Lecture 5 Flashcards
pH relevance to soil
- 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
world soils and pH
- most of the world soils are acidic (low pH)
- most ares that are not acidic are mountains, drylands, permafrost or deserts/semideserts
soil acidity and rainfall
high rainfall increases soil acidity through leaching of base cations
highly leached acid soils are rich in ___ and ____ which strongly absorb _____
rich in aluminium and iron oxides which strongly absorb phosphorous
the acidity of soil is comprised by 2 components
1) active acidity in soil solution
2) exchange acidity/ reserve acdidicety associated with colloids
causes of acidity in soil
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
hydrogen movement between soil and root
- to obtain NH4+ (ammonium) roots pump out H+
- to obtain NO3- (nitrate) roots pump out OH-
process whereby ammonium is oxidised to nitrate =
nitrification
-process releases H+
in acid soils, (pH <4.5) nitrification is normally ____
inhibited
how do you get H+ in acid soils
- ammonium main up take, as this is a cation it releases H+
- not through nitrification
how do you get H+ in alkali soils
nitrification
organic acid release by roots
-plants produce localised areas of low pH to gain extra nutrients
most plants release substantial amounts of organic acid as ___
why are they important
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
Base depletion:
-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
leaching by high rain fall causes…
acidification
aluminium chemistry
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
at what pH does Al3+ become soluble
below pH 4.5
Al3+ effect on roots
is highly toxic directly to roots and binds to PO42- (phosphorous) making the phosphorous unavailable
are there plants adapted to cope in <4.5 acidic pH systems
YES, but some plants are very sensitive to Al3+ and cannot survive and some that can tolerate v high concentrations
nastiest soils in the world =
acid sulphate soil
-contain minerals such as pyrite –> this oxidises releasing sulphuric acid
acid rain and soil
it forms sulphuric acid H2SO4
soil acidification results in loss of
species diversity of plants
soils with high base saturation support ____ plant communities and ___ of our rarest species
support very diverse plant communities and harbour many of our rarest species
benefit of ploughing with pH and an e.g.
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
down sound of using lime on soil pH
- uses energy
- releases CO2 back into atmosphere when you apply it
adding lime to surface (no till) effect on maize, soy bean and wheat
soy bean and wheat yields increase
-wheat roots effected by liming
alternative to lime to reduce soil acidity and decrease CO2 emissions
use of Ca silicate rock
basalt volcanic rock in reducing acidity and CO2 emissions
volcanics soils are fertile soils because volcanic ash & basaltic lava are rich in plant nutrients and rapidly weather
basalt e.g.
on cocoa plant - in high acidic soil
-greater yields
most areas with high rainfall and therefore where water crop growing is plentiful suffer from
acid soils
adding lime to soil benefits and cons
Benefits
- cheap
- effective way of raising pH
cons
- contributes to CO2 release
- only replenishes Ca
adding Ca-rich volcanic rock dust such as basalt has the potential to
- reduce soil acidity
- supply nutrients
- improve crop yields
- with less GHG releases
acid soils present a syndrome of problems:
- low nutrients (N,P,K,Ca,Mg)
- often toxic concentrations elements like Al, Mn and Fe
- crops vary in sensitivity to acidity
