SOILS
I. SOIL CONSTITUENTS
The atmosphere below ground in the soil difference substantially from that aboveground. The soil atmosphere is higher in CO2 and lower in O2 [Why?]
Soil water will be covered under Resources.
II. MINERAL (INORGANIC) FRACTION
Inorganic materials make up the bulk of most soils. Important characteristics of soil particles are their chemical composition (mineralogy) and size.
Primary minerals are small pieces of the original parent materials (either rocks or materials deposited by wind or water); secondary minerals have undergone chemical weathering (modification) in the soil.
Soil texture refers to its particle size distribution:
Coarse-textured soils are high in sand and gravel fractions; fine-textured soils are high in silt and clay fractions. Coarse-textured soils have large pore spaces but a smaller total pore volume. Finer textured soils have a higher water-holding capacity (more total pore volume).
Generally, gravels, sands and silts are primary minerals; clays are secondary minerals. The amount and kind of clay particles in a soil determine many of its important qualities for plant growth.
Clays are secondary minerals, produced by chemical weathering of primary minerals:
Silicate clays have a structure with Si+4 in tetrahedral layers and Al+3 in octahedral layers;
Hydroxide clays are composed entirely of Al and Fe hydroxides; all silica has been removed.
Primary minerals typically weather as:
Primary minerals --> 2:1 clays --> 1:1 clays --> Hydroxide clays
Cation-exchange capacity (CEC) refers to the fact that most clays have a net negative charge (caused by substitution of Al+3 or Fe+3 for Si+4, or Mg+2 for Al+3 in the crystal structure). Cations are attracted to clay particles, including K+, Na+, H+, Mg+2, Ca+2, NH4+, and Al+3. The cation exchange capacity is a measure of the quantity of cations (as positive charges) that can be held on the surface of clay particles:
CEC is variable, depending somewhat on the soil's pH. High CEC is associated with fertile soils, because many of these cations are important plant nutrients (K+, Mg+2, Ca+2, NH4+). Soil organic matter (SOM, see below) also is negatively charged and therefore has a CEC.
III. SOIL ORGANIC MATTER
Soil organic matter is composed mostly of stable, decomposed plant and animal remains called humus with C:N ~ 10-14, C:P ~ 100, and C:S ~ 80-100. Humus is very stable due to complex structure, usually composed of many polycyclic compounds (much derived from lignin). Humic acid has a MW of 20000-50000 and is about 60% C. Fresh plant residues are typically 42% C.
Decomposition of plant residues proceeds through several stages:
Residues --> "Light fraction" --> "Sand-sized fraction" --> Humus.
The "light fraction" is also called the "coarse fraction"; the "sand- sized fraction" is also referred to as the "fine fraction" or "heavy fraction." The light or coarse fraction typically accounts for 10-30% of total SOM; the fine fraction and humus for 70-90% of total SOM.
C:N decreases as SOM decomposes; amino N groups are used first; N in humus probably resides in heterocyclic N compounds.
Functions of SOM:
SOM Dynamics [dSOM = change in SOM]
dSOM = inputs (plant litter and residues; manures) - losses (to decomposition)
Decomposition rate = f(quality of inputs, temperature, moisture, tillage).
Measures of input quality include C:N ratio, lignin:N ratio, or polyphenolics:N ratio.
Other soil physical properties influenced by SOM include:
Example of bulk densities from Netherlands inceptisol: grassland soils, 1.4 g cm-3; organic farms, 1.5 g cm-3; and conventional farms, 1.6 g cm-3
IV. SOIL STRUCTURE
Aspects of soil structure include:
V. SOIL TYPES
Soil classification is based on characteristics of the soil profile:
O-organic horizons
A-Horizon properties determined by degree of leaching, primarily of clays
B-Horizon properties determined by deposition of clays, carbonates, hydroxides
C-Horizon, mostly unweathered parent materials
Selected Soil Orders (most important in agriculture)
Alfisols, ultisols, and oxisols form a weathering sequence. All tend to form below forests or savannas with moderate to high amounts of rainfall.
World Distribution of Soil Orders. Look at a soil map (provided in class) and be able to answer the following questions:
VI. SOIL ORGANISMS
Soil organisms can be classified by taxonomic groups, size, and or functional roles (most useful):
Taxonomic classification; most common soil organisms include:
Functional Classification:
1. Mode of nutrition:
2. Aerobic vs. anaerobic
3. Free-living, pathogenic, symbiotic
Distribution (habitats) in the soil. Soil habitats are determined mostly by (1) the availability of energy (carbon) resources, and (2) soil structure. Important habitats include:
Rhizosphere bacteria are supported by root exudates-about 5% of C assimilated by higher plants (Killham 1994)
Mycorrhizal Fungi
Mycorrhizae-symbiotic associations between plant roots and certain soil fungi. There are two main types: ectomycorrhizae, associations be basidiomycetes (mushroom-forming fungi) and certain trees, particularly conifers, and vescicular-arbuscular mycorrhizae (VAM or AM), associations between certain phycomycetes (Family Endogonaceae) and most herbs and many trees.
There are about 200 species of VAM fungi and more than 5000 species of ectomycorrhizal fungi. In general, VAM are not host-specific, and the roots of plants, either the same species or different species, tend to be connected belowground by the hyphae of shared VAM fungi.
VAM colonize about 80% of land plant families; certain families-- Chenopodiaceae, Brassicaceae--are not mycorrhizal. Species with finer root systems are less mycotrophic.
Functions of mycorhizae (operate mostly from extending the surface area of roots); hyphae density in the range of 10-100 m cm-3 in agricultural and natural ecosystems:
VAM probably are not capable of utilizing complex organic-N pools (SOM), but ectomycorrhizae appear to have this capability.
The host plant allocates 4-36% of C to mycorrhizae; but more C is available due to increased assimilation in mycorrhizal plants (Pedersen & Sylvia 1996).
Mycorrhizae are most beneficial where nutrients are low and the soil is not disturbed frequently. Therefore their importance is agricultural systems characterized by high levels of fertilizers and frequent tillage (plowing, disking, hoeing, etc.) is probably reduced.
I.e., mycorrhizae are most important in resource-poor agroecosystems where they function to greatly increase resource-uptake efficiency (U/S)
VII. SOIL pH
Optimum pH in most soils is 6.0 - 7.0
High pH > 8.0 associated with high salinity and high alkalinity (exchangeable Na+ > 15%). High Na+ disperses soil particles, breaking down soil structure and decreasing infiltration. Volatilization of NH3 increases at high pH.
Low soil pH (4.0 - 5.0) a greater problems for agriculture soils worldwide:
Acidification. Many nutrient cycling processes (see Resources) lead to soil acidification, including all processes associated with removal of cations from the soil:
Alkalinization. Some processes can raise the pH of soils:
URL: http://ag.arizona.edu/~spmcl/lecturenotes/soilnotes.html
Revised 27 January 2003