactured fertilizers—also a source of toxic elements in soils • Use waste materials from industrial processes (Fe and Zn) • Also have high concentrations of other toxic elements (As, Cd, Cr)

Pesticides • Pesticides • Kill insects that eat plants • Microbes that spread plant disease • Herbicides—kill unwanted plants—weeds • Pesticides are most commonly manufactured chemicals • Many are now designed to degrade after use • Some persist as residues in soils for decades • Two-edged sword—example: DDT • Effective at preventing disease by killing insects (Nobel Prize) • Negative ecological impact • Fish accumulate DDT in their bodies—pass it up food chain • Concentration in individual organisms increases (biomagnification) • 24 years passed after Nobel Prize, DDT is banned for general use in the U.S.

Depletion in Soils Biodiversity depletion Constantly churning and tilling soil Growing only one type of crop (monoculture farming) Applying pesticides—decrease a soil’s biodiversity Nutrient depletion Nutrients taken up by return to soil when plants die and decay Leaching—water dissolves minerals, carries nutrients away Harvesting—prevents recycling of nutrients into the soil Monoculture farming—depletes select nutrients quickly Churning up soil helps crops: by increasing porosity, mixing nutrients, increasing oxygen levels.But higher oxygen content Accelerates chemical reactions that release nutrients Causes organic matter to decompose more rapidly Decreases humus levels

Farming Practices That Protect Soil Resources • Contour farming—tilling perpendicular to surface slopes—creates plowed furrows that catch soil and water rather than letting them run off freely. • Terracing—converting steeper slopes into a series of flat terraces • Tilling fields when storms are not likely can decrease erosion • Machinery that can aerate, plant, weed fields without churning the soil • Strip farming—cultivating crops in parallel strips that can be harvested and tilled at different times—ensures that some areas will always be covered with vegetation. • Planting barriers that protect fields from wind—can help control erosion • Rotating crops in a field can prevent selective nutrient depletion • Reducing tillage and adding organic wastes helps maintain or increase the organic content of soils

Contour and Terraced Farming Figure 11-35 Contour Farming Plowing fields parallel to slopes decreases erosion. This Wisconsin farm has plowed and planted alfalfa hay (green) and corn (yellow) in contoured strips. Figure 11-36 Terracing Terraces, horizontal benches along slopes, significantly decrease erosion and have sustained soils on steep slopes for thousands of years. These terraced rice farms are on the lower slopes of Mount Batukaru in Bali.

Strip Farming Alternating crops in parallel strips (corn, wheat, and soybeans, for example) is a type of crop rotation that helps maintain soil nutrients. It is commonly combined with contour farming to decrease erosion. This farm is in Pennsylvania. Figure 11-37 Rotating different crops in a field through the years can help prevent selective nutrient depletion. Corn and soybeans are a good example. Corn decreases soil nitrogen levels, but soybeans harbor nitrogen-fixing bacteria on their roots. If soybeans and corn are planted alternately over time in the same field, the soil’s nitrogen levels can be maintained. Reducing tillage and adding organic wastes helps maintain or increase the organic content of soils.

Soil Remediation • Bioremediation—Using organisms to clean up contamination • Bacteria species that will eat toxic substances • Isolating the bacteria requires considerable laboratory work • Need to be assisted (e.g. adding oxygen) to increase their populations • Phytoremediation—Using plants to clean up soil • Certain plants selectively take up and concentrate toxic substances • Then degrade or release to the atmosphere in modified form • Some plant species = excellent accumulators of specific metals (As, Cd, Pb) • Growing these plants—then harvesting them—reduces metal concentrations in soil • Metal-bearing plants are usually burned and the ash deposited in landfills

Soil Remediation (cont.) • Phytomining—using specific plant varieties for mining applications • Desalination of soil • Irrigating with less salty water • Using minimum amounts of irrigation water • Reducing water evaporation rates

SUMMARY Soil is a mix of air, water, minerals, organic material, and life. Soils are formed by physical, chemical, and biological processes that affect geosphere materials on Earth’s surface. The soil-forming processes combine to develop horizons that define a soil’s profile. upper A horizon = material is leached and removed; middle B horizon = material accumulates from above lower C horizon = remnants of the weathered geosphere parent materials Examples of the 12 major orders recognized by soil scientists—are spodosols (forest soils), aridisols (desert soils), mollisols (prairie soils), and oxisols (tropical soils).

SUMMARY (cont.) • Physical properties of soils: texture, structure, density, shear strength, and compressibility. • Compositional properties: % water and % air in pores, pH, nutrients available to plants. • Biological properties: biodiversity; amount of decaying organic matter. • Soils may become degraded, eroded, contaminated, depleted in biodiversity and nutrients. • Farming practices such as contour tilling, terracing, crop rotation, and no-till farming help conserve soil. Both bioremediation and phytoremediation techniques have been used to clean soils.

Soil Resources Chapter 11