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  • Soil Degradation

    Ann Verdoodt

    Academic Year 2011-2012

    Compilation of course notes by Prof. Donald Gabriëls

    and new materials by Prof. Ann Verdoodt

    Baringo Rangelands, Kenya

    Photo by Stephen Mureithi

  • Soil Degradation | Introduction i

    Table of Contents

    1. Introduction ................................................................................................................. 1

    1.1. What is land degradation....................................................................................... 1

    1.1.1. Definition of land .......................................................................................... 1

    1.1.2. Ecosystem functions and services ................................................................. 1

    1.1.3. Definitions of land degradation ..................................................................... 2

    1.1.4. Causes of land degradation ........................................................................... 3

    1.1.5. Extent of land degradation ........................................................................... 4

    1.2. What are the main land degradation types and processes? .................................... 4

    1.2.1. Soil degradation ............................................................................................ 5

    1.2.2. Vegetation degradation ............................................................................... 11

    1.2.3. Water resource degradation ........................................................................ 12

    1.2.4. Soil and water pollution .............................................................................. 13

    1.3. Why should we combat land degradation? ........................................................... 13

    1.4. How to assess land/soil degradation? ................................................................... 15

    1.5. How to remediate land/soil degradation .............................................................. 15

    2. Soil degradation assessment ...................................................................................... 17

    2.1. Conceptual frameworks ....................................................................................... 17

    2.1.1. DPSIR ......................................................................................................... 17

    2.1.2. Ecosystem services framework.................................................................... 19

    2.1.3. Sustainable livelihoods approach ................................................................ 21

    2.2. Soil quality Indicators ......................................................................................... 24

    2.2.1. Definition .................................................................................................... 24

    2.2.2. Characteristics of good indicators ............................................................... 25

    2.2.3. Selection of indicators ................................................................................. 27

    2.2.4. Interpretation of indicators ......................................................................... 29

    3. Soil structural degradation ........................................................................................ 32

    3.1. Process of aggregate formation ............................................................................ 33

    3.1.1. Physico-chemical processes ......................................................................... 33

    3.1.2. Activities of soil organisms ......................................................................... 35

    3.2. Factors affecting aggregation............................................................................... 36

    3.2.1. Impact of raindrops ..................................................................................... 36

    3.2.2. Flooding ...................................................................................................... 36

    3.2.3. Drying and wetting ..................................................................................... 37

    3.2.4. Freezing and thawing ................................................................................. 37

    3.2.5. Biotic factors ............................................................................................... 37

    3.2.6. Influence of Tillage ..................................................................................... 37

    3.2.7. Soil amendments ........................................................................................ 38

    3.3. Assessment of soil structure ................................................................................ 38

    3.3.1. Laboratory methods .................................................................................... 39

    3.3.2. Other indices............................................................................................... 41

    3.4. Impact of soil structural degradation ................................................................... 42

    4. Soil compaction .......................................................................................................... 44

  • Soil Degradation | Introduction ii

    4.1. Soil compaction process ....................................................................................... 44

    4.1.1. Soil strength ............................................................................................... 44

    4.1.2. Factors affecting soil strength ..................................................................... 45

    4.1.3. Propagation of external stress with depth................................................... 45

    4.2. Susceptibility to compaction ................................................................................ 46

    4.3. Resilience to compaction ...................................................................................... 48

    4.4. Causes of soil compaction .................................................................................... 49

    4.5. Consequences of soil compaction .......................................................................... 50

    4.5.1. Influence on soil physical properties ........................................................... 50

    4.5.2. Influence on soil chemical properties .......................................................... 51

    4.5.3. Influence on soil biological properties ......................................................... 51

    4.5.4. Influence on soil functions .......................................................................... 52

    4.6. Assessing soil compaction .................................................................................... 52

    4.6.1. Field observations ....................................................................................... 52

    4.6.2. Soil profile descriptions ............................................................................... 52

    4.6.3. Direct laboratory methods .......................................................................... 53

    4.6.4. Indirect methods ......................................................................................... 54

    4.7. Prevention and remediation ................................................................................ 56

    4.7.1. Preventing soil compaction ......................................................................... 56

    4.7.2. Remediating soil compaction....................................................................... 56

    5. Soil surface sealing and crusting ................................................................................ 58

    5.1. Sealing versus crusting ........................................................................................ 58

    5.2. Types of crusts ..................................................................................................... 58

    5.3. Surface sealing and crusting process ................................................................... 59

    5.3.1. Morphology of structural crusts .................................................................. 59

    5.3.2. Morphology of depositional crusts ............................................................... 60

    5.3.3. Formation of soil crusts............................................................................... 60

    5.4. Factors affecting slaking and deflocculation ........................................................ 62

    5.4.1. Rainfall factor ............................................................................................. 62

    5.4.2. Weather ...................................................................................................... 62

    5.4.3. Soil Properties ............................................................................................ 62

    5.4.4. Antecedent soil moisture and wetting rate.................................................. 64

    5.4.5. Microrelief................................................................................................... 65

    5.5. Characterisation of soil crusts ............................................................................. 65

    5.5.1. Direct methods ............................................................................................ 66

    5.

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