the reactive soil

Question 1

what are the three mechanism for nutrient uptake

Answer 1

1. root interception
2. mass flow
3. diffusion

Question 2

what is diffusion and what nutrients primarily use this uptake route

Answer 2

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

Question 3

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

Answer 3

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

Question 4

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

Answer 4

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

Question 5

what is a nutrient pool

Answer 5

the combination of both the mineral pool and organic pool

Question 6

what is the mineral pool

Answer 6

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

Question 7

what is the organic pool

Answer 7

nutrients released by decay ( P, N, S)

Question 8

what is pedogenesis

Answer 8

process of soil formation

Question 9

what is the significance of clay on infiltration and reactivity

Answer 9

decreases infiltration

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

Question 10

secondary minerals consist of what

Answer 10

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

Question 11

what are two mechanisms the result in surface charge

Answer 11

isomorphous substitution

dissociation of h+/oh- ions

Question 12

what is isomorphous substitution

Answer 12

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

Question 13

what is dissociation of h+/oh- ions

Answer 13

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

Question 14

consequence of acidifying soil

Answer 14

decreased cation exchange capacity

Question 15

what is cation exchange capacity

Answer 15

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

Question 16

what is the diffuse layer

Answer 16

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

Question 17

why is the diffuse layer a thing

Answer 17

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

Question 18

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

Answer 18

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

Question 19

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

Answer 19

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

Question 20

what net charge does the diffuse layer have

Answer 20

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

Question 21

why would we want to decrease the diffuse layer

Answer 21

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

Question 22

what is point zero net charge (PZNC)

Answer 22

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

Question 23

what is the importance of surface charge

Answer 23

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

Question 24

deflocculated (dispersed) particles leads to

Answer 24

elluviation as they are able to move down the soil

also leads to poorly drained/ aerated soil due to the absence of aggregates

Question 25

flocculated (aggregated ) particles leads to

Answer 25

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

Question 26

what is cation exchange

Answer 26

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

Question 27

what soils particles can facilitate anion adsorption and how

Answer 27

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

Question 28

what is non-specific anion adsorption

Answer 28

- 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

Question 29

what is specific anion adsorption

Answer 29

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

Question 30

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

Answer 30

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

Question 31

problem with soil acidity

Answer 31

- 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 -

Question 32

how does soil acidity increase

Answer 32

- respiration - organic acid synthesis - imbalanced cation/ anion uptake nitrification