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what are the three mechanism for nutrient uptake

1. root interception
2. mass flow
3. diffusion


what is diffusion and what nutrients primarily use this uptake route

movement of nutrients down a concentration gradient from soil surfaces into the soil solution which allows up take by plants
responsible for majority of p and k transport to the roots
is short range


what is mass flow and what nutrients primarily use this uptake route

movement of nutrients in soil solution adsorped by the plant roots for transpiration
mainly used b n, mg, s and ca uptake


what is root interception and what nutrients is it primarily responsible for

it is the movement of nutrients on soil particle surfaces by direct contact with plant roots
not that common but mainly used for Ca


what is a nutrient pool

the combination of both the mineral pool and organic pool


what is the mineral pool

nutrients weathered by hydrolysis (ca, k, na, mg, zn)


what is the organic pool

nutrients released by decay ( P, N, S)


what is pedogenesis

process of soil formation


what is the significance of clay on infiltration and reactivity

decreases infiltration
increases reactivity = increased ability of nutrients ions being able to diffuse into soil solution


secondary minerals consist of what

layer silicates (clay minerals) that are made up of octahedral and tetrahedral units


what are two mechanisms the result in surface charge

isomorphous substitution
dissociation of h+/oh- ions


what is isomorphous substitution

structural units within the octahedral and tetraedral sheets of clay minerals can be substituted by cation of similar size but different charge
which results in a bulk soil negative charge


what is dissociation of h+/oh- ions

changes in the soil solution ph leads to the removal of h+/OH- ions from exposed soil surfaces of some clay minerals , fe-al hydrous oxides and om

decreased ph = more acidic (adds h+) so surface charge decreases
increased ph = more basic (adds OH-) so surface charge is more negative


consequence of acidifying soil

decreased cation exchange capacity


what is cation exchange capacity

the amount of negative charge available for the retention of cations = number of exchange sites
high cec = high cation retention ability


what is the diffuse layer

the distance between the soil surface and the point in the soil where the cation and anion charges in the bulk solution are equal


why is the diffuse layer a thing

clay surface has a negative charge thus adsorbs cations from the soil solution to balance this moving down the profile the initial high conc of cations which are attracted to the surface deceases and anions begins to increase at the point where the cons are equal is the end of the diffuse layer after this the rest of the soil contains and equal proportion of cations and anions


how does the change in concentration of cations in the soil affect the diffuse layer

depth of diffuse layer decrease and conc of cations in bulk solution increases as number of cations required to neutralise negative charge at clay surface can be found much closer to the surface


how does the change in valency of cations in the soil solution affect the diffuse layer?

increased valency = decreased diffuse layer as the negative charges on the clay surfaces are able to be neutralised with less cations


what net charge does the diffuse layer have

a net positive charge to cancel out the net negative charge of the clay surface


why would we want to decrease the diffuse layer

as it increases flocculation cos colliods are able to get closer togeter so that the van der waals attractive forces can work


what is point zero net charge (PZNC)

the ph at which the net surface charge = 0

above = more negative charge due to increased oh- ions present = attract cations
below = more positive due to increased h+ present = attract anions


what is the importance of surface charge

it controls flocculation/ deflocculation
at PZNC particles have no charge so can come together (flocculate) but the acidity for this to happen is impractical so we just try to get as close to PZNC as possible


deflocculated (dispersed) particles leads to

elluviation as they are able to move down the soil
also leads to poorly drained/ aerated soil due to the absence of aggregates


flocculated (aggregated ) particles leads to

illuviation as they cant move in the soil
they form stable aggregates which leads to well drained and aerated soils
key is having appropriates ca concentration


what is cation exchange

= the adsorption of cation onto negatively charged soil surfaces (clay an OM) driven by equilibrium reactions


what soils particles can facilitate anion adsorption and how

fe and al oxides can as they are able to have both positive and negative charges which depend on the soil ph


what is non-specific anion adsorption

- simple electrostatic attraction of anions onto a positively charged fe or al oxide surface
- reversible and controlled by soil factors that dictate ph these are weak bonds
- important for cl, no3, so24


what is specific anion adsorption

ions in a soil solution form a chemical bond with functional groups of a soil structure
- occurs without factors that control non specfic
- strong and essentially irreversible at a particular ph
-important for dihydrogen phospahte


how does the addition of lime (CaCO3) facilitate ph buffering

good at buffering cos after dissolution in water it releases OH- ions which then neutralise the H+ in the acidic soil solution and accordingly a concentration gradient will form and there will be less h+ ions in the soil solution compared to that on the colloid surfaces thus these colloids will release h+ ions to reestablish equilibrium and resist a change in ph
- soils with a finer texture require more lime to shift ph