Soil Science SimplifiedPpt 1 - Intro

BHS Envirothon Competition Team

Soils/Land Use Objectives

This ppt will cover the following objectives:• U. Recognize soil as an important

resource.• V. Describe basic soil properties (and

formation factors)• X. Determine basic soil properties and

limitations, such as mottling and permeability, by observing a soil pit or soil profile

Why Study Soil?• Great Integrator!

– Medium of crop production– Filter water and waste– Producer and absorber of gases – Home to organisms– Medium for plant growth– Waste decomposer– Source material for construction, art, medicine, etc.– Snapshot of geological, climatic, biological, and

human history– Essential natural resource.

Nature and Function of Soil

• Soil – the unconsolidated cover of the earth, made up of mineral and organic components, water and air and capable of supporting plant growth. Most important function: GROW PLANTS

• Photosynthesis – plant ability to combine CO2 and H2O from the ground into sugar (C6H12O6). Light furnishes the energy for this reaction. Nitrogen, sulfur and phosphorus are required for synthesis of plant proteins too.

• As a medium for plant growth, soil performs four

functions:– Anchors roots– Supplies water– Provides air– Furnishes minerals for plant nutrition

• The pore space between the solids is taken up by water and air.

• Air takes up part of the pore space not occupied by water. As the water increases, the air content decreases. In respiration (opposite of photosynthesis), plant roots use oxygen and give off carbon dioxide. So, soil usually contains less oxygen and more carbon dioxide than atmospheric air does.

• Millions of microbes live in each ounce of fertile soil. Without them, soils would become inactive and lose their ability to support plants.

Physical Properties of Soils• Soils consist of solid,

liquid, gaseous and biotic components.

• Sand and silt are merely broken down rock frags (consists of quartz, feldspar, mica, or other minerals). Chemically they are essentially inert compared with clay and organic matter, which are responsible for most of the chemical reactions in the soil.

Soil fraction

Diameter Description

Gravel Larger than 2 mm

Coarse

Sand 0.05 - 2 mm

Gritty

Silt 0.002 – 0.05 mm

Floury

Clay Smaller than 0.002 mm

Sticky when wet

• Clay particles are plastic and sticky when wet. They are highly adsorptive of water, gas, and dissolved substances. Clays are minute, plate-shaped, aluminosilicate crystals consisting of silicon, aluminum, iron, magnesium, oxygen, and hydrogen.

• There are several types of clay. Two of the most important ones are kaolinite and smectite. Smectite clays have the ability to swell on wetting and shrink when dry. Smectite enter chem rxns more than kaolinitic clays.

• Clay is a negatively charged colloid.

This negative charge is the reason that positively charged cations surround each clay particle. The individual cations can be exchanged for each other.

• If the cations can get close to the surface of the clay, the neg charge on the clay is largely neutralized and the clay particles will cling together or flocculated. Calcium and magnesium are dominant in this area. They are small and effective in holding clay particles together.

Soil Texture• Texture – relative proportion of the

various grain sizes in a soil.

• To describe soil texture, names, such as loamy sand, silt loam, clay loam and silty clay are used.

• The best soils are generally those which contain 10 to 20% clay, with silt and sand in approximately equal amounts, and a fair amount of organic matter.

Soil Texture Triangle

• The content of sand, silt, and clay for the twelve main soil texture classes can be found on this triangle. Ex. Point A is in the sandy loam texture class with 65% sand, 25% silt, and 10% clay. Not that soils with relatively small clay contents (<40) are in the clay texture class bc the properties of clay readily predominate over the coarser fractions.

Surface Area• In comparing clay with sand and silt, it is important

to be aware of the relative amount of surface area of these particle size groups, bc it is on the surface that many chemical and physical processes take place.

• Smaller = more surface area (clay is tiny!)

Kind of Particle

Diameter of Particle

# of Particles in 1 gram

Surface area of 1 gram

Sand 2 mm 90 11 cm2

Silt 0.02 mm 90,000,000

(9x107)

1130 cm2

Clay 0.0002 mm 9x1013 113,000 cm2

Soil Structure• Structure – arrangement of individual particles in

relation to each other. Soil structure is the arrangement of particles into small groups, or aggregates.

• Aggregates may be bound together with other aggregates in larger masses called peds.

• Peds come in different shapes that roughly resembel sphere, blocks, columns and plates.

• If the individual particles are arranged in small aggregates with rounded edges, we speak granular structure. This is very desirable for plant growth bc it provides both large and small pores.

• Some soils lack structure. Sandy soils the individual grains act independently of each other. No binding substances hold the particles together, so the soil has no peds.

Pore Space

• Large pores are readily drained of water and filled by air after a heavy rain. They are valuable as an aeration system.

• Small pores hold water against gravity and pull water up from a water table by capillary action. They are necessary for the water supply of plants.

• Ideal structure includes large and small pores in proportions that corresponds to the water/air needs of the crop plants given for that culture or climate.

Soil Temperature• Just as important to plant growth as air temp.

The temp of the surface soil fluctuates greatly both during a 24 hr period and with the seasons.

• Where soil is covered by a dense growth of plants or a thick layer of mulch, temperature variations are much less severe and do not penetrate deeply

• Soil temperature has a direct effect on plant growth and also influences microbial activity.

• Freezing and thawing of the soil water also affects soil structure. Slow and occasional freezing and thawing (like under mulch) is beneficial for soil structure.

Soil Color• Color of soils tells us much about some of its other properties. • The color of a surface soil horizon depends mainly on its organic matter

content – the darker the soil, the more organic matter it contains. This organic matter imparts favorable properties to the soil, such as better aggregation and a high water-holding capacity. Also, dark soils absorb more radiation during the day, and radiate more heat during the night.

• In subsoil horizons, soil color indicates the wetness and aeration conditions of the soil.

• In general, reddish and brownish subsoils indicate good aeration and little water logging (you see the iron red)

• Grayish and olive colors indicate much water logging and a chemical reduction of iron.

• A mottled subsoil, one with a splotchy pattern of brownish and grayish colors, is indicative of a fluctuating ground water table.

Munsell Color System

• Defined color in terms of Hue, Value and Chroma.

• Hue was defined as the actual color, red, blue, green, etc.

• Value was defined as how light or dark a color is.

• Chroma was defined as how strong or weak a color is.

• Soil scientists use the Munsell Color book to determine the color of the soil to help determine in what type of soil it is along with using texture.