sustainable soil for crops (acidity) Flashcards
5 drivers of soil degradation
- climate change
- deforestation
- urbanization and expansion
- pollution and waste
- unsustainable soil management
what are the 2 main constraints of the Australian system soil degradation
acidity and salinity
6 consequences of soil degradation
- water scarcity
- food and nutrient scarcity
- rapid climate change (Co2)
- poverty and social insecurity
- migration
- reduction in ecosystem services
what are the 8 main types of degradation of the soil
- soil erosion
- decline in organic C
- nutrient imbalances
- salinization and sodification
- loss of biodiversity
- contamination
- acidification
- compaction
percent of drylands on the earth surface and how much is susceptible to wind erosion
40% of the earth land surface
430 million hectares are susceptible
how is soil nutrients imbalanced
continuous harvesting (lots of output of nutrients)
continuous fertilizers of N and or P
this leads to excess macronutrients and lack of micro nurtients
why is Cacl2 used to measure pH instead of water
water has high variability due to natural presence of salts therefore Cacl2 reduces effect of salts on pH. however this can displace some H ions making the solution more acidic than normal.
what is pH 7 a measure of
this is when oH- and H+ ions are in equilibirum in concentrations
6 sources of acidity in soils and which are the most important
- product removal
- metabolic activity of microorganisms
- leaching of nitrate from ammonium fertilisers and N fixation
- pH imbalance from nutrient uptake
- mine spoil
- acid rain
(1-3 are most important)
how does product removal increase acidity
acidification is a natural process that occurs over time through weathering of minerals and rainfall etc however agriultrual removal of biomass increases this process quickly through removing base cations (alkaline) through harvest increasing Concentration of H ions. Hay is largest contributor of removal of base cations therefoe need large about of lime to replace
how does microbes increase acidiy
the breakdown of organic C and other metabloc processes increases Co2 and other organic acids in soil
how does nitrification increase acidification
reaction of ammonium into nitrate is an acidic processs however plants usually uptake this nitrate in exchnage for hydroxl = equilibium. however when No3 is lost through leaching the system is left with H ions (acidic)
how does nutrient uptake cause pH imbalance (rhizosphere)
ammonium (cation) or nitrate (anion) are uptaken by plants and exchanged for either H+ ions or OH- ions. nitrate feeding plants raise pH (OH-) and ammonium feeding plants lower pH (H+ acidification)
when can pH be toxic to plants
5.5 and less
5 effects of low pH (less than 7)
- decrease in base cations (calcium, sodium, potassium, and magnesium)
- Al3+ increases
- negative charge on humus decreases
- availability of toxic elements increases eg Al and Mn
- activity of microorganisms reduced
ideal pH range
5.5-8.5
what happens in soils with pH greater than 8.5
bicarbonate and carbonate toxicity nutrient deficiency and toxicity and low biological activity. metal deficiency
what happens in soils with pH lower than 5.5
H, Mn and Al toxicity nutrient deficiency and toxicity and low biological activity. Ca and P deficiency
4 forms of soil acidity
- active
2/ exchangeable - nonexchangeable (residual)
- reserve
what is active acidity
all free al3+ and H+ dissolved in solution this is what is measured. this is a small proportion of all soil acidity
what is exchangeable acidity
H+ and Al3+ on exchange sites (H+ in organic soils and Al3+ in mineral soils) largely Al3+ as these replace base cations on exchange sites in acidic soils
there is an equilibrium between active and exchangeable
what is reserve acidity
(non-exchangeable and exchangeable) this represents the buffer capacity of soils (if soil pH is manipulated this will return to original pH)
associated with solid phase
what is total acidity
(reserve and active acidity)
what is non-exchangeable acidity
only released when pH is greatly altered eg adding lime
how will adding lime affect soil acidity
active acidity will be affected (raised) however the reserve acidity will not be affected and will buffer back to original acidity
what are the 3 soil pH buffering mechanisms in soil and why are they important
if soils do not have buffering capacity plants and microbes will die under sudden changes
- carbonate mineral buffering
- exchangeable base cation buffering
- buffering by alumino-silicate mineral deposition
explain carbonate mineral buffering
in pH 8 (basic)
carbonate minerals react with acid (H+) to neutralize incoming acid creates bicarbonate and calcium salts.
explain exchangeable base cation buffering
buffering between pH 5.5-7 (neutral)
Al3+ and H+ replace base cations (Ca2+, K) on exchange sites
does not neutralize acidity but stores it on exchange sites and keeps out of soil solutions.
explain buffering by alumino-silicate mineral deposition
in pH <5.5 (acidic)
variable charge mineral surfaces and silicate layer edges accept protons on functional groups and stored them. below pH 4 dissolution of clay minerals
what are reaction constants for Al
Al3+ and monomers act as a proxy for pH in soils (what forms are present) there are different constants at different pH.
Al3+ loses protons with increasing pH
why is pH significant for plants which pH is minimum best
AL3+ is toxic to plants, not pH! Al3+ species becomes almost minimal at pH 5.5
how was lime work
CaCo3 carbonates (lime) added to soil, neutralization by dissolution reaction ( gives exchange site ca + water + co2)
