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CHAPTER 11. Soil: The Foundation for Land Ecosystems. A rich soil is much more than dirt. Soil : solid material of geological and biological origin Chemical, biological, and physical processes change soil Giving it the ability to support plant growth
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CHAPTER 11 Soil: The Foundation for Land Ecosystems
A rich soil is much more than dirt Soil: solid material of geological and biological origin Chemical, biological, and physical processes change soil Giving it the ability to support plant growth In productive soil, detritus feeders and decomposers constitute a biotic community Facilitating the transfer of nutrients Creating a soil environment favorable to root growth Productive topsoil involves dynamic interactions among organisms, detritus, and mineral particles of the soil
Soil formation Animation: Soil Formatioin
Soil characteristics Most soils are hundreds of years old They change very slowly Soil science is at the heart of agriculture and forestry Soil is classified by profile, structure, and type Soil texture: relative proportions of each soil type Parent material: mineral material of the soil Soil has its origin in the geological history of an area Weathering: gradual physical and chemical breakdown of parent material It may be impossible to tell what the parent material was
Classification of soil Soil separates: small fragments smaller than stones Sand: particles from 2.0 to 0.063 mm Silt: particles range from 0.063 to 0.004 mm Clay: anything finer than 0.004 mm Gravel, cobbles, boulders: particles larger than sand You can see the individual rock particles in sand Clay particles become suspended in water Clay is “gooey” because particles slide around each other on a film of water
Soil makeup Animation: Soil Makeup
Proportions Sand, silt, and clay constitute the mineral part of soil If one type of particle predominates, the soil is sandy, silty, or clayey Loam: a soil with 40% sand, 40% silt, and 20% clay To determine a soil’s texture: Add soil and water to a test tube and let the soil settle Sand particles settle first, then silt, then clay Scientists classify soil texture with a triangle It shows relative proportions of sand, silt, and clay
Properties Soil properties are influenced by its texture Larger particles have larger spaces separating them Small particles have more surface area relative to their volume Nutrient ions and water molecules cling to surfaces These properties profoundly affect soil properties Infiltration, nutrient- and water-holding capacity, aeration Workability: the ease with which soil can be cultivated Clay soils are hard to work with: too sticky or too hard Sandy soils are easy to work with
Soil profiles Horizons: horizontal layers of soil from soil formation Can be quite distinct Soil profile: a vertical slice through the soil horizons Reveals the interacting factors in soil formation O horizon: topmost layer of soil Dead organic matter (detritus) deposited by plants High in organic content Primary source of energy for the soil community Humus: decomposed dark material at the bottom of the O horizon
Subsurface layers A horizon (topsoil): below the O horizon A mixture of mineral soil and humus Permeated by fine roots Usually dark May be shallow or thick Vital to plant growth Grows an inch or two every hundred years E horizon: pale-colored layer below the A horizon Eluviation: process of leaching (dissolving) minerals due to downward movement of water
Subsurface layers B horizon (subsoil):below the E horizon Contains minerals leached from the A and E horizons High in iron, aluminum, calcium, other minerals, clay Reddish or yellow colored from oxidized metals C horizon: parent mineral material Weathered rock, glacial deposits, volcanic ash Reveals geologic process that created the landscape Not affected by biological or chemical processes
Soil and plant growth For best growth, plants need a root environment that supplies Mineral nutrients, water, oxygen The proper pH and salinity Soil fertility: the soil’s ability to support plant growth The presence of proper amounts of nutrients and all other needs Farmers refer to a soil’s ability to support plant growth as tilth
Mineral nutrients Initially become available through rock weathering Phosphate, potassium, calcium, etc. Much too slow to support normal plant growth Breakdown and release (recycling) of detritus provides most nutrients Leaching: nutrients are washed from the soil by water Decreases soil fertility Contributes to water pollution Nutrient-holding capacity: the soil’s capacity to bind and hold nutrient ions until they are absorbed by roots
Fertilizer Agriculture removes nutrients from the soil Fertilizer: nutrients added to replace those that are lost Organic fertilizer: plant or animal wastes or both Manure, compost (rotted organic material) Leguminous fallow crops (alfalfa, clover) Food crops (lentils, peas) Inorganic fertilizer: chemical formulations of nutrients Lacks organic matter Much more prone to leaching
Water is crucial for plants Transpiration: water is absorbed by roots and exits as water vapor through pores (stomata; singular = stoma) in the leaves Oxygen enters, and carbon dioxide exits, through stomata Loss of water through stomata can be dramatic Wilting: a plant’s response to lack of water Conserves water Shuts off photosynthesis by closing stomata Severe or prolonged wilting can kill plants
Water and water-holding capacity Water is resupplied to the soil by rainfall or irrigation Infiltration: water soaks into the soil Water runoff is useless to plants and may cause erosion Water-holding capacity: soil’s ability to hold water after it infiltrates Poor holding capacity: water percolates below root level Plants must depend on rains or irrigation Sandy soils Evaporative water loss depletes soil of water The O horizon reduces water loss by covering the soil
Aeration Novice gardeners kill plants by overwatering (drowning) Roots must breathe to obtain oxygen for energy Land plants depend on loose, porous soil Soil aeration: allows diffusion of oxygen into, and carbon dioxide out of, the soil Overwatering fills air spaces Compaction: packing of the soil Due to excessive foot or vehicular traffic Reduces infiltration and runoff Strongly influenced by soil texture
Relative acidity (pH) pH refers to the acidity or alkalinity of any solution The pH scale runs from 1 to 14 7 is neutral (neither acidic or alkaline) Different plants are adapted to different pH ranges Most do best with a pH near neutral Many plants do better with acidic or alkaline soils Blueberries do best in acidic soils
Salt and water uptake Buildup of salt in the soil makes it impossible for roots to take in water High enough salt levels can draw water out of a plant By osmosis Dehydrates and kills plants Only specially adapted plants grow in saline soils None of them are crops Irrigation can lead to salt buildup in soil (salinization)
The soil community To support plants, soils must Have nutrients and good nutrient-holding capacity Allow infiltration and have good water-holding capacity Resist evaporative water loss Have a porous structure that allows aeration Have a near-neutral pH Have low salt content According to the principle of limiting factors, the poorest attribute is the limiting factor
Limiting factors in plant growth Sandy soils dry out too quickly to be good for agriculture They have poor water-holding capacity Clay soils do not allow infiltration or aeration The best soils are silts and loams They moderate limiting factors Soil texture limitations are improved by the organic parts of the soil ecosystem Detritus Soil organisms
Organisms and organic matter in the soil Dead leaves, roots, other detritus on and in the soil Support a complex food web Bacteria, fungi, mites, insects, millipedes, spiders, earthworms, snails, slugs, moles, etc. Millions of bacteria are in a gram of soil Humus: residue of partly decomposed organic matter In high concentrations at the bottom of the O layer Extraordinary capacity for holding water and nutrients Composting: fosters decay of organic wastes Is essentially humus
Soil structure and topsoil Animals feeding on detritus also ingest mineral soil particles Castings: earthworm excrement of stable clumps of “glued” inorganic particles plus humus Burrowing of animals keeps clumps loose Soil structure: refers to the arrangement of soil particles Soil texture: refers to the size of soil particles A loose soil structure: best for infiltration, aeration, and workability Topsoil: clumpy, loose, humus-rich soil Loss of topsoil reduces crop yield by 85–90%
Interactions between plants and soil biota Mycorrhizae: a symbiotic relationship between the roots of some plants and certain fungi Fungi draw nourishment from the roots Fungi penetrate the detritus, absorb nutrients, and pass them to the plant Nutrients are not lost to leaching Bacteria add nitrogen to the soil Nematodes: small worms that feed on roots Detrimental to plants May be controlled by other soil organisms (e.g., fungi)
Soil enrichment Most detritus comes from green plants So green plants support soil organisms Soil organisms create the chemical and physical soil environment beneficial to plants Green plants further protect the soil by reducing erosion and evaporative water loss So keep an organic mulch around garden vegetables The mutually supportive relationship between plants and soil is easily broken Keeping topsoil depends on addition of detritus
Mineralization If detritus is lost, soil organisms starve Soil will no longer be kept loose and nutrient-rich Humus decomposes, breaking down the clumpy aggregate structure of glued soil particles Water- and nutrient-holding capacities, infiltration, and aeration decline Mineralization: loss of humus and collapse of topsoil All that remains are the minerals (sand, silt, clay) Topsoil results from balancing detritus and humus additions and breakdown
Soil degradation Turnover of plant material produces detritus When humans cut forests, graze livestock, or plant crops, the soil is managed or mismanaged Soil degradation: occurs when key soil attributes required for plant growth or other ecosystem services deteriorate Some reports on soil degradation are incorrect or outdated 75% of the land in Burkina Faso was said to be degraded But agricultural yields have increased due to soil and water conservation
Erosion Erosion: the process of soil and humus particles being picked up and carried away by water and wind Occurs any time soil is bared and exposed Soil removal may be slow and gradual (e.g., by wind) or dramatic (e.g., gullies formed by a single storm) Vegetative cover prevents erosion from water Reducing the energy of raindrops Allowing slow infiltration Grass is excellent for erosion control Vegetation also slows wind velocity
Splash, sheet, and gully erosion Splash erosion: begins the process of erosion Raindrops break up the clumpy structure of topsoil Dislodged particles wash between other aggregates Decreases infiltration and aeration Sheet erosion: the result of decreased infiltration More water runs off, carrying away fine particles Gully erosion: water converges into rivulets and streams Water’s greater volume, velocity, energy remove soil Once started, erosion can turn into a vicious cycle Less vegetation exposes soil to more erosion
Desert pavement Another devastating feature of wind and water erosion: differential removal of soil particles Lighter humus and clay are the first to be carried away Rocks, stones, coarse sand remain The remaining soil becomes coarser Deserts are sandy because wind removes fine material Desert pavement: occurs in some deserts Removal of fine material leaves a thin surface layer of stones and gravel This protective layer is easily damaged (e.g., by vehicles)
Drylands and desertification Clay and humus are the most important parts of soil For nutrient- and water-holding capacity Their removal results in nutrients being removed Regions with sparse rainfall or long dry seasons support grasses, scrub trees, and crops only if soils have good water- and nutrient-holding capacity Erosion causes these areas to become deserts Desertification: a permanent reduction in the productivity of arid, semiarid, and seasonally dry areas (drylands) Does not mean advancing deserts
Causes of erosion: overcultivation Plowing to grow crops exposes soil to wind and water erosion Soil remains bare before planting and after harvest Plowing causes splash erosion Destroying soil’s aggregate structure Decreasing aeration and infiltration Tractors compact soil Reducing aeration and infiltration Increasing evaporative water loss and humus oxidation Rotating cash crops with hay and clover is sustainable
No-till planting No-till agriculture: a technique allowing continuous cropping while minimizing erosion Routinely practiced in the U.S. After spraying a field with herbicide to kill weeds A planting apparatus cuts a furrow through the mulch Drops seeds and fertilizer Closes the furrow The waste from the previous crop becomes detritus So the soil is never exposed Low-till farming uses one pass (not 6–12) over a field
Inorganic fertilizer Can provide optimal amounts of nutrients efficiently But it lacks organic matter to support organisms and build soil structure It can keep nutrient content high under intensive cultivation (two or more cash crops/year) But mineralization and soil degradation proceed Additional fertilizer leaches into waterways Chemical fertilizers have a valuable place in agriculture Organic fertilizers may not have enough nutrients Growers must use each fertilizer as necessary
Reducing soil erosion Contour strip cropping: plowing and cultivating at right angles to contour slopes Shelterbelts: protective belts of trees and shrubs planted along plowed fields The U.S. Natural Resource Conservation Service (NRCS) Established in response to the Dust Bowl Regional offices provide information to farmers and others regarding soil and water conservation practices U.S. soil erosion has decreased through conservation Windbreaks, grassed waterways, vegetation to filter runoff
