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Lecture 7

EESA01H3 Lecture Notes - Lecture 7: Soil Texture, Hardpan, Soil Ph


Department
Environmental Science
Course Code
EESA01H3
Professor
Carl Mitchell
Lecture
7

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Soil colour: Indicates chemical composition, organic content (E.g. high Fered) Colour variation occurs
in any location.
Soil texture: Determined by the size of particles and is the basis on which soils are assigned to one of
three general categories. Soil with even mixture of clay, silt & sand particle sizes is loam. Texture also
greatly influences the soil’s porosity volume of spaces between particles in the soil, and permeability.
Soil Structure: lumpiness of soil. Needed to encourage soil productivity, and organic matter, activity,
and clay help promote the structure. However, large clumps discourage plant roots from establishing if
particles tightly compacted. Repeated tilling compacts less H20 absorption. When farmers repeatedly
till the same field, same depth, they end up forming a crusty hardpan layer that resists H20infiltration and
the root penetration.
Soil pH: Plants die in soils that are too acidic or alkaline, moderate variation can influence the availability
of nutrients for plant roots. During leaching, acids from organic matter remove nutrients from exchange
sites between plant roots and soil particles. Water leaches these nutrients & carries them to deeper
levels less for the plants
Ion exchange: Gain nutrients by exchanging cations & anions between soil/soil solution. Negatively
charged sites prevail in clay/humus. Nutrients are resupplied to exchange sites by mineral weathering and
the decomposition.
Cation exchange capacity: A soil’s ability to hold cations from leaching away available to plants.
Indicates fertility for plant growth. Soils rich in fine texture (clay) & organic matter have the best CEC.
Low pH diminishes CEC, nutrients leach, and soil supplies them with harmful aluminum ions acid
precipitation can harm soils and plant communities.
Many pollutants are + charged Cd, PB, As, and Hg b/c of the attraction to /-/ clay and humus in soil.
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