Sub-Concept: Soil Formation
Agricultural Context: Soil formation factors
Exercise: Locate Major Types of Parent Materials
Goals:
Materials:
References: Beyrouty, C.A., Hanson, M.G., Slaton, N. (1994). Manual for Soils (AGRN 2201). University of Arkansas, Fayetteville, AR.
Soil Forming Factors
1. Parent Material Parent material and topography are the initial factors
2. Topography present in the soil formation process. Climate and
3. Climate vegetation act upon the conditions created, and time
4. Vegetation is required to change the parent materials into a soil
5. Time profile. Variations between soils in the field are
directly related to variations in the factors of soil
formation.
Parent Material
- Original mineral or organic matter from which the A and B horizons are formed.
- Starting point in soil formation
- Determines range in texture a soil will have
- Initial chemical and mineralogical properties will affect the fertility of the soil formed
- Parent materials fall into two categories: transported and residual.
Transported parent materials: brought to an area from another location
1. Glacial - occurs mostly in north central and northeastern US
a. Glacial till - mixture of rock debris deposited by glaciers
b. Glacial outwash - sorted sediments deposited by glacial streams
2. Lacustrine - sediments initially deposited in fresh lake water; may or may not be glacially derived
3. Eolian - wind deposited
a. Dune sand
b. Loess (pronounced "luss") - silt sized material deposited by wind
4. Alluvial - deposited by streams
5. Marine - deposited under ocean and exposed with falling sea
6. Colluvial - deposited by gravitational forces such as mud slides
Residual parent materials: accumulated by weathering of rock in place
- 3 groups of rocks:
1. Igneous - cooled molten rock; example is granite
2. Sedimentary - sediments deposited in water and consolidated; examples are sandstones, shales, and limestones
3. Metamorphic - an igneous or sedimentary rock altered through the action of heat and
pressure; examples include quartzite and slate
Climate
- Moisture and temperature significantly impact the types of soil which will form in a particular region
- Climate determines vegetative distribution
- Figure below illustrates relationship between climate and types of vegetation
- Note that precipitation generally increases from west to east across the US, while temperature increases from north to south
- The result is that more highly weathered soils are found in the Southeast US where climate is
wet and hot
Vegetation
- Vegetation varies in size and activity of root systems, rate of organic matter production, nutrient requirements, and moisture demands
- These variables contribute to differences in types of soil formed
- Forest soils have shallower topsoils and less humus than prairie soils because tree vegetation contributes less organic matter to a shallower depth than grass vegetation, where most of the root systems die and regenerate annually
- Grasses recycle more nutrients than trees because of their dense matting fibrous root systems
- More basic nutrients (Ca, Mg, K, Na) are taken up by grass leaves then redeposited on the soil
surface when the grass dies and leaf tissue decomposes
Topography
- Topographic variations are largely responsible for the variety of soils found within a few miles in many areas
- Differences in slope determine to a large extent the amount of runoff water as well as the degree of erosion and sedimentation that occurs
- Topography also affects the location of the water table, soil drainage conditions and accumulation of organic matter
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