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
