1. APES
Chapter 8

2. 8.1 Plants and Soil
Soil production involves dynamic interaction among mineral particles, detritus, and members of the detritus food web.

3. Most soils are hundred of years old and change very slow.
The process of soil formation creates a vertical gradient of layers.
These horizontal layers are call horizons.
Soil Profiles:
O horizon  dead organic matter (detritus) deposited by plants – leaves, stems, fruits, seeds, etc.  This horizon is high in organic content and is the primary energy source for the soil ecosystem.
Humus  bottom of O horizon. Dark well decomposed material.
A horizon  Mixture of mineral soil from below and humus from above and is familiarly termed topsoil.
E horizon  eluviation which refers to the process of leaching of many minerals due to the downward movement of water.
B horizon  characterized by the deposition of minerals that have leached from the A and E horizons. Frequently referred to as subsoil.
High in iron, aluminum, calcium, and other minerals.
C horizon  the parent mineral material originally occupying the site, representing weathered rock, glacial deposits, or volcanic rock.

4. Soil Texture
As rock weathers it breaks down into smaller and smaller fragments
Sand: 2.0 – 0.02 mm
Silt: 0.02 – mm
Clay: anything finer than 0.002mm
Loam: 40% sand, 40% silt, 20% clay.
Soil texture refers to the relative proportions of each of the above in soil.
Soil texture affects workability (the ease with which a soil can be cultivated).
Large particles have larger spaces separating them than smaller particles (hold less water)
Smaller particles have more surface area relative to their volume than larger particles
Nutrient ions and water molecules tend to cling to surfaces

5. Soil Classes Order  Suborder  Groups  Subgroups  Families  Class
We will focus on Orders (11 major classes total)
4 to know
Mollisols  Very fertile. Found in temperate grassland biomes. Worlds best agricultural soils.
Found in the Midwestern U.S., across temperate Ukraine, Russia, and Mongolia.
Oxisols  Soils of the tropical and subtropical rain forests. Have a layer of iron and aluminum oxides in the B horizon and have little O horizon, due to rapid decomposition of plant matter.
Limited fertility for agriculture
Alfisols  widespread, moderately weathered forest soils. Not deep, but they have well developed O, A, E, and B horizons.
Typically moist, temperate forest biome. Suitable for agriculture if supplemented with organic matter or mineral fertilizers.
Aridsols  very widespread soils of drylands and deserts. Unstructured vertically. May support some enough vegetation for rangeland animal husbandry.
Irrigation of these soils leads to salinization, as high evaporation rates draw salts to surface horizons, where they accumulate to toxic levels.

6. Soil and Plants
Plants need a root environment that supplies optimal amounts of mineral nutrients, water, and air. The pH and salinity of the soil are also critically important.
Soil fertility  the soil’s ability to support plant growth; this includes the soil’s ability to meet the plants other needs as well. Farmers refer to a soils ability to support plant growth as tilth.
Mineral Nutrients and Nutrient holding Capacity
Weathering  physical and chemical break down of rock.
Leaching  nutrients washed from soils as water moves through.
Soils ability to hold nutrients until they are absorbed by roots is the nutrient holding capacity.
Organic fertilizer, inorganic fertilizer

7. Water Holding Capacity
Water is resupplied to the soil naturally by rainfall or artificially by irrigation.
Three attributes of the soil are significant:
Soils ability to allow water to infiltrate or soak in.
Soils ability to hold water after it infiltrates (water holding capacity)
Evaporative water loss from the soil surface
Aeration
Relative Acidity
Most plants require a pH near neutral and most environments naturally provide this.
Salt and Water uptake

9. Soil Degradation
Erosion  How topsoil is lost. In this process soil and humus particles are picked up and carried away by water or wind. This occurs anytime soil is bare or exposed to the elements.
Splash erosion  impact of falling rain drops breaks up clumpy structure of topsoil
Sheet erosion  Decreased infiltration results in more water runoff and carrying away of the fine particles of the surface.
Gully erosion  further runoff leads to convergence into rivulets and streams which have greater volume, velocity, and energy  greater capacity to remove soil.
Desertification  regions with sparse rainfall or long, dry seasons support grass, scrub trees, or crops only insofar as soils have good water holding and nutrient holding capacity.
As these diminish by erosion of topsoil, these areas become deserts.