Unit 4.3: Soil and Agriculture

Rock Cycle

Weathering The breaking down and changing of rocks at or near the Earth’s surface 2 Types: Mechanical Chemical

Mechanical Weathering Occurs when physical forces break rock into smaller and smaller pieces without changing the rock’s mineral composition Mechanical weathering increases surface area

Three processes that cause mechanical weathering Frost Wedging caused by the expansion of freezing water in cracks and crevices. wedged loose rocks may tumble into large piles called talus, found at the bases of steep, rocky cliffs

Three processes that cause mechanical weathering 2. Unloading/Exfoliation Reduced pressure on igneous rock causes it to expand and allows slabs of outer rock to break off in layers in a process called exfoliation

Three processes that cause mechanical weathering 3. Biological Activity The activity of organisms, including plants, burrowing animals, and humans

Chemical Weathering Is the transformation of rock into one or more new compounds

Processes of chemical weathering Water (dissolved gases) oxygen (Fe + O2 = rust!) CO2 (carbonic acid) sulfur/nitrogen oxides (acid rain)

Erosion Erosion – major external process: loosened material is dissolved, or worn away from one part of the Earth's surface and deposited in another Streams – the most important agent of erosion produce ordinary valleys and canyons; may form deltas when they flow into lakes and oceans Wind Erosion – produces dunes or loess Human activities can accelerate erosion: activities that destroy vegetation

Soil

Soil Regolith – layer of rock and mineral fragments produced by weathering Soil – part of regolith that supports plant growth

Soil Composition Mineral matter, humus (decayed, organic material), air, water

Soil Texture Texture refers to % of particle sizes Sand (large size) Silt Clay (small size) Loam (a mixture of all three sizes) is best suited for plant life

Soil Texture

Soil Forming Factors “what makes soils look different” Parent material Topography Time processing Soil Properties Biology Climate

Soil Formation: Topography steep – more erosion = thin/nonexistent soil flat – little erosion = thick soils

Soil Formation: Time Time (years) millions 1 2 3 4 millions In this diagram you can see how the soil forming factors have affected the weathering of bedrock to form a soil. Time- It takes millions of years t forma soil. Parent material ( bedrock) is broken down by rain and ice.. Vegetation starts to grow on the soils. The roots break up the soil. Biological life arrives later and acts on the soil further braking up organic matter and soil particles. 1 2 3 4

Soil Formation: Organisms plants – main source of organic matter, acids from decay increase weathering animals – burrow and mix materials (earthworms)

Soil Formation: Climate Greatest affect on soil formation hot/wet – thick/chemically weathered soil cold/dry – thin/mechanically weathered soil

Soil Profile O horizon - organic matter A Horizon – (topsoil) accumulation and decomposition A Horizon – (topsoil) mixture of mineral and organic matter, insects, fungi, microorganisms  B Horizon – (subsoil) fine clay particles leached out of A horizon  C Horizon - partially weathered parent rock

Soil scientists categorize soils in to 12 soil orders Soil scientists categorize soils in to 12 soil orders. There are over 50,000 soils in the US.

Soil Types Pedalfer – (temperate) – eastern U.S.- rich in iron oxide and aluminum clays (forest floor) Pedocal – (dry) – southwest U.S. – less clay, rich in calcium carbonate (grasslands) Laterite - Hot, wet, tropical climates, intense chemical weathering

Soil Erosion/Fertility/Food Production

Soil Erosion Losing topsoil makes a soil less fertile and less able to hold water The sediment clogs ditches, boat channels, reservoirs, and lakes Soil can be renewed: in tropical and temperate areas: 200 – 1000 years depending on climate for 1 inch of new topsoil to form

Soil Erosion Global Soil Erosion World is losing 7-21% of its topsoil from actual or potential cropland each decade. Soil Erosion in the U.S. About 1/3 of nation's original topsoil has been washed or blown into streams, lakes and oceans – mostly as the result of overcultivation, overgrazing and deforestation. Soil on cultivated land is eroding 16 times faster than it can form.

How Do Excess Salts and Water Degrade Soils? Salinization – thin layer of dissolved salts in the topsoil as a result of irrigation stunts crop growth , lowers yields Cures take land out of production for 2-5 years install underground network of perforated drainage pipe flush soil with large amount of low-salt water Waterlogging – water used to flush topsoil accumulates underground, raising the water table. Saline water lowers root productivity, eventually killing them

Reducing Soil Erosion Terracing – reduces erosion on steep slopes (converted into a series of broad nearly-level terraces that run across the land contour

Reducing Soil Erosion Contour Farming – on gently sloping land plowing and planting crops in rows across rather than up and down the sloped contour of the land.

Reducing Soil Erosion Strip Cropping A row crop (corn) alternates with another crop (a grass or grass-legume mixture). catch and reduce water runoff help prevent the spread of pests and plant diseases. Soybeans and alfalfa help restore soil fertility.

Reducing Soil Erosion Alley Cropping (agroforestry) - several crops are planted together in strips or alleys between trees or shrubs that provide fruit or fuelwood The trees provide shade (helps to retain moisture). Trimmings from the trees and shrubs provide mulch (green manure) for the crops

Reducing Soil Erosion Windbreaks/Shelterbelts reduce wind erosion long rows of trees

Soil Fertility Fertilizers – partially restore plant nutrients lost by erosion, crop harvesting and leaching. organic fertilizer – from plant and animal materials (animal manure, green manure, compost) commercial inorganic fertilizer -produced from various minerals (nitrogen, potash, phosphorus)

Soil Fertility Will Inorganic Fertilizers Save the Soil? Advantages: Easy to transport, store, apply Disadvantages : Do not add humus to the soil - will become compacted. Decreased soil porosity leads to reduced oxygen content and fertilizer from being taken up efficiently Usually supply only two or three of the needed 20 or so nutrients Cause water pollution (eutrophication)

Soil Fertility Crop Rotation Plant corn, tobacco or cotton one year following year plant legumes to add nitrogen to the soil (soybeans, oats, rye, barley, sorghum) Also helps reduce crop losses to insects by presenting them with a changing target