the reactive soil Flashcards
what are the three mechanism for nutrient uptake
- root interception
- mass flow
- 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
problem with soil acidity
- ca can be displaced from cation binding sites as it is a base cation and leach = replaced with lower valency cations which will increase the diffuse layer which will increase deflocculation
- decreases solubility of h2po4
- ## increases solubilty of undesirable cations eg al
how does soil acidity increase
- respiration
- organic acid synthesis
- imbalanced cation/ anion uptake
nitrification
