Soils & Soil Horizons APES – Ch. 8

Weathering of Minerals

Soils  Links the rock cycle to the biosphere (plants & animals).  Defined as a mix of geologic and organic components.  Form on the Earth’s surface.  Medium for plant growth  Filter of water  Provides habitat for decomposers & recyclers of organic matter

Formation of Soil  Soil is the result of physical & chemical weathering of rocks and gradual accumulation of detritus from biosphere.  Takes thousands of years to form.  Processes that form soil work in two directions:  Weathering of bedrock provides raw material for soil from below (clays & quartz)  Deposition of organic matter from organisms and their waste contributes to soil formation from above

Young vs. Mature Soil Fewer nutrients

Properties of Soil  Five factors that determine the properties of soil are:  Parent Material – rock underlying the soil (inorganic).  Quartz-rich soils lack nutrients whereas volcanic soils are high in nutrients  Climate – Soils do not form well in below freezing conditions. Impedes decomposition & water movement  Soils in polar regions are composed mostly under-decomposed organic material whereas soils in humid/tropical areas develop rapidly

Properties of Soils  Topography – surface slope & arrangement of landscape  Soils that form on steep slopes are constantly subjected to erosion whereas soils that form at the base of steep slopes continually accumulate material.  Organisms – Plants remove nutrients from the soil & excrete acids. Animals that live in the soil mix the soils uniformly (bioturbation).  Time – older soils tend to have more nutrients than younger soils. Really old soils can be nutrient poor.

Soil Horizons & Properties  Some soils are good for growing crops while other soils are good for building structures.  It is important to classify soils by soil horizons & properties  Soil horizons are layers that develop in the soil  Specific composition of horizons depends largely on climate, vegetation, & parent material.

Soil Horizons A Horizon: Called the topsoil. Zone of organic materials and minerals that have been mixed. O Horizon: Organic detritus such as leaves, needles, twigs, animal bodies in various stages of decomposition. Most pronounced in forest biomes. Horizon where plants grow B Horizon: Known as the subsoil. Composed mostly of minerals with very little organic material. Nutrients are present in the B horizon Transported by leaching C Horizon: Least weathered horizon. Always beneath the B horizon. Similar to the parent material (bedrock).

E Horizon: Found in acidic soils Zone of leaching or eluviation. Forms between A and B horizon Fe, Al, & dissolved organic acids from above are transported through & removed from the E horizon. Removed materials accumulate in the B horizon

Physical Properties of Soil  Texture is determined by the % of sand, silt, and clay.  Sand, silt and clay are mineral particles of different sizes  Sand - largest size (consistency of sugar)  Silt – in between sand & clay (consistency of corn meal)  Clay – smallest size (consistency of baby powder)  Mixture of silt & clay is called MUD (according to geologist)

Soil Sample contained: 40% sand 40% silt 20% clay Where would it plot on this diagram?

 Porosity of a soil:  How quickly soil drains depends on texture.  Ex: Water can move easily through sand, whereas water cannot move easily clay. Physical Properties of Soil Roots can also spread quickly through sand (aquifer). Clay has less pore space and roots cannot easily move through it (aquitard).

Permeability  Soils can be porous but not permeable. This means the pore-spaces are not connected to each other.  Permeability describes how connected the pore-spaces are.

 Best soil for agriculture is a mix of all three (loam).  Balanced water drainage  Best for water retention  Soils can also influence how environment acts with pollution  Sandy soils are porous and therefore pollutants can quickly spread into the groundwater  Landfills are lined with clay to prevent pollutants from spreading. Physical Properties of Soil

Chemical Properties of Soil  Clay particles contribute most to chemical properties. Why?  There attractive! Negatively charged so they attract cations. Cations can be released as nutrients.  Cation Exchange Capacity (CEC):  Function of the types of clay particles present  Soils with high CEC are good for agriculture  Over 20% clay can cause plant roots to rot & lack of oxygen  Base Saturation  Na, Mg, Ca, K = soil bases; neutralize acids (H, Al)  Measure of the proportion of acids & bases

Three groups of organisms account for 80-90% of soil organisms: Fungi Bacteria protozoans

Soil Degradation & Erosion  The loss of some or all of a soils ability to sustain plant growth  Caused by overuse by agriculture, forestry, or other human activity  Ex: Dust Bowl in OK  Major cause is soil erosion – topsoil removed by water.