Lecture 5

Question 1

pH relevance to soil

Answer 1

• one of the most important soil properties

- pH optimum range for most plants is near neutral as this is when plants most easily uptake nutrients

Question 2

world soils and pH

Answer 2

• most of the world soils are acidic (low pH)

- most ares that are not acidic are mountains, drylands, permafrost or deserts/semideserts

Question 3

soil acidity and rainfall

Answer 3

high rainfall increases soil acidity through leaching of base cations

Question 4

highly leached acid soils are rich in ___ and ____ which strongly absorb _____

Answer 4

rich in aluminium and iron oxides which strongly absorb phosphorous

Question 5

the acidity of soil is comprised by 2 components

Answer 5

1) active acidity in soil solution

2) exchange acidity/ reserve acdidicety associated with colloids

Question 6

causes of acidity in soil

Answer 6

1) H+ ion release by plant roots and micro-organisms
2) organic acids
3) base depletion & leaching
4) aluminium chemistry
5) weathering and oxidation
6) acid rain and anthropogenic emissions

Question 7

hydrogen movement between soil and root

Answer 7

• to obtain NH4+ (ammonium) roots pump out H+

- to obtain NO3- (nitrate) roots pump out OH-

Question 8

process whereby ammonium is oxidised to nitrate =

Answer 8

nitrification

-process releases H+

Question 9

in acid soils, (pH <4.5) nitrification is normally ____

Answer 9

inhibited

Question 10

how do you get H+ in acid soils

Answer 10

• ammonium main up take, as this is a cation it releases H+

- not through nitrification

Question 11

how do you get H+ in alkali soils

Answer 11

nitrification

Question 12

organic acid release by roots

Answer 12

-plants produce localised areas of low pH to gain extra nutrients

Question 13

most plants release substantial amounts of organic acid as ___
why are they important

Answer 13

exudates and decomposition products

• -carboxylic acids
• -amino acids
• -phenolic acids

these weak acids are important components of acidity especially in organic soils
-main acid components of humic (charged) substances

Question 14

Base depletion:

Answer 14

-linked to rainfall & washing out of ions

where soil exchange sites are occupied by Ca, Mg, K and Na and other base cations –> NOT Al & H

Question 15

leaching by high rain fall causes…

Answer 15

acidification

Question 16

aluminium chemistry

Answer 16

in strongly acidic mineral soils Al is present as main Al3+, AlOH2+ and Al(OH)2+ which are associated with cation exchange sites on colloids

These 3 + H2O = H+ molecules

Question 17

at what pH does Al3+ become soluble

Answer 17

below pH 4.5

Question 18

Al3+ effect on roots

Answer 18

is highly toxic directly to roots and binds to PO42- (phosphorous) making the phosphorous unavailable

Question 19

are there plants adapted to cope in <4.5 acidic pH systems

Answer 19

YES, but some plants are very sensitive to Al3+ and cannot survive and some that can tolerate v high concentrations

Question 20

nastiest soils in the world =

Answer 20

acid sulphate soil

-contain minerals such as pyrite –> this oxidises releasing sulphuric acid

Question 21

acid rain and soil

Answer 21

it forms sulphuric acid H2SO4

Question 22

soil acidification results in loss of

Answer 22

species diversity of plants

Question 23

soils with high base saturation support ____ plant communities and ___ of our rarest species

Answer 23

support very diverse plant communities and harbour many of our rarest species

Question 24

benefit of ploughing with pH and an e.g.

Answer 24

ploughing can help get insoluble/ not v soluble i.e. lime into soil to increase pH

• legumes and their nitrogen fixation is sensitive to pH
• adding lime can increase N fixation where the soil toxicity is a problem causing Al or Mn toxicity
• cheaper than buying N fertiliser

Question 25

down sound of using lime on soil pH

Answer 25

• uses energy

- releases CO2 back into atmosphere when you apply it

Question 26

adding lime to surface (no till) effect on maize, soy bean and wheat

Answer 26

soy bean and wheat yields increase

-wheat roots effected by liming

Question 27

alternative to lime to reduce soil acidity and decrease CO2 emissions

Answer 27

use of Ca silicate rock

Question 28

basalt volcanic rock in reducing acidity and CO2 emissions

Answer 28

volcanics soils are fertile soils because volcanic ash & basaltic lava are rich in plant nutrients and rapidly weather

Question 29

basalt e.g.

Answer 29

on cocoa plant - in high acidic soil

-greater yields

Question 30

most areas with high rainfall and therefore where water crop growing is plentiful suffer from

Answer 30

acid soils

Question 31

adding lime to soil benefits and cons

Answer 31

Benefits

• cheap
• effective way of raising pH

cons

• contributes to CO2 release
• only replenishes Ca

Question 32

adding Ca-rich volcanic rock dust such as basalt has the potential to

Answer 32

• reduce soil acidity
• supply nutrients
• improve crop yields
• with less GHG releases

Question 33

acid soils present a syndrome of problems:

Answer 33

• low nutrients (N,P,K,Ca,Mg)
• often toxic concentrations elements like Al, Mn and Fe
• crops vary in sensitivity to acidity