Chapter 13: Soil and Its Uses 13.1 The Study of Soil as a Science 13.2 Geologic Processes 13.3 Soil and Land 13.4 Soil Formation 13.5 Soil Properties 13.6 Soil Profile 13.7 Soil Erosion 13.8 Soil Conservation Practices 13.9 Conventional Versus Conservation Tillage Protecting Soil on Nonfarm Land

13.1 The Study of Soil as a Science Soil science is the study of soil as a natural resource on the surface of the earth including soil formation, classification and mapping; physical, chemical, biological, and fertility properties of soils; and these properties in relation to the use and management of soils.

13.2 Geologic Processes The crust of the Earth is an extremely thin, less-dense solid covering over the mantle. The mantle makes up the majority of the Earth, and surrounds a small core of iron. The outermost portion of the mantle is solid. The crust and solid outer mantle are collectively known as the lithosphere. The asthenosphere is a thin layer below the outer mantle capable of plastic flow. The core consists primarily of iron and nickel and has a solid center and a liquid outer region.

Structure of the Earth

Plate tectonics Plate tectonics is the concept that the outer surface of the Earth is made of large plates of crust and outer mantle (lithosphere) that are slowly moving over the surface of the liquid outer mantle (asthenosphere).  Heat from the Earth causes the slow movement of the outer layer (as when we heat a liquid on the stove).  Plates are pulling apart in some areas, and colliding in others.  Where the plates are pulling apart from one another, the liquid mantle moves up and solidifies. Thus, new crust is formed (half of the surface of the Earth have been formed in this way, such as the bottom of the Atlantic Ocean).  Where plates collide, often one plate slides under the other and is melted (volcanoes and mountains are formed, such as in the west coast of America).  When two continental plates collide, the crust buckles to form mountains (The Himalayan, Alp, and Appalachian mountain ranges).

Tectonic plates

Weathering Weathering processes are important in reducing the size of particles that can then be dislodged by moving water and air. Mechanical weathering results from physical forces that reduce the size of rock particles without changing the chemical nature of the rock.  Freezing and thawing cycles  Heating a large rock can cause it to fracture  Glaciers cause rock particles to grind against one another, resulting in smaller fragments.  Actions of plants (roots) and the burrows of animals  Wind and moving water remove small particles and deposit them at new locations, exposing new surfaces to the weathering process.

Physical fragmentation by freezing and thawing

Chemical weathering Chemical weathering involves the chemical alteration of rock in such a manner that it is more likely to fragment or be dissolved.  Rock fragments exposed to the atmosphere may oxidize (combine with oxygen) or hydrolyze (combine with water).  Oxidized or hydrolyzed molecules are more readily soluble in water. The process of loosening and redistributing particles is called erosion.

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13.3 Soil and Land Land is the portion of world not covered by water. Soil is a mixture of minerals, organic material, living organisms, air, and water that together support growth of plant life.  Good agricultural soil:  45% Mineral  25% Air  25% Water  5% Organic Matter This combination provides good drainage, aeration, and organic matter.

The components of soil

13.4 Soil Formation Soil forming factors include the following:  Parent Material  Climate  Topography  Biological Factors  Time (it takes about 500 years to form 1 inch of soil in dry or cold climates)

Parent material Soil formation begins with fragmentation of parent material.  Parent material consists of ancient layers of rock, or more recent deposits from lava flows or glacial activity.  Residual soils have the same general chemistry as the original rock.  Fast-flowing water leaves gravel, rocks, and sand. Slow- flowing water and lakes leave fine textured materials (clay and silt)

Biological factors The first organisms to gain a foothold in modified parent material also contribute to soil formation.  Lichens form pioneer communities.  Decomposition of dead lichens further alters underlying rock.  Animals and microorganisms mix soils and form burrows and pores.  Plant roots open channels in the soil.

Topography Slope affects the moisture and temperature of soil. Steep slopes facing the sun are warmer. Steep soils may be eroded and lose their topsoil as they form.

Humus Humus is the organic material resulting from the decay of plant and animal remains.  It mixes with top layers of mineral particles, and supplies needed nutrients to plants.  It creates a crumbly soil that allows adequate water absorption and drainage. Burrowing animals such as earthworms bring nutrients up from deeper soil layers, improving soil fertility. Dry or cold climates develop soils very slowly, while humid and warm climates develop them more rapidly. Cold and dry climates have slow rates of accumulation of organic matter. Chemical weathering proceeds more slowly at lower temperatures and in the absence of water.

13.5 Soil Properties Soil texture is determined by the size of mineral particles within the soil.  Too many large particles, such as gravel (larger than 2 millimeters in diameter) and sand (between 2 mm and 0.05 mm) lead to extreme leaching (carrying dissolved organic matter and minerals to lower layers).  Too many small particles, such as silt (0.05 mm to mm) and clay (less than mm) lead to poor drainage.

Soil texture

Soil Properties Soil structure refers to the way various soil particles clump together. Particles in sandy soil do not attach to one another (granular structure). Particles in clay soil tend to stick to one another to form large aggregates.  An ideal soil for agricultural use is loam, which combines the good aeration and drainage properties of large particles with the nutrient retention and water-holding ability of clay particles.  In good soils, one-half to two-thirds of spaces contain air after excess water has drained.  A good soil is friable, which means that it crumbles easily.  Protozoa, nematodes (wireworms), earthworms, insects, algae, bacteria, and fungi are typical inhabitants of soil.

13.6 Soil Profile The soil profile is a series of horizontal layers of different chemical composition, physical properties, particle size, and amount of organic matter. Each recognizable layer of the profile is known as a horizon.

Soil Profile A horizon is the topsoil, or the uppermost layer. It contains most of the soil nutrients, small mineral particles, and living organisms (dark color). O horizon is made of litter (undecomposed or partially decomposed organic material). Forest soils have an O horizon. E horizon is formed from leaching darker materials (from A). Usually very nutrient poor. B horizon is the subsoil. It contains less organic matter and fewer organisms, but accumulates nutrients leached from topsoil. It supports a well-developed root systems. Soils in woodlands that receive high rainfall have B horizon. C horizon is weathered parent material below the subsoil. It has no organic materials. R horizon is bedrock (limestone, granite, etc).

Soil Profile

Major soil types

Soil Profile Over 15,000 separate soil types have been classified in North America. Most cultivated land can be classified as either grassland or forest soil.  Grassland soils usually have a deep topsoil layer. A lack of leaching results in a thin layer of subsoil.  In forest soils, which are typically high rainfall areas, the topsoil layer is relatively thin, but topsoil leachate forms a subsoil that supports substantial root growth. Two features of tropical rainforests have great influence over the nature of the soil:  High temperatures lead to rapid decomposition of organic matter, with little litter.  High rainfall leads to excessive leaching of nutrients.

13.7 Soil Erosion Erosion is the wearing away and transportation of soil by wind, water, or ice. Worldwide, erosion removes 25.4 billion metric tons of soil per year.  Made worse by deforestation and desertification.  Poor agricultural practices increase erosion and lead to the transport of associated fertilizers and pesticides. Most current agricultural practices lose soil faster than it can be replenished. Wind erosion may not be as evident as water erosion, but is still serious.  It is most common in dry, treeless areas.  Great Plains of North America have had four serious bouts of wind erosion since European settlement in the 1800s.

13.8 Soil Conservation Practices When topsoil is lost, fertility is reduced or destroyed, thus fertilizers must be used to restore fertility.  This practice raises food costs, and increases sediment load in waterways.  Over 20% of U.S. land is suitable for agriculture, but only 2% does not require some form of soil conservation practice.

Soil Conservation Practices Agricultural Potential  Worldwide:  11% of land surface is suitable for crops.  An additional 24% is in permanent pasture.  United States:  20% land surface suitable for crops.  25% in permanent pasture.  African Continent:  6% land surface suitable for crops.  29% can be used for pasture.

Soil Conservation Practices Soil Quality Management Components:  Enhance organic matter.  Avoid excessive tillage.  Manage pests and nutrients efficiently.  Prevent soil compaction.  Keep the ground covered.  Diversify cropping systems.

Soil Conservation Practices Contour farming is tilling at right angles to the slope of the land. Each ridge acts as a small dam.  Useful on gentle slopes.  One of the simplest methods for preventing soil erosion. Strip farming is the practice of alternating strips of closely sown crops (hay, wheat) with strips of row crops (corn, cotton, soybeans) to slow water flow, and increase water absorption.

Contour farming

Strip farming

Soil Conservation Practices Terracing is the practice of constructing level areas at right angles to the slope to retain water.  Good for very steep land. Terraces

Soil Conservation Practices Waterways are depressions in sloping land where water collects and flows off the land.  Channels movement of water. Windbreaks are plantings of trees or other plants that protect bare soil from full force of the wind.  Windbreaks reduce wind velocity, decreasing the amount of soil that can be carried.

13.9 Conventional Versus Conservation Tillage Plowing has multiple desirable effects:  Weeds and weed seeds are buried.  Crop residue is turned under, where it will contribute to soil structure.  Leached nutrients brought to surface.  Cooler, darker soil brought to top and warmed.

Conventional Versus Conservation Tillage Each trip over the field is an added expense to the farmer, and at the same time increases the amount of time the soil is open to erosion via wind or water. Reduced tillage is a practice that uses less cultivation to control weeds and prepare soil, but generally leaves 15-30% of soil surface covered with crop residue after planting.

Conventional Versus Conservation Tillage Conservation tillage further reduces amount of soil disturbance and leaves 30% or more of soil surface covered with crop residue.  Mulch tillage: Tilling entire surface just prior to planting.  Strip tillage: Tilling narrow strips that will receive seeds.  Ridge tillage: Leaves ridges; the crop is planted on the ridge with residue left between ridges.  No till farming: Involves special planters that place seeds in slits cut in the soil.

Conventional Versus Conservation Tillage Positive Effects of Reduced Tillage:  Wildlife gain winter food and cover.  Less runoff results in reduced siltation of waterways.  Row crops can be planted in sloped areas.  Fewer trips over the field means lower fuel consumption.  Two crops may be grown on a field in areas that had been restricted to a single crop.  Fewer trips over the soil means less soil compaction.

Conventional Versus Conservation Tillage Drawbacks of Conservation Tillage  Plant residue may delay soil warming.  Crop residue reduces evaporation and upward movement of water through the soil, which may retard the growth of plants.  Accumulation of plant residue can harbor plant pests and diseases, requiring more insecticides and fungicides.