1. 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

2. Soil Formation
Video: 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 yrs.

3. Soil Formation

4. Soil Formation
Complex mixture of organic and inorganic components Parent material = starting material that affects the composition of the resulting soil Weathering = the processes that break down rocks and minerals (1st step in soil formation) Erosion = the movement of soil from one area to another
38% of Earth’s surface has been converted for agriculture
cultivating soil, producing crops, and raising livestock for human use and consumption

5. 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

7. Soil Composition

8. Soil Texture determined by the proportion of particle size classes
Video: 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 ( mm)
silt ( mm)
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

9. 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).

10. Silt
Silt is rock worn into tiny pieces (coarser than clay, but finer than sand).
Silt is usually 1/20 millimeter or less in diameter.

11. 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.

12. 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)

13. 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

14. 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.

15. Soil Horizons
Video: Soil Profile

16. 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
Humus = any organic matter that has reached a point of stability, will not break down any further, contributes to moisture and nutrient retention

17. Soil Horizons

18. Soil Profiles (Biomes)

19. Soil Acidity (pH) pH - concentration of H+ ions
Scale - From 0 (very acidic) to 14 (very basic, or alkaline)
pH of most healthy soils
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

20. Soil Nutrients (NPK)

21. 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.

22. Fertilizers Organic… manure adds N and soil bacteria & fungi
green manure
compost
mushroom spores
Inorganic…1/4th of the world’s crops depend on this.
N, K, P
Experimental data comparing methods!
Click on the picture!

23. Irrigation

24. Salinization & Waterlogging

25. Salinization & Waterlogging

26. Soil Degradation Natural Artificial Wind, weather, water
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

29. Desertification

30. Soil Conservation Soil moves….due to water, wind, and people
Loss of topsoil…the most fertile
Soil ends up as sediment in water

31. Soil Conservation Drought and degraded farmland produced the 1930s
DUST 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

32. Soil Conservation Legislation in recent years:
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

33. 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