Acidity and Salinity

PLSC204 exam #3

the pH of natural rain is _____

5, so it is acidic 

H⁺ and Al³⁺ are _____

acidic cations

non acidic or basic cations do not produce ___

H⁺

an increase in base saturation leads to _____

an increase in pH

Soil pH is controlled largely by

% base saturation

reactions involved in producing acidity

• Carbonic and various organic acids

• Atmospheric CO2, CO2 from plants and microbes > H2CO3,

• Production of organic acids by plants and microbes

• Plant uptake of cations (acidifying) relative to anions (alkalinizing)

• Oxidation of ammonium cations to nitric acid

• Oxidation of reduced sulfur to sulfuric acid

• Acids in precipitation

• Accumulation of organic matter from certain types of plants

some sources of soil acidity include

acid rain and exposure of reduced sulfur to oxygen produces acidity

buffering capacity increase with ___

increasing CEC

pH buffering in soils is important in _____

determining the effect of pH-active amendments added to the soil

pH buffering in soils is _____

the ability of a soil to resist changes in pH

reserve acidity equation

RA = residual acidity + exchangeable acidity

in total soil acidity, active acidity only represents ____

a small percentage of the total acidity

residual acidity has ____

Al and H tightly bound to clay and humus

salt replaceable or exchangeable holds _____

Al³⁺, Al(OH)_x ^y+ and H⁺ ions near clay and humus surfaces

active acidity has ____

Al³⁺, Al(OH)_x ^y+ and H⁺ ions in the solution

biological effects of soil acidity include 

• Nutrient availability

  • Fe and Mn become more/less available

    • Mn levels can become toxic at low pHs

• Aluminum toxicity (at low pHs)

• Microbial effects

• Mobility of organic pollutants

At low pH __

• bacteria decreases 

• N, K, S decrease 

• Al, Fe, Mn increase 

at high pH ___

• Ca, Mg increase

• Al, Fe, Mn decrease

at pH of 6.5

P increases

bacteria are more sensitive to low pH than ____

fungi

soil liming definition

reducing the acidity of the soil by increasing the pH level

liming requirement (amount of lime to apply) is influenced by

• change in pH desired (target around ~6.5)

• buffering capacity of the soil

• purity and fineness of the liming material

liming requirement is based on

current active acidity and buffer pH

to be effective lime must be 

mixed into the soil

to raise pH to target level, more lime is added to soil with _____

higher CECs and lower initial pHs

lime should be applied ____

6-12 months ahead of most pH sensitive crops and reapplied every 3-5 years

other means to ameliorate the effects of high soil acidity include adding ____

1. soil organic matter (humus)

2. gypsum

3. calcium-rich chicken manure 

saline soils have ___

high salt content

sodic soils have ____

high percentage of exchangeable Na⁺

alkalinity arises due to _____

lack of leaching (high % base saturation) and presence of carbonate/bicarbonate anions from lime or carbonate minerals

classes of salt/sodium affected soils

1. saline soils

2. sodic soils

3. saline-sodic soils

salinity is determined by _____

measuring the electrical conductivity of the soil solution, generally a saturated soil paste extract.

soil sodicity is commonly expressed as ___

either exchangeable sodium percentage ESP or sodium adsorption ratio SAR

soils with an ESP _____ are considered sodic

>15

majority of salt affected soils lie ___

in the vast dry regions of the western US

in saline and saline-sodic soils, osmotic potential of the soil water is ___-

lowered (more negative) making it more difficult for plants to take up water (additive with matric potential)

for plant growth in saline and saline-sodic soils, __

Seed germination is often greatly reduced and Nutrient imbalances are common

salt in irrigation water can accumulate thus, 

the drainage water added to groundwater is 4x more concentrated than the original irrigation water.

shallow-rooted annual crops can develop ____

saline seeps

For saline soils, excess salts are removed by ____

careful leaching with irrigation water that is low in soluble salts

For sodic and saline-sodic soils, reclamation is a 2 step process:

1. Displacement of sodium, generally by application of gypsum

2. Removal of sodium ions and any excess salts by careful leaching with irrigation water

____ can alleviate sodicity

gypsum