Soil acidity and management

Question 1

One of the most important soil properties is pH

Answer 1

The pH optimum range for most plants is near neutral

Question 2

Most of the areas on Earth that are not acid are mountains, drylands, permafrost or deserts / semideserts

Answer 2

Places with little rain

Question 3

High rainfall increases soil acidity through leaching of base cations

Answer 3

Highly leached acid soils are rich in aluminium and iron oxides which strongly sorb phosphorus

Question 4

Soil acidity is measured by pH which is a log scale of units of H+ moles in a solution

Answer 4

pH 4 to 3 = 9 x 10^-4 moles H+
pH 7 to 6 = 9 x 10^-7 moles H+

Three orders of magnitude different

Question 5

H+ is a cation - very strongly attracted to negatively charged sites on clays and organic matter

Answer 5

The acidity of soil is comprised in two components

1) Active acidity in soil solution
2) Exchange acidity/reserve acidity associated with colloids

Question 6

CAUSES OF ACIDITY

Answer 6

(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 7

Plants release H+ ions into the soil via their roots and micro-organisms

Answer 7

The process whereby ammonium is oxidized to nitrate is called nitrification:
NH4+ NO3- This can release H+ ions.

In acid soils (pH <4.5) nitrification is normally inhibited

Since N is the mineral nutrient required in largest amounts by plants, and NH4+ is a cation, where NH4+ is the main N source there is ongoing release of H+ linked to NH4+ uptake.

Question 8

Most plant roots release substantial amounts of organic acids as exudates and decomposition products.

Carboxylic acids
Amino acids
Phenolic acids

Answer 8

These weak acids are important components of acidity especially in organic soils.

They are the main acid components of humic substances – which have exchange acidity of 500-14000 moles H+ per Kg dry weight.

Question 9

Base Depletion –
(often linked to rainfall and flushing out of ions)

Processes leading to loss of bases e.g. leaching by high rainfall causes acidification

Answer 9

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.

Percentage base saturation = % cation exchange sites occupied by base cations.

Question 10

In strongly acid mineral soils Al is present mainly as Al3+ and AlOH2+ and Al(OH)2+ cations which are often associated with the cation exchanges sites on colloids

Answer 10

When mixed with water can release H+, increasing acidity of soil

At pH below 4.5 in mineral soils Al3+ becomes soluble.
Al3+ is highly toxic to roots of most plants and binds to PO42- making phosphorus unavailable.

Question 11

Most plants have intermediate pH and AL3+

Answer 11

tolerance

Question 12

(5) Weathering and oxidation of acid-generating minerals like pyrite

Answer 12

“Acid sulphate soils are the nastiest soils in the world”

These are soils that contain minerals such as pyrite which oxidizes to release sulphuric acid- and can have pH values as low as 2.0

Question 13

Highly acid soils suffer toxic concentrations of Fe, Al, Mn, and other trace metals, and

Answer 13

are very low in available and exchangeable nutrients N, P, Ca, Mg.

Question 14

Acid rain –an anthropogenic impact on soil pH

Answer 14

Non-marine sulphur –
Historically a major component of acid rain due to coal burning.

It forms sulphuric acid: H2SO4

Question 15

Soil acidification results in loss of species diversity

Answer 15

Soil acidification in the UK from late 1950’s to 1990 increased by 1.8 pH

Question 16

Soils with high base saturation often support very diverse plant communities

Answer 16

and harbour many of our rarest species.

Question 17

Growth of legumes and their nitrogen fixation is sensitive to pH.

Answer 17

Adding lime can increase N fixation where the soil acidity is a problem causing Al or Mn toxicity.

Question 18

Applying aglime to soils reacts

Answer 18

with nitric acid and other acid sources to release CO2 in the atmosphere

Question 19

Lime improves yield by improving acidity at surface level

Answer 19

but is’t getting mixed in and is slow in ameliorating subsoil activity

Question 20

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

Answer 20

Volcanic soils are

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

Question 21

Most areas with high rainfall – and therefore where water for crop growing is plentiful suffer from acid soils.

Acid soils present a syndrome of problems-
Low nutrient (N, P, K , Ca, Mg) content,
Often toxic concentrations elements like Al, Mn and Fe
Crops vary in their sensitivity to acid soils

Answer 21

Adding lime has traditionally been a cheap and effective way of raising soil pH but it contributes to CO2 release and does not replenish nutrients other than Ca.

Adding Ca-rich volcanic rock dust such as basalt has the potential to reduce soil acidity, supply nutrients and improve crop yields. With less greenhouse gas releases