SSC 220

Colloids

Small size (diameter of 0.1 to 0.001µm), high surface area with some internal surface area, surface charge, absorption of cations/anions, and water

Inorganic colloids (Phyllosilicate)

Clay minerals constructed from tetrahedral (Si4+, O2-) and octahedral (Al3+, O2-) sheets

2:1 phyllosilicate type clays

two tetrahedral sheet to one octahedral sheet. Example illite. Younger soils (Mollisols, Alfisols). Shrink swell soils (Vertisols)

1:1 phyllosilicate type clays

one tetrahedral sheet to one octahedral sheet. Example kaolinite. Older/ more weathered soils (Spodosols, Ultisols).

Iron and aluminum oxides

just octahedral sheets. Common in highly weathered soils (Oxisols).

Humus

the organic matter fraction that has decomposed sufficiently so that the source material is no longer recognizable.

Cation exchange capacity (CEC)

sum of total cations that a given soil can absorb.

Highest CEC

humus → 2:1 clays → 1:1 clays → oxides → sand (sand is not chemically reactive) (Lowest CEC)

Isomorphic Substitution

process in which one element substitutes another of comparable size in the crystalline structure. Permeant charge.

pH dependent charge

broken ends of clays carry a charge. Charge can change based on the pH. Lower pH = more H+ will attach to the broken ends.

Organic matter only has

pH dependent surface charge

Buffering capacity

soils ability to resist change in pH.

Active acidity

H+ or Al3+ in the water of soil and a measure of how acidic the soil is.

Exchangeable acidity

H+ and Al3+ that are on exchange sites of clay or OM negative sites.

Residual acidity

H+ or Al3+ that are bound to clay or OM and NOT exchangeable.

Calcium Carbonate Equivalent (CCE)

Neutralizing value of any liming material compared to pure calcium carbonate.

Symbiotic biological N fixation

bacteria fix N2 to plant-available forms for plants to use.

Mineralization

Biological conversation of unavailable organic N into plant available inorganic N

Immobilization

Biological conversation of plant available N into unavailable organic N

Ammonification

Conversion of organic N to ammonium by microorganisms. Need warm temperatures, good soil moisture, and oxygen supply. Makes the soil more basic

Nitrification

Conversion of ammonium to nitrite and then to nitrate. Ammonium to nitrite is mediated by Nitrosomonas. Nitrite to nitrate is mediated by Nitrobacter. Need warm temperatures and well drained soil. Makes the soil more acidic.

Volatilization

Removal of N from the soil by truing it into a gas that leave the soil.

Denitrification

Conversion of nitrate to a gas form of N (ideally N2 ). – Need warm temperatures, low oxygen, organic matter (carbon source).

Crop uptake and removal

Removal of N by plants and then removing plant residues from the field.

Law of the minimum

“If one growth factor/nutrient is deficient, plant growth is limited, even if all other vital factors/nutrients are adequate… plant growth is improved by increasing the supply of the deficient factor/nutrient.” (1873)

Mass Flow

dissolved nutrients move to the root in soil water that is flowing towards the roots.

Diffusion

nutrients move from higher concentration in the bulk soil solution to lower concentration at the root

Root interception

roots obtain nutrients by physically contacting nutrients in soil solution or on soil surfaces