Soil

Not just dirt!

Soil…

• is a thin surface layer of the Earth’s crust.• is affected by agents such as weather, wind,

water, and organisms.• is (approximately) composed of :

– Mineral particles (45% )– Organic matter (5%)– Water (25%)– Air (25%)

Importance of soil

• Organisms (mainly microorganisms) depend on it for shelter, food & water.

• Plants anchor themselves into it, and receive nutrients and water from it.

• Humans could not exist without soil.• Soil is a slowly renewed resource.

• Soil formation begins when bedrock is broken down by physical & chemical processes called weathering.

• It can take 200-1000 years to form 1 inch of soil depending on the geographical location.

• Mature soils, or soils that have developed over a long time are arranged in a series of horizontal layers called soil horizons.

Soil Formation

Weathering

• Physical weathering: (wind, water, ice, etc.)

• Chemical weathering: the CO2 produced during respiration diffuses into soil, reacts with H2O & forms carbonic acid (H2CO3). This eats parts of the rock away.

– 0.05 to 2mm = sand (the largest soil particles) can be seen easily with the eye.

– 0.002 to 0.05mm = silt – about the size of flour and barely visible with the eye.

– <.002mm = clay (has the greatest surface value) – only seen under an electron microscope.

– Over 2mm = gravel/stones. (not considered to be one of main 3 particle sizes)

Soil particles:

Soil particles:

• The percentages (by weight) of different sized particles of sand, silt and clay that it contains.

Soil Texture:

Moisten a soil sample and rub it between thumb and fingers: (note: these are not necessarily scientific terms)

• “Gritty” – it has a lot of sand.• “Silky” - or smooth (like flour), it has a lot

of silt.• “Sticky”- it has a high clay content and you

should be able to roll it into a clump .

Soil Texture:

• Friability: How easily the soil can be crumbled.

• Porosity: A measure of the average distances between particles.

• Permeability: The rate at which water and air moves from upper to lower soil layers.

Soil Characteristics:

Soil Characteristics:

• Infiltration: the downward movement of water through soil.

• Leaching: dissolving of minerals and organic matter in upper layers carrying them to lower layers.

Shrink-Swell Potential

• Some soils (particularly clay soils), swell when water gets in them, then they dry and crack as water evaporates.

Soil pH

• The pH of most soils ranges from 4.0 to 8.0. • Plants are affected by soil pH because it

influences the solubility of nutrient minerals.• Extremes in soil pH: The Pygmy Forest in

California is extremely acidic (2.8-3.9) and in Death Valley, California, it is very basic (10.5).

Soil pH

• Slope: Steep slopes often have little or no soil on them because of gravity & erosion from precipitation runoff.

• Depth: soils can be as shallow as 2” while others may be over 3 feet in depth.

• Color: can indicate high organic content (darker color) or low organic content (lighter color)

Other soil factors

Soil horizons (layers)

• The uppermost layer; it is rich in organic material.

• Plant litter accumulates in the O-horizon and gradually decays.

• It is minimal or absent in desert soils.

O

Soil horizons (layers)A

• It is dark and rich in accumulated organic matter and humus.

• It has a granular texture and is somewhat nutrient-poor due to the loss of many nutrient minerals to deeper layers and by leaching.

Soil horizons (layers)B

• It is often a zone where nutrient minerals have leached out of the topsoil and litter accumulate.

• It is typically rich in iron & aluminum compounds and clay.

Soil horizons (layers)C

• contains weathered pieces of rock and borders the solid parent material.

• Most roots do not go down this deep and it is often saturated with groundwater.

Fig. 3-24a, p. 69

Mosaic of closely packed pebbles, boulders

Weak humus-mineral mixture

Dry, brown to reddish-brown with variable accumulations of clay, calcium and carbonate, and soluble salts

Alkaline, dark, and rich in humus

Clay, calcium compounds

Desert Soil Grassland Soil

Soil horizons (layers)

Fig. 3-24b, p. 69

Tropical Rain Forest Soil

Acidic light-colored humus

Iron and aluminum compounds mixed with clay

Forest litter leaf mold

Humus-mineral mixture

Light, grayish-brown, silt loam

Dark brown firm clayDeciduous Forest Soil

Soil horizons (layers)

• Erosion is the movement of soil, especially surface litter and topsoil, from one place to another.

• 6.4 billion tons of soils are eroded from the U.S. each year.

• Farming, logging, construction, overgrazing , off-road vehicles, deliberate burning of vegetation etc., destroy plant cover and leave soil vulnerable to erosion.

• Soil erosion lowers soil fertility and can overload nearby bodies of water with sediment.

Soil Erosion

• Types of Erosion by Water:– Sheet erosion: surface water

or wind peel off thin layers of soil.

– Rill erosion: fast-flowing little rivulets of surface water make small channels.

– Gully erosion: fast-flowing water join together to cut wider and deeper ditches or gullies.

Soil Erosion

• Soil is eroding faster than it is forming on more than one-third of the world’s cropland.

Figure 13-10Figure 13-10

Soil Erosion

• Erosion by wind:–Saltation – one particle hitting

another and being blown across the surface of the soil.

–Suspension – airborne soil.

Soil Erosion

• Surface Creep – mountains/sand dunes; surface moving very slowly.

Soil Erosion

• Landslides are an example of a very fast surface creep.

Agriculture

• Since 1950, high-input agriculture has produced more crops per unit of land.

• In 1967, fast growing dwarf varieties of rice and wheat were developed for tropics and subtropics.

Figure 13-17Figure 13-17

The Green Revolution

• Lack of water, high costs for small farmers, and physical limits to increasing crop yields hinder expansion of the green revolution.

• Since 1978 the amount of irrigated land per person has declined due to:– Depletion of underground water supplies.– Inefficient irrigation methods.– Salt build-up.– Cost of irrigating crops.

The Green Revolution

• Modern agriculture has the potential for a greater harmful environmental impact than any human activity.

• Loss of a variety of genetically different crop and livestock strains might limit raw material needed for future green and gene revolutions.– In the U.S., 97% of the food plant varieties available in

1940 no longer exist in large quantities.

The Green Revolution

Soil Conservation in agriculture

• Contour Farming – planting crops along the contours of the land to minimize runoff.

• Terracing – used with contour farming. Terraces run along contour of the land. This helps to retain water for crops at each level and reduce soil erosion by controlling runoff.

• Minimum Tillage - disturbance to the soil is less than conventional plowing. Special tillers break up and loosen the soil without turning over the topsoil, previous crop residues, or any cover vegetation.

• No-till farming – a way of growing crops with minimal soil disturbance. Crops are planted directly in small seed trench. The potential for erosion is greatly reduced. It is very effective on steeper slopes.

Soil Conservation in agriculture

Soil Conservation in agriculture

• Strip cropping – a row crop alternates in strips with another crop that completely covers the soil, reducing erosion. This method of farming catches water and reduces runoff . It also helps prevent the spread of pests and plant diseases.

• Cover cropping – several crops are planted together in strips or alleys between trees and shrubs. This provides shade and helps reduce water loss by evaporation.

• Shelterbelts – can reduce wind erosion. Long rows of trees are planted to partially block the wind. They can also help retain soil moisture, supply some wood for fuel, and provide habitats for birds.

Soil Conservation in agriculture

• Fertilization can help restore soil nutrients, but runoff of fertilizer can cause water pollution. (eutrophication)

• Types of fertilizers:– Organic fertilizers: manure, compost, crop

residues, bone meal.– Commercial inorganic fertilizers: contain

nitrogen, phosphorous, and potassium (N-P-K ) and sometimes includes micronutrients. Leaches

quicker from soil than organic fertilizers.

Soil fertilization

Irrigation in agriculture• Conventional center-pivot irrigation- allows

80% of the water input to reach crops (but much of the water evaporates).

• Gravity-flow irrigation- Valves send water down irrigation ditches.

• Drip irrigation- Can raise water efficiency to 90-95% and reduce water use by 37-70%.

• Floodplain irrigation- allowing the natural floods to irrigate the crops. Soils in flood zones tend to be nutrient rich and fertile.

Irrigation in agriculture

• Repeated irrigation can reduce crop yields by causing salt buildup (salinization) in the soil and waterlogging of crop plants.

• Traditional Agriculture - Row crops, deep plowing.• Poly-varietal cultivation - planting several genetic

varieties.• Intercropping - two or more different crops grown

at the same time in a plot.• Agroforestry - crops and trees are grown together.• Polyculture - different plants are planted together.• Hydroponics – plants are grown directly in

fertilized water. (ex: cranberries)

Farming techniques

• Food Production & Population: The number of people the world can support depends mostly on their per capita consumption of grain and meat and how many children couples have.– Research has shown that those living very low

on the food chain or very high on the food chain do not live as long as those that live somewhere in between.

Food Production

• Industrialized agriculture uses about 17% of all commercial energy in the U.S. and food travels an average 2,400 kilometers from farm to plate.

Figure 13-7Figure 13-7

Food Production

Food Production

• People in urban areas could save money by growing more of their food.– Urban gardens provide

about 15% of the world’s food supply.

• Much of the world’s food is wasted.

Figure 13-26Figure 13-26

• About half of the world’s meat is produced by livestock grazing on grass.

• The other half is produced under factory-like conditions called feedlots (see next slide)– Densely packed livestock are fed grain or fish

meal.

• Eating more chicken and farm-raised fish and less beef and pork reduces harmful environmental impacts of meat production.

Food Production

Fig. 13-21, p. 289

Trade-Offs

Animal Feedlots

Advantages Disadvantages

Increased meat production

Need large inputs of grain, fish meal, water, and fossil fuelsHigher profits

Concentrate animal wastes that can pollute water

Less land use

Reduced overgrazing

Reduced soil erosion

Antibiotics can increase genetic resistance to microbes in humans

Help protect biodiversity

• Efficiency of converting grain into animal protein.

Figure 13-22Figure 13-22

Food Production

Fig. 13-18, p. 285

Biodiversity Loss Soil Water Air Pollution Human Health Loss and degradation of grasslands, forests, and wetlands

Erosion Water waste Greenhouse gas emissions from fossil fuel use

Nitrates in drinking water

Loss of fertility Aquifer depletion

Pesticide residues in drinking water, food, and air

Salinization Increased runoff and flooding from cleared land

Other air pollutants from fossil fuel use

Fish kills from pesticide runoff

Waterlogging

Sediment pollution from erosion Greenhouse gas

emissions of nitrous oxide from use of inorganic fertilizers

Contamination of drinking and swimming water with disease organisms from livestock wastes

Desertification

Killing wild predators to protect livestock

Fish kills from pesticide runoff

Surface and groundwater pollution from pesticides and fertilizers Belching of the

greenhouse gas methane by cattle

Loss of genetic diversity of wild crop strains replaced by monoculture strains

Bacterial contamination of meat

Overfertilization of lakes and rivers from runoff of fertilizers, livestock wastes, and food processing wastes

Pollution from pesticide sprays

• Solutions & sustainability: We can increase global food security by slowing population growth, reducing poverty, and slowing environmental degradation of the world’s soils and croplands.

Food Production

• To increase crop yields, we can mix the genes of similar types of organisms and mix the genes of different organisms.– Artificial selection has been used for centuries to

develop genetically improved varieties of crops.– Genetic engineering develops improved strains at an

exponential pace compared to artificial selection.

• Controversy has arisen over the use of genetically modified food (GMF).

The “GENE” Revolution

Mixing Genes

• Genetic engineering involves splicing a gene from one species and transplanting the DNA into another species.

Figure 13-19Figure 13-19

• Raising large numbers of fish and shellfish in ponds and cages (feed lots) is the world’s fastest growing type of food production.

• Fish farming involves cultivating fish in a controlled environment and harvesting them in captivity.

• Fish ranching involves holding anadromous species that live much of their life in saltwater and then return to spawn in freshwater (this is when they are harvested).

Aquaculture

Fig. 13-24, p. 292

Trade-Offs

Aquaculture

Advantages Disadvantages

High efficiency Needs large inputs of land, feed, and water

High yield in small volume of water

Large waste output

Destroys mangrove forests and estuaries

Can reduce overharvesting of conventional fisheries Uses grain to feed

some species

Low fuel use Dense populations vulnerable to disease

Tanks too contaminated to use after about 5 years

High profits

Profits not tied to price of oil