How can acid, alkaline, saline, and sodic soils be managed?

a) Raising pH of acid soils
b) Lowering pH of alkaline soils
c) Reducing levels of salt in saline soils
d) Reducing levels of sodium in sodic soils

Soils tend to become more ________ as they weather and develop

acidic

What are 2 toxic metals (to plants) in acid soils?

Toxicity of soil metals to plants is common in acid soils
• Aluminum (Al3+)
• Manganese (Mn2+)

Most plants, ________  grow poorly in acid soils

bacteria

 Cadmium, lead, copper, zinc

Some _________ are not available to plants in acid soils

nutrients

formula for pH?

pH = ‐log (H+)

 where (H+) is the activity of H+ in solution

How do acid soils form?

• (acid rain) CO₂ dissolved in rain water forms carbonic acid (H₂CO₃) – pH ~ 5.6

• S and N oxides add acidity to precipitation
– CO2 produced by soil biological activity dissolves in soil water, forming H₂CO₃

 Some fertilizers are acidic.

What non‐acidic cations are leached from the soil during weathering, and are replaced by what acidic cations

non acidic: Na⁺, K⁺, Mg⁺², Ca⁺²

acidic: Al³⁺ and H⁺

an acid soil has a pH _______.

<6.5

Where are acid soils (most likely) found?

Where soils have been exposed to long periods of soil weathering like: high rainfall climates or warm climates

OR in soils formed from acidic parent materials

United States:
– mostly in the eastern states

– coastal areas of western states
– some mountain soils

What element is abundant in soils?

Aluminum

Which are the non‐acidic cations and what happens to them in soil?

Aluminum ___________ releasing acidity
over time, Al³⁺ may occupy much of the soil’s cation exchange capacity

hydrolyzes

Soil pH is largely controlled by which exchangeable cations (there are 7)?

 Fe³⁺, Al³⁺, H⁺ are acidic cations 

Ca²⁺, Mg²⁺, K⁺, Na⁺ are non‐acidic cations

The fraction of the CEC filled with acidic versus non‐acidic cations controls soil ___.

pH

What is acid saturation?

The percentage of the soil CEC that is taken up by the acid cations: Al⁺³ and H⁺

What is non‐acidic or base saturation?

The percentage of the soil CEC that is taken up by the non‐acid cations: Na⁺, K⁺, Mg⁺² and Ca⁺²

Higher % base saturation means _______ pH

higher

Higher % acid saturation has ________ pH

lower

Alkalinity is almost always associated with soil salinity (salt accumulation) due to:

Moderate ________ is associated with accumulation of Ca salts (CaCO₃) 

Alkalinity

sodium salts like NaHCO₃^‐

_________ __________ is the ability to resist pH changes.

Buffering capacity

In moderately acidic soils, buffering is also known as?

reserve acidity

In alkaline soils, buffering is due to reactions of __________ ____________.

calcium carbonates

– Amount of solid‐phase carbonates (also called ‘free lime’) determines the amount of buffer capacity

What is happening in this reaction?

CaCO₃^‐2 + H⁺ ↔ HCO₃‐ + Ca⁺²

Calcium is released and hydrogen bonds to form bicarbonate....

What is happening in this reaction?

Al⁺³ + OH^‐ ↔ AlOH⁺²

If base is added, it is neutralized by exchangeable Al⁺³

 The greater the soil buffering capacity, the more _______________ that will be needed to change the soil H.

amendment/Lime/Calcium Carbonate

all acceptible answers

Both exchangeable acidity and active acidity must be ______________ to change soil pH

neutralized

To raise the pH of acid soils, it is necessary to:

1. Neutralize active acidity

(the H⁺ and Al³⁺ in the soil solution, this is what pH measures)
2. Neutralize reserve acidity (exchangeable Al³⁺ and H⁺ )
– Higher CEC means more amendment is needed to raise the soil pH

• Lime is used to neutralize soil acidity

Soils with greater CEC have greater buffering capacity, and therefore require more _______ to raise the soil pH

lime

What bases are used to neutralize soil acidity

1. Lime or Agricultural Lime

• Ground CaCO₃ (calcite) is most common

• CaMg(CO₃)₂ (dolomite) is also used
2. Quicklime or burnt lime
• CaO made by heating ground limestone
3. Hydrated lime or slaked lime
• Quicklime is reacted with water to make calcium hydroxide