Soil Reactions

Question 1

Charges

Answer 1

+vely and -vely charged things in soil

Clay and humic material mostly -ve so attract cations; attraction extends far so +ve ions near surface, -ve repelled away; equal anions and cations far away as attractive and repelling force decreases w distance

Question 2

Cation exchange

Answer 2

Weak bonds between cations and charged surfaces
Cations move in and out of solution and form dynamic equilibrium
Ions available for plant uptake

Basic: Na, Ca, K, Mg
Acidic: H, Al

Question 3

Soil type

Answer 3

Structure of soil particles determine how strongly they bind

Sand: few charged surfaces
Clay: some charged surfaces
Org material and rock: more charged surfaces

High clay, low sand: hard to work, nutrients unavailable, potential to get acidified
Low clay, high sand: easier to work, nutrients leach easity
Loamy soils best

Question 4

Displacement of cations

Answer 4

Plants displace cations through root hairs using low weight org acids - roots release org acids and CO2 forms carbonic acid
Release of protons into solution from root, exchange for mineral cation

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Question 5

Soil acidification

Answer 5

Natural soil pH dependent on parent matierial and weathering

Other factors influencing:
Rainfall - acidification
Plant growth - release of protons
Root respiration - releases CO2 which forms carbonic acid
Fertiliser use - releases protons from nitrification
Oxidative weathering - release of protons

Question 6

Alkalinity

Answer 6

Weathering of minerals w lots of basic cations
Adding limestone to manage acidification
Addition of high bicarbonate water

Question 7

Soil pH changes nutrient availability

Answer 7

Al: acidic; toxic; more soluble in low pH; limits above and below ground growth and water uptake

Higher pH = more available -vely charged surfaces so higher cation exchange capacity

Question 8

Anions

Answer 8

-vely charged nutrients
Nitrate, phosphate, sulphate and chloride
Don’t have much to stick to - stick to +vely charged impurities

Question 9

Soil nutrient mobility

Answer 9

Nutrients enter through weathering, plants and anthropogenic
Leave through leaching, immobilisation, uptake

Dynamic equilibriums:
+vely charged surfaces (e.g. Fe and Al oxides) trap -vely charged nutrients
-vely charged surfaces (e.g. clay and humic material) bind +vely charged surfaces

Leaching occurs when nutrients are mobile

Question 10

Mobility

Answer 10

High mobility: phosphate and nitrate ions are repelled by charged clay surface; nowhere to stick and move quickly - plant draws them in quick or leached out
Low mobility: ammonium attracted to charged clay surface; stick to soil surfaces and stay in soil profile; not always immediately plant available

Impurities in soil can reduce mobility of phosphate and ntirate so more to stick to - reduce leaching