Weber State University      Department of Botany

                                                    BOTANY  LS1203 - PLANT  BIOLOGY

Soil  

For plant growth, soil provides
        air (O₂)
        water
        essential elements as mineral nutrients
        beneficial microorganisms

Three soil particle sizes from breakdown of parent rock: sand, silt, clay
Ways to break down rock:
        freeze/thaw cycle
        water and wind erosion
        lichens
        root growth

Some of the particles in soil will slowly dissolve to provide mineral nutrients

Sizes of the soil particles provide air spaces, water holding
Largest spaces from sand: highest aeration and draining; lowest water holding
Smallest spaces from clay: highest water holding; lowest aeration and drainage
A space in soil at any one time will have air OR water. By having a mixture of space sizes, you can have a mixture of air-filled spaces and water-filled spaces in a certain soil volume.

sand 0.05-2 mm
silt 0.002-0.05 mm
clay <0.002 mm
loam == roughly 40% sand, 40% silt, and 20% clay

Organic matter (humus [decaying organic matter], compost, manure)
improves the functions of any soil:  water holding, aeration,  nutrient holding
a slow release of nutrients thanks to decomposers

Soil organisms
        bacteria and fungi who provide nutrient cycling, decomposition of organic matter
        mutualistic and commensal symbiotic relationships
        nitrogen fixers
        earthworms (great little aerators) and a big deal for literally eating dirt
                ingest soil (humus and small inorganic particles), excrete casts. This process involves mixing of organic materials and soil minerals. Consequences of worm activity in soils:
                        improved aeration
                        better water drainage
                        stabilizing of soil aggregates
                        enhanced nutrient availability
        other misc. burrowing animals
        plants themselves: root growth followed by decomposition opens air spaces; rhizosphere provides a microhabitat for critical soil organisms  

Review

What factors does soil provide for plant growth?
How is rock weathered to form soil particles?
What are the three soil particles that result from the weathering of rock? Which is the largest? The smallest? How does each particle affect the aeration and water holding capacity of a soil? What is the ideal mixtures of the three particles called? What is the approximate ratio of the three particles in this mixture?

How does the presence of organic matter affect a soil? What are some sources of organic matter for soils?

What are some examples of soil organisms? How do your examples contribute to the properties of a soil?

Suggested Online Reading

Soil (Ross Koning at Eastern Connecticut State University) 

How Earthworms Can Help Your Soil   (NSW Agriculture - Australia)

Earthworms: under-valued, under-rated and unloved  (McGill University)

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5 April 2007.  Links checked 20 March 2011.