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soils test > the reactive soil > Flashcards

the reactive soil Flashcards

1
Q

what are the three mechanism for nutrient uptake

A
  1. root interception
  2. mass flow
  3. diffusion
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2
Q

what is diffusion and what nutrients primarily use this uptake route

A

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

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3
Q

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

A

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

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4
Q

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

A

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

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5
Q

what is a nutrient pool

A

the combination of both the mineral pool and organic pool

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6
Q

what is the mineral pool

A

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

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7
Q

what is the organic pool

A

nutrients released by decay ( P, N, S)

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8
Q

what is pedogenesis

A

process of soil formation

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9
Q

what is the significance of clay on infiltration and reactivity

A

decreases infiltration

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

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10
Q

secondary minerals consist of what

A

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

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11
Q

what are two mechanisms the result in surface charge

A

isomorphous substitution

dissociation of h+/oh- ions

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12
Q

what is isomorphous substitution

A

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

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13
Q

what is dissociation of h+/oh- ions

A

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

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14
Q

consequence of acidifying soil

A

decreased cation exchange capacity

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15
Q

what is cation exchange capacity

A

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

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16
Q

what is the diffuse layer

A

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

17
Q

why is the diffuse layer a thing

A

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

18
Q

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

A

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

19
Q

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

A

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

20
Q

what net charge does the diffuse layer have

A

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

21
Q

why would we want to decrease the diffuse layer

A

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

22
Q

what is point zero net charge (PZNC)

A

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
23
Q

what is the importance of surface charge

A

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

24
Q

deflocculated (dispersed) particles leads to

A

elluviation as they are able to move down the soil

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

25
Q

flocculated (aggregated ) particles leads to

A

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

26
Q

what is cation exchange

A

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

27
Q

what soils particles can facilitate anion adsorption and how

A

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

28
Q

what is non-specific anion adsorption

A
  • 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
29
Q

what is specific anion adsorption

A

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
30
Q

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

A

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

31
Q

problem with soil acidity

A
  • 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
32
Q

how does soil acidity increase

A
  • respiration
  • organic acid synthesis
  • imbalanced cation/ anion uptake
    nitrification

soils test (5 decks)

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