3.4 The Soil System

The Soil System Soil is a complex mixture of eroded rock, mineral nutrients, decaying organic matter, water, air and billions of living organisms (microscopic decomposers).

Soil formation is a slow process: Weathering of rock (mechanical). Deposition of sediments by erosion (mechanical). Decomposition of organic matter in dead organisms (chemical).

soil system integrates the biotic and abiotic

Mature soils are arranged in a series of zones called SOIL HORIZONS: “O” HORIZON = freshly fallen and partially decomposed leaves, twigs, animal waste. You can find fungi and other organic materials. “A” HORIZON = porous mixture of partially decomposed organic matter (humus) and some inorganic mineral particles.

These top two layers are most fertile, have the highest concentration of organic matter, and contain large amounts of living organisms. Flatworm Rove beetle Ant Centipede Mite Pseudoscorpion Ground beetle Adult fly Millipede Fly larvae Sowbug Earthworm Slug Snail Roundworms Protozoa Bacteria Organic debris Beetle Fungi Springtail Actinomycetes

“B” (subsoil) and “C” (parent material) HORIZON contain most of the soil’s inorganic matter, broken-down rock.

Soil Content Clay (very fine particles) Silt (fine particles) Sand (medium-size particles) Gravel (coarse to very coarse particles) SOIL TEXTURE is determined by the relative amounts of the different types and sizes of mineral particles.

Increasing percentage sand 100%clay Increasing percentage silt percentage clay 20 40 60 80 100%sand 100%silt Increasing percentage sand sandy clay silty silty clay loam silt sandy clay loamy sand

Soil texture helps determine SOIL POROSITY, a measure of the volume of spores or spaces per volume of soil and the average space between those spaces. INFILTRATION is the downward movement of water through soils. As the water seeps down, it dissolves various soil components in upper layers and carries them down to lower layers in a process called LEACHING.

SOIL PERMEABILITY is the rate at which water and air move from upper to lower soil layers. High permeability Low permeability

Water-Holding Capacity Properties of Soils with Different Textures Texture Nutrient Capacity Infiltration Water-Holding Capacity Aeration Workability Clay Good Poor Silt Medium Sand Loam medium

Absorption of nutrients Nitrogen fixing by lightning Commercial inorganic fertilizer 10-6-4 N-P-K Organic fertilizers, animal manure, green manure, compost Crop plant Dead organic matter Application to land by bacteria Weathering of rock Nutrient removal with harvest Decomposition Supply of available plant nutrients in soil Nutrient loss by bacterial processes such as conversion of nitrates to nitrogen gas from soil erosion Absorption of nutrients by roots Pathway of plant nutrients in soil.

Soil erosion is the movement of soil components, especially surface litter and topsoil. The two main agents of erosion are wind and flowing water. Loss of plant cover by farming, logging, construction, overgrazing by livestock, off-road vehicles, deliberate burning of vegetation and other activities leave soil vulnerable to erosion.

Two major harmful effects of soil erosion: Loss of soil fertility and its ability to hold water Runoff of sediment that pollutes water, kills fish and shellfish, and clog irrigation ditches, boat channels, reservoirs, and lakes. serious concern some concern Stable areas

Desertification is the enlargement of deserts through human activities. Consequences Causes Worsening drought Famine Economic losses Lower living standards Environmental refugees Overgrazing Deforestation Surface mining Erosion Salinization Soil compaction

Irrigation water contains small amounts of dissolved salts. SALINATION Irrigation water contains small amounts of dissolved salts. Evaporation and transpiration leave salts behind. Salt builds up in soil. WATERLOGGING Precipitation and irrigation water percolate downward. Water table rises. Both result in stunted plant growth, lower crop yields, dead plants and ruined land.

Evaporation Transpiration Waterlogging Less permeable clay layer

Soil Conservation involves reducing soil erosion and restoring soil fertility.

Advantages and disadvantages of using Conservation Tillage. Reduces erosion Saves fuel Cuts costs Holds more soil water Reduces soil compaction Allows several crops per season Does not reduce crop yields Can increase herbicide use for some crops Leaves stalks that can harbor crop pests and fungal diseases and increase pesticide use Requires investment in expensive equipment

Contour planting and strip cropping: each row acts as a small dam to help hold soil and slow water runoff.

Alley cropping or agroforestry: several crops are planted together in strips or alleys between trees and shrubs that can provide fruit or fuel-wood, shade, help retain and slowly release soil moisture, and fodder for livestock.

Windbreaks or shelterbelts of trees reduce wind erosion, help retain soil, supply wood for fuel, and provide habitats for birds, pest-eating and pollinating insects, and other animals.

Terracing retains water for crops at each level and reduces soil erosion by controlling runoff.

Soil Restoration Organic fertilizer Manure Compost crop rotation No till farming Contour farming Terracing Nitrogen fixation-legumes