Lecture 11: Chapter 7- Soil Acidity, Alkalinity, Salinity, Sodicity

Question 1

Where are alkaline soils found relative to climate?

Answer 1

Grasslands and dryer climates have higher soil pH (alkaline)

Question 2

how to fix saline soils

Answer 2

-overirrigation so water can wash salt out of the rooting zone
-adding gypsum provides competition from calcium
-plant more salt tolerant plants
-plant deep rooted plants to add more air

Question 3

Explain how an alkaline soil pH affects the availability/toxicity of plant macro- and micronutrients

Answer 3

UNSTABLE SOIL STRUCTURE, FE AND P DEFICIENCY
-Alkaline soils have available plant nutrients
-Increased amounts of Ca, Mg, Mo
-Phosphorus, Iron, and Manganese precipitate

Question 4

Explain how alkaline soils can be improved for agricultural use or managed to reduce to risk of erosion

Answer 4

-adding organic matter or compost

Question 5

Discuss how soil pH affects the activity and composition of the soil microbial community and availability of plant nutrients

Answer 5

-Fungi easily available with wide range of pH
-Bacteria and plant nutrients less available at lower pH
-Phosphorus, Iron, and Manganese ten to precipitate at low and high pHs (not at neutral)
-Increase of Al, Fe, Mn, and Zn at lower pHs

Question 6

Explain the term base saturation and why Al3+ is considered an acidic cation

Answer 6

base saturation:
Al3+ ions have a strong tendency to hydrolyze, splitting water molecules into H+ and OH- ions, and to form hydroxy Al ions.

Question 7

active acidity

Answer 7

defined by the H+ ions activity on the soil solution. determines the solubility of many substances and provides the soil solution environment to which plant roots and microbes are exposed

Question 8

List the 3 dominant soil pH buffering systems and their approximate pH range

Answer 8

Buffered by aluminum compounds: pH 3-4.5
Buffered by cation exchange and pH dependent charge sites: pH 4.5-7
Buffered by carbonates: pH 7-9

Question 9

Describe where acid soils are found relative to climate, and how the typical range of soil pH associated with specific vegetation types/land use

Answer 9

Humid and Temperate climates that have a wet climate, like the east coast, have acidic soils

Question 10

List processes that result in the acidification of soils and how they have been accelerated by humans

Answer 10

-ADDING ORGANIC MATTER
-OXIDATION OF N AND S FERTILIZERS
-ALUMINUM
-CATIONS LOSS VIA PLANT UPDATE OR LEACHING
-WET AND DRY ACID DEPOSITION FROM THE ATMOSPHERE
-ACID RAIN

Question 11

Explain how soil acidity affects the availability/toxicity of plant macro- and micronutrients

Answer 11

LOSS OF MACRONUTRIENTS (Ca, Mg,K) Al TOXICITY, P DEFICIENCY

Question 12

Explain how acid soils can be improved for agricultural use or managed to protect ground and surface waters

Answer 12

-raising soil pH by liming
-amelioration of Al toxicity= not changing pH by adding gypsum or organic matter to get rid of Al
-for agricultural use: ash creates a lot of Ca (calcium) so people have been adding ash to their soils to increase pH for farming

Question 13

Acidifying process equation

Answer 13

CO2 + H2O -> H2CO3 <-> HCO3 - + H+ <-> CO3 2- + H+
(Carbon dioxide-> carbonic acid-> bicarbonate->carbonate)

Question 14

How is carbon dioxide produced?

Answer 14

-[in atmosphere] respiration of plants and animals and combustion processes (~400ppm)
-[in soil] respiration

Question 15

Acidification of soils: organic matter

Answer 15

-adding organic matter to soils acidifies them (like adding mulch, fertilizer, and sewage sludge)
-soil organic matter contains a lot of functional groups from which H+ ions can dissociate. they can form soluble complexes with non-acidic cations (Ca2+, Mg2+) and facilitate their leaching

Question 16

Acidification of soils: aluminum

Answer 16

-weathering destroys minerals and releases Al which makes soils more acidic
-dissolved Al 3+ is highly toxic to most organisms and responsible for much of the deleterious impact of soil acidity on plants and aquatic organisms
-Al 3+ ions have a strong tendency to hydrolyze, splitting water molecules into H+ and OH- molecules and to form hydroxy Al ions

Question 17

Acidification of soils: cations loss via plant uptake or leaching

Answer 17

-plants add a lot of H+ ions to take nutrients so they also acidify soils

Question 18

Acidification of soils: wet and dry acid deposition from the atmosphere

Answer 18

-SO2 emissions from combustion of coal causes acid rain
-NOx comes from atmosphere and combustion of carbon because it gets so hot it splits NO2 from air to form NOx

Question 19

salt replaceable (exchangeable) acidity

Answer 19

involving the aluminum and hydrogen that are easily exchangeable by other cations in a simple unbuffered salt solution.

Question 20

residual acidity

Answer 20

accompanied by hydrogen and aluminum ions. residual acidity usually a lot larger than the active or salt replaceable acidity. can be neutralized by limestone or other alkaline materials

Question 21

total acidity

Answer 21

total acidity = active acidity + salt replaceable acidity + residual acidity