Lecture 5 - soil acidity

Question 1

what is the optimum ph range for most plants?

Answer 1

near neutral - 6.5-7.5

Question 2

what happens at low soil ph?

Answer 2

some of the ions become toxic to plants

Question 3

what increases soil acidity and how?

Answer 3

high rainfall- through leaching of base cations - big link between levels of rainfall and particular soil types seen across the globe

Question 4

Phosphorous availability is one of the biggest issues in soils - what is the issue with acidic soils and phosphorous?

Answer 4

-Highly leached acid soils are rich in aluminium and iron oxides which strongly sorb phosphorus - i.e. when you add phosphorous to soils it binds with the ions and become unattainable - a solution to this is to change the PH of the soil

Question 5

describe the link between PH and hydrogen ions

Answer 5

A fall in PH has a massive effect on the number of hydrogen ions available

Question 6

what two components is the acidity of soil comprised of?

Answer 6

1) active acidity in soil solution

2) exchange acidity/reserve acidity associated with colloids

Question 7

6 causes of acidity

Answer 7

(1)	H+ ion release by plant roots and micro-organisms

(2)	Organic acids

(3)	Base depletion and leaching	

(4)	Aluminium chemistry

(5)	Weathering and oxidation

(6) 	Acid rain and anthropogenic emissions

Question 8

what effect do plant roots have on soil PH?

Answer 8

-Root systems have a major effect on the PH surrounding them
- The process whereby ammonium is oxidized to nitrate is called nitrification:
This can release H+ ions.
- In acid soils (pH <4.5) nitrification is normally inhibited

Question 9

how do organic acids affect soil PH?

Answer 9

Organic acid can also be released by roots to reduce the surrounding PH and in the process lose phosphorous - produce localised and temporary changes in PH - very advanced root systems that can do this most cant -Most plant roots release substantial amounts of organic acids as exudates and decomposition products

Question 10

describe base depletion

Answer 10

• often linked to rainfall and flushing out of ions
• Where soil exchange sites are occupied by Ca, Mg, K and Na and other base cations (not Al and H), the sites will have little or no exchange acidity.
• Process leading to loss of bases e.g. leaching by high rainfall causes acidification

Question 11

difference between cation and anion

Answer 11

cation = positively charged 
anion = negatively charged

Question 12

how does aluminium chemistry effect soil PH?

Answer 12

Aluminium contains a potential store of a lot of acidity: At pH below 4.5 in mineral soils Al3+ becomes soluble

Question 13

what is the issue with Al3+?

Answer 13

is highly toxic to roots of most plants and binds to PO42- making phosphorus unavailable

Question 14

how do plants respond to acid soils?

Answer 14

• Plants specialised on acid soils have high Al3+ tolerance
• some plants have intermediate
  pH and Al3+ tolerance
  -Plants absent from acid soils are often highly Al3+ sensitive

Question 15

what can weathering and oxidation of minerals like pyrite cause?

Answer 15

acid sulphate soils that contain minerals such as pyrite which oxidizes to release sulphuric acid- and can have pH values as low as 2.0
-Highly acid soils suffer toxic concentrations of Fe, Al, Mn, and other trace metals, and are very low in available and exchangeable nutrients N, P, Ca, Mg.

Question 16

what is acid rain?

Answer 16

• an anthropogenic source impacting on soil PH

- Non-marine sulphur – Historically a major component of acid rain due to coal burning - It forms sulphuric acid: H2SO4

Question 17

what helps to buffer soils against acid rain?

Answer 17

soil rich in chalk and limestone - high base saturation - these types of soils often support very diverse plant communities and harbour many of our rarest species

Question 18

2 ways to manage soil acidity

Answer 18

1) the addition of lime - reacts with water to form slaked lime
2) growth of legumes

Question 19

what is the potential problem of adding lime?

Answer 19

can potentially be a source of adding CO2 back into the atmosphere

Question 20

what can calcium silicates be used for?

Answer 20

• Use of Ca silicate rock instead of lime to reduce soil acidity and decrease CO2 emissions

Question 21

why are volcanic soils fertile?

Answer 21

• Volcanic soils are fertile soils because volcanic ash and basaltic lava are rich in plant nutrients and rapidly weather

Question 22

describe how australias highly weathered oxisol was ammended with ground basalt?

Answer 22

• the basalt increased available P
• basalt reduces P sorption
• basalt reduces soil acidity