Definition of Soil- The outermost solid layer of the Earth- A natural product formed from

weathered rock by the action of climate & living organisms

- A collection of natural bodies of earth that is composed of mineral & organic matter, & is capable of supporting plant growth

Soil FormationVideo: Soil Formation

Soil formation - Parent material is slowly broken down by biological, chemical and physical weathering.

Biological - Respiration of plant roots and other organisms produce CO2, which reacts with soil water to produce carbonic acid (H2CO3).

Chemical - Acids crack rocks water seeps in breaks down particles.

Physical - Weathering introduces water that breaks down particles.

Time scale - Formation of 2.5 cm of topsoil takes 200 - 1000 yrs.

Soil Formation

Soil Formation

Complex mixture of organic and inorganic components

Parent material = starting material that affects the composition of the resulting soilWeathering = the processes that break down rocks and minerals (1st step in soil formation)Erosion = the movement of soil from one area to another     

Soil Composition

Soil Composition

45% Mineral particles (broken down pieces of rock)

5% Organic matter (humus - from dead organisms, worm castings, leaf litter)

25% Water (precipitation)

25% Air (More with sandy soil, less with clay soil)

Soil organisms - Millions in one teaspoon of fertile agricultural soil!

- bacteria, fungi, algae, microscopic worms.

provide ecological services such as worm castings, decomposition to humus, breaking down of toxic materials, cleansing water, nutrient cycling from decomposers or upon death

Soil Composition

Soil Texture– determined by the proportion of particle size classes– texture is partly inherited from parent material and partly a result of

the soil forming process– particle classification:

• rock fragments (> 2.0mm diameter)• sand (0.05-2.0mm)• silt (0.002-0.05mm)• clay (<0.002mm)

- texture is important for indicating pore space- important in the movement of water and air, penetration by roots, and

water storage capacity- Particles equal 50% of the soil volume and the other 50% is pore space.

- 45% mineral matter- 5% organic matter (humus*)- 25% air- 25% water

Video: Soil Texture

Clay

Clay is formed as products of chemical weathering of other silicate minerals at the earth‘s surface.

They are found most often in shales, the most common type of sedimentary rock.

Plants grown in clay soils are more susceptible to waterlogging, and oxygen depletion (think small pore size, low porosity).

SiltSilt is rock worn into tiny pieces (coarser than clay, but finer than sand).

Silt is usually 1/20 millimeter or less in diameter.

Sand

Sand is quartz or silica worn down over time.

Sand grains have diameters between 0.06 mm to 2 mm.

Plants grown in sandy soils are more susceptible to mineral deficiencies and drought.

Loam

Loam is soil containing a mixture of clay, sand, silt and humus (organic matter).

Loam is good for growing most crops - ideal agricultural soil.

40% sand (larger - structural support, aeration, permeability)

40% silt (smaller - holds nutrient minerals and water)

20% clay (even smaller - holds nutrient minerals and water)

Soil Porosity and Permeability

• Porosity - volume of water that “fits between” the soil particles

• Permeability - rate of flow of water through soil

• % water retention - how much water is “trapped” by soil, the amount of water per unit volume a soil can hold is influenced by texture, clay minerals, organic content, particles, & soil structure

• Porosity and Permeability are directly related - when one is high, the other is high as well

• % water retention is inversely related to both

Soil Profile• Soil profile – distinctive layering of

horizons in the soil.

• Soil horizon – developmental layers in the soil with its own characteristics of thickness, color, texture, structure, acidity, and nutrient concentration.

Soil Horizons

Video: Soil Profile

O = Organic or litter layer

A = Topsoil; mostly inorganic minerals with some humus (organic materials); crucial for plant growth

E = Eluviation horizon; leaching = a process whereby solid materials are dissolved and transported away

B = Subsoil; zone of accumulation of leached minerals and organic acids from above

C = Slightly altered parent material

R = Bedrock

Soil Horizons

Soil Profiles (Biomes)

Soil Acidity (pH)

pH - concentration of H+ ions

Scale - From 0 (very acidic) to 14 (very basic, or alkaline)

pH of most healthy soils - 4 - 8

pH matters because it affects solubility of nutrient minerals

Aluminum and Manganese are more soluble in low pH - roots sometimes absorb too much (toxic levels)

Soil pH affects leaching - high pH increases leaching of important ions such as K+

Causes of changes - acid rain, decomposition, leaf litter, mining (acid sulfate soils)

Remediation

- Too high pH, add acidic leaf litter

- Too low pH, add lime

Soil Nutrients (NPK)

Soil Nutrients (NPK) Organic - animal manure, bone meal, compost (slow-acting, long-lasting)

Delay in availability to plants, needs time for the organic material to decompose

Delay causes low level of nutrient leaching

Improves water holding capacity

Inorganic - Manufactured from chemical compounds (fast-acting, short-lasting)

Highly soluble so immediately available to plants

High solubility also makes it leach quickly (pollutes water)

Suppresses growth of microorganisms

Source of nitrogen gases that increase air pollution

Production requires much energy from fossil fuels, increasing CO2 emissions.

Fertilizers

Organic…manure adds N and soil bacteria & fungigreen manurecompostmushroom spores

Inorganic…1/4th of the world’s crops depend on this.N, K, P

Experimental data comparing methods!

Click on the picture!

Irrigation

Salinization & Waterlogging

Salinization & Waterlogging

Soil Degradation

• Most of the world’s soil is not ideal for agriculture

• We are losing 5–7 million hectares (12–17 million acres) of productive cropland per year

Natural Wind, weather, water

Artificial Over cultivating, poor planning Overgrazing rangeland Deforestation, especially on slopes

Desertification

Soil Conservation• Soil moves….due to water, wind, and people

• Loss of topsoil…the most fertile

• Soil ends up as sediment in water

http://www.usd.edu/anth/epa/dust.html

Soil ConservationDUST BOWL:

Drought and degraded farmland produced the 1930s

Storms brought dust from the U.S. Great Plains all the way to New York and Washington

Soil Conservation

As a result of the Dust Bowl, the U.S. Soil Conservation Act of 1935 and the Soil Conservation Service (SCS) were created

Local agents in conservation districts work to educate farmers and help them conserve soil

Legislation in recent years:– Food Security Act of 1985– Conservation Reserve Program, 1985– Freedom to Farm Act, 1996– Low-Input Sustainable Agriculture Program,

1998

Soil Testing

• Soil Texture• Soil Porosity• Soil Permeability• Soil Moisture• Soil Fertility – pH, NPK• Soil Salinity – we will set this up next

week as a separate lab