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 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 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 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

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 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 (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

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).

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.

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

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 & 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!

Irrigation

Salinization & Waterlogging

Salinization & Waterlogging

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

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 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

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

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