soil chemistry

what charge do most soil colloids carry

a negative charge

what is adsorption of cations

attraction of positively charged ions (cations) to negatively charged colloid surfaces

name common cations adsorbed on colloids

al3+, ca2+, mg2+, k+, h+, na+

what influences the specific cations present on colloids

soil parent material and climate (especially leaching)

what is adsorption of anions

attractions of anions (cl-, no3-, so4 2-) to positive sites on colloids

define ion exchange capacity

a measure of how many cations/anions soil particles can retain on their surfaces

why is ion exchange capacity important

it affects nutrient mobility and retention in soils

what is specific surface

the total surface area of soil particles per unit mass; higher in clay and humus

what determines whether a soil is acidic, neutral or alkaline

the relative concentrations of h3o+ (h+) and oh- ions

what does ph measure

the acidity or alkalinity of a solution based on the concentration of h+

what does a 1 unit different in ph represent

a tenfold change in h+ concentration

what climatic conditions promote soil acidity

humid regions where precipitation > evapotranspiration

why do humid regions become acidic

non acid cations are leached and replaced by acidic cations (H+, al3+)

what climatic conditions promote soil alkalinity

arid and semi arid regions (precipitation < evapotranspiration)

why are dry region soils alkaline

non acid cations accumulate because there’s not enough water to leach them

what is a major natural source of h+ in soils

carbonic acid from co2 dissolving in soil water

what organic acids contribute to soil acidity

carboxylic and phenolic acids from humus

what is salinization

the accumulation of soluble salts in soil to levels harmful for plants

name four sources of salts leading to salinization

mineral weathering, salt laden groundwater, sea spray/seawater, fossil salt deposits

how does groundwater cause salinization

it dissolves salts and moves them upward or horizontally, depositing them in low areas

how does irrigation increase salinization

brings more water (raising water table) and adds dissolved salts

what happens when too much water raises the water table

capillary rise brings salts into the root zone

why do salts accumulate in surface soils in dry regions

rainfall is insufficient to flush salts downward

how does vegetation change affect salinization

replacing deep rooted perennials with annuals reduces evapotranspiration - raises water table - mobilizes salts

what happens if a shallow impermeable layer restricts drainage

salty ground water flows sideways - rises to surface -evaporates - salts accumulate

what are management practices to reduce salinization

maintain perennial cover, cover crops; grow salt tolerant crops

why is drainage important for irrigation management

it helps leach salts out of the root zone

what are saline soils

high soluble salt content

what are sodic soils

high exchangeable sodium

what are saline sodic soils

high in salts and exchangeable sodium