Soil

Soil  Formed by 1- weathering of rocks, 2- deposition of sediment, and 3- decomposition of organic material  Soil Composition  Minerals (45%) - Weathered rock  Organic Material (5%) - Leaves, animal dung, dead stuff  Water (25%)  Air (25%)

SOIL LAYERS Leaf litter – leaves, animal waste, insects & decomposers Topsoil – humus (decomposed organic matter); inorganic soil; root systems; insects & decomposers Subsoil – inorganic matter; mixture of sand, silt, clay, and gravel Weathered parent rock

Nutrient Cycling  Nutrients are cycled between plants, animals and soil  Biogeochemical Cycles  Carbon  Nitrogen  Hydrologic  Phosphorus

Soil Properties  Soil Acidity  4-8 pH = most soils  pH affects solubility of plant nutrients  6-7 pH = Optimum soil (nutrients are maximally available)

Soil Properties  Soil Texture  Sand: 2mm–0.05 mm  Silt: 0.05mm–0.002 mm  Clay: <0.002 mm  Loam: a roughly equal concentration of sand, silt and clay

Water High permeabilityLow permeability Infiltration/Percolation – downward movement of water through soil Leaching – as water seeps down, it dissolves soil nutrients in upper layers & carries it to lower layers Soil texture determines porosity & permeability

Soil Properties

Types of Soil  Clay – Very fine particles  Low permeability to water, prone to waterlogging  Compacts easily  Nutrient-rich due to negatively charged surface (able to hold onto important plant nutrients: K +, Ca 2+, NO 2- )

Types of Soil  Silt  Particles intermediate in size between sand and clay  Erodes easily  Often found on riverbanks  Has a silky feel (like flour)

Types of Soil  Sand  coarser than silt  Water flows through too fast for most crops

Types of Soil  Loams  A mixture of clay, sand, silt and humus  Best soil for crops From left to right Sandy Loam Clay

Soil Triangle

Soil  Rate of Soil Formation - Factors:  Parent Material  Time  Climate  Organisms  Topography

Tropical Soils  Infertile soils  Topsoil often thin (nutrients in overlying plant life)  Prone to chemical weathering (humus + water = acidic solution)

Desert/Arctic Soils  Thin  Made up of mostly rock fragments (evidence of mechanical weathering)

Soil Problems: Erosion  Soil Erosion  Causes: water runoff, wind, steep slope  Why a problem?  Loss of soil fertility as organic material & nutrients are eroded  More fertilizers must be used to replace nutrients

Case Study: The American Dust Bowl 1930s  Great Plains region subject to drought  Natural grassland vegetation had been removed  Replaced by shallow- rooted annual crops  Winds blew soil as far east as NYC  Inspired Soil Conservation Act

Soil Problems: Nutrient Mineral Depletion

 Often in arid /semi- arid areas  Elevated salt concentrations toxic to plants Soil Problems: Salinization  Soil Salinization  Gradual accumulation of salt in the soil, usually due to improper irrigation techniques

Salinization Solutions PreventionCleanup Reduce irrigation Switch to salt- tolerant crops (such as barley, cotton, sugar beet) Flushing soil (expensive, water intensive) Not growing crops for 2-5 years Install under- ground drainage systems (expensive)

Soil Problems: Desertification The degradation of once-fertile rangeland, agricultural land, or tropical forest into nonproductive desert

Desertification Consequences Causes Worsening drought Famine Economic losses Lower living standards Environmental refugees Overgrazing Deforestation Surface mining Erosion Salinization Soil compaction

 Crop Rotation  Plant a series of different crops in the same field over a period of years  Corn legumes (ex: soybean) Soil Conservation  Conservation Tillage / “no-till” - Minimize disturbance of soil

Strip Cropping Terracing Soil Conservation  Contour Plowing  Plowing around hill instead of up-down  Strip Cropping  Alternate strips of different crops along natural contours  Terracing  Creating terraces on steep slopes to prevent erosion  Windbreaks

Preserving Soil Fertility  Organic fertilizers  Animal manure, crop residue, bone meal, compost  Slow release of nutrients available as material decomposes  Inorganic fertilizers  Manufactured from chemical compounds (high fossil fuel use)  Soluble  Fast acting, short lasting  Mobile: easily leach into groundwater supplies

Soil Conservation Policies in US  Soil Conservation Act 1935  Authorized formation of Soil Conservation Service, now called Natural Resource Conservation Service (NRCS)  Assess soil damage and develop policies to improve soil  Food Security Act (Farm Bill) 1985  Requires farmers with highly erodible soil to change farming practices  Instituted Conservation Reserve Program  Pays farmers to stop farming highly erodible land