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Desert Soils - Part 2

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Soil Formation in a Moist

Environment

Water,Clay particles,Organic matter,

Salts

Water

Water,

Salts

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Water transportssalts, clays, and

organic materialsin the soil profile

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Soil Formation in an Arid

Environment

Na+

Na+

K+

Na+

Mg++

Ca++Ca++ Ca++Ca++ SO4=

Na+K+ Na+Na+

H2O

H2OH2O

H2O

H2O

H2O

H2O

H2O

HCO3-

SO4=

SO4=

Cl-

Cl-

Cl-

Cl-

H2OH2O H2O

H

H2OH2ONa+

H2O

CO3=

HCO3-

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Water Transports Salts

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Caliche

A layer of cemented Ca or Mg carbonates

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Soil Horizons

Soil horizons or layers are formed by: Addition of organic materials

Movementof salts, clays, organic materialsilluviation (leaching)

eluviation (deposition)

Loss of salts (leaching)

Transformations

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Soil Horizons

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Arid Soil Horizons

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Properties of Desert Soils

Shallow limited rainfall and soil development

Low organic matter content*

High pH

accumulation of carbonates

Caliche

cemented soil layers High levels of soluble salts

limited leaching

high evapotranspiration

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Soil Organic Matter Content

Highorganicmatter

soils

Loworganicmatter

soils

slow plant growth in arid climate

rapid decomposition in warm soils

rapid decomposition in

well-drained soils

rapid decomposition in tilled soils

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Soil Organic Matter

Organic wetland soil Well-drained mineral soils

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Soil Biota & Aerobic Respiration

C6

H12

O6

+ 6 O2

6 H2

O + 6 CO2

Oxygen (gas)

Organic material

Carbon dioxide (gas)

Water

Soil microbes

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Acidity

O H

H

H+ (Acid)

OH- (Base)

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The pH Scale

Neutral

Range of AlkalinityRange of Acidity

0 2 4 6 8 10 12 14

Plant Growth

Each pH unit is 10 times more acid or alkalinethan the next unit

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Soil pH

Most arid western soils

Optimum for most plants

0 2 4 6 8 10 12 14

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Alkalinity in Arid Soils

Na+

Na+Na+

Mg++

Ca++

Ca++

Ca++Ca++Na+Na+

H2

O

H2OH2O

H2OH

2

O

H2O

H2O

H2O

HCO3-

H2OH

2O

H2O

H

H2OH2ONa+

H2O

CO3=

HCO3-

HCO3--

CO3=

CO3= HCO3

-Ca++

Ca++ Ca++

CO

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Alkalinity in Arid Soils

Calcium bicarbonate

Ca++

HCO3- + H2

OCa++

OH-

(moderatelystrong base)

CO2

High pH (8

HCO3-

H2O

OH-

CO2

i l l i

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Essential Plant Nutrients

C - Carbon

H - Hydrogen

O Oxygen

* Cations-Anions

MacroN Nitrogen-

P Phosphorus-

K Potassium*S Sulfur -

Ca Calcium*Mg Magnesium*

MicroFe Iron*

Mn Manganese*Cu Copper*Zn Zinc*B Boron-

Cl - Chloride

Mo - Molybdenum

Essential: A plant can not complete its life cycle withoutthese nutrients

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Soil pH

Alters nutrient availability

Affects microbial activity

Can affect disease susceptibility

Controls organic matter solubility

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Effects of pH on Nutrient Availability

The thicker the bar, the more available the nutrient

4.0

5.0

9.0

8.0

7.0

6.0

8.5

7.5

6.5

5.5

4.5

4.0

5.0

9.0

8.0

7.0

6.0

8.5

7.5

6.5

5.5

4.5

pH pH

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Salt-Affected Soils

Saline - excess salts

good structure

moderate pH

Sodic - excess Na

poor structure

high pH (>8.5)Saline-sodicexcess salts

excess Na

good structure

high pH

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Tests for Soil Salts

SAR - sodium adsorption ratio (SAR>13=sodic soil ESP - exchangeable sodium percentage

(ESP>15=sodic soil) measures of the amount of sodium

used to identify sodic soils EC - electrical conductivity

measures all soil salts used to identify saline soils

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Salts Affect Soil Structure

Na+- -

-

--

- -- -Na

+

Na+

- - -----

--

Na+Na+

Na+

Na+ Na+Ca

++

- - ---

--

--

- - ---

--

--

Ca

++

- - ---- -- -

Na+ Na+- - ---

--

--

A little sodium makes particles repel one another.

A lot of sodium or a little calcium

makes particles attract one another.

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Salts Affect Soil Structure

0

10

20

30

40

50

0.5 1.0 1.5 2.0 2.5 3.0

Electrical conductivity (dS/m)

ES

P

Unstablesoil

Stablesoil