Introduction
to Soils
Functions of Soils
Medium for plant growth
Regulate water supply
Nutrient cycling
Habitat for soil organisms
Engineering medium
For plant growth soils provide mechanical support, water, nutrients and root aeration. Soils also buffer temperature in the root zone.
As regulator of water quantity and quality, soils limit surface runoff, recharge ground and surface water supplies and filter and purify water.
Via recycling of nutrients soils prevent irreversible sequestering of C, N, P and other nutrients, thereby ensuring a continued supply of these nutrients to plants.
As a habitat for diverse organisms soil is an integral component of terrestrial ecosystem, indispensable to plants and water quality.
Soils also affect engineering design due to variable bearing strength, compressibility, shear strength and stability.
Soils as Natural Bodies
Distinct from geologic deposits of unconsolidated material, soils exist due to action of pedogenic processes over time. Soils exhibit later lateral variability across the landscape and show internal (vertical) variability. Soils overlie consolidated rock and may have formed from rock weathered in place or formed in unconsolidated material transported from elsewhere.
General soils map of East
Baton Rouge Parish
showing small scale
variability in soil associations
(groups of soils that
typically occur together).
Detailed soils map of LSU
campus showing large
scale variability in mapping
units. Published 1964.
The unconsolidated material above rock is called the regolith. Soils form at the surface of the regolith. As soil development proceeds, horizons evolve. The solum is typically limited to upper 1 to 2 m.
Profile of a
horizons. See brief general
descriptions below.
Soil Profile
The sequence of horizons that make up a soil profile is clearly revealed in a soil pit or road cut. This sequence may include some or all of the master soil horizons listed below.
O Organic matter above the surface of a mineral soil.
A Zone of
accumulation of organic matter at and below the mineral soil surface.
It is darker
than horizon(s) below it.
E Zone of
maximum leaching. It contains
little organic matter and is lighter than
horizons below it.
B Zone of maximum
accumulation of clay or salts translocated from above. It is
typically
brighter color than the other horizons.
C Shows little or no
development.
General Composition of Soil
Soil is composed of mineral and organic particles and pore space. The latter is filled with air and water.
Mineral Particles
These vary in size and mineralogy. Typically, individual particles are aggregated together.
Size classes divided among coarse fragments (> 2 mm) and fine earth (< 2 mm). The latter are divided among sand, silt and clay separates.
Sand 2.00 to
0.050 mm
Silt 0.05 to
0.002
Clay
< 0.002
The relative proportion of these various sized particles is called soil texture. The smaller clay (and soil organic matter) is colloidal. Colloids remain in suspension and exhibit a high surface area per unit mass. Soil colloids carry electrostatic charges and adsorb ions and water.
Primary and secondary minerals occur in soil. Sand and silt are dominated by primary minerals (precipitated as molten magma cooled) such as quartz, micas and feldspars. The clay size fraction is dominated by secondary minerals formed by weathering of less resistant primary minerals. Secondary clay minerals include silicate clays and oxides.
Individual particles may be aggregated to produce larger units of soil structure. The relatively large pores between units of structure increase air and water movement in soil.
Organic Matter
Mineral soil typically contains 1 to 6 % organic matter. It consists of living biomass, partially decomposed plant and animal residue and products of microbial biosynthesis.
The dark colored colloidal fraction of soil organic matter is called humus. Soil organic matter is important because it increases the water holding capacity of soil and provides nutrients to plants and other soil organisms.
Soil Water
Soil water contains various inorganic and organic solutes. Accordingly, it is referred to as the soil solution.
Soil Air
Air-filled pore space and water-filled pore
space vary inversely. The gasesous
composition of soil air depends on the metabolic activity of soil organisms. Compared to the above ground atmosphere,
soil air typically is higher in water vapor (relative humidity near 100%),
higher in carbon dioxide and lower in oxygen. Aeration is affected not only by water
content but also by soil pore size distribution. If most pores are small
(micropores), aeration is poor.
Nutrient Content in Soil and Uptake by Plant Roots
Sources of Nutrients
Structural component of minerals and soil
organic matter
Adsorbed on mineral and organic colloids
Dissolved in the soil solution
The release of nutrients from mineral and organic combination depends on chemical weathering and microbial decomposition. Release from colloidal surfaces occurs by a physicochemical process known as ion exchange. Weathering is very slow, decomposition slow and ion exchange fast. Nutrients in the soil solution are directly available for plant uptake.
Processes that Bring Roots in Contact with Nutrients
Root interception root growth into an unexploited zone.
Mass flow nutrients move with soil water toward a root that is
absorbing water.
Diffusion from
a zone of high to low concentration.
Soil environmental conditions such as low water content or soil compaction adversely affect these mechanisms Poor aeration or temperature extremes may also affect the active uptake of nutrients.