Manual for describing land degradation indicators

Table of Contents

1. Editor
2. Introduction
3. Classification of indicators
4. Land degradation indicators
5. Evaluation and short-list of indicators
6. Explanations for filling the information sheets
   1. General information
   2. Climate indicators
   3. Water indicators
   4. Soil indicators
   5. Vegetation indicators
   6. Water runoff indicators
   7. Forest fires indicators
   8. Agricultural indicators
   9. Cultivation indicators
   10. Husbandry indicators
   11. Land management indicators
   12. Land use indicators
   13. Water use indicators
   14. Tourism indicators
   15. Social indicators
   16. Institutional indicators
7. Farm survey research
8. Focus group approach
9. References

Soil indicators

Drainage: Soil drainage conditions are defined on the basis of the depth of hydromorphic features such as iron or magannese mottles or gray colors, and depth of the groundwater table. The following drainage classes are distinguished:

• Very well to well drained soils. Soils with any Fe or Mn mottles or gray colors at some depth greater than 100 cm from the soil surface. The soil is not wet enough near the soil surface or the soil does not remain wet during the growing period of the plants. Water is removed from the soil rapidly.
• Moderately well to somewhat poorly drained soils. Fe, Mn or gray mottles are present in the soil, at some depth between 30 and 100 cm from the soil surface. The soil is wet enough near the soil surface or the soil remains wet during the early growing period of the plants. Water is removed from the soil slowly.
• Poorly to very poorly drained soils. Mottles of Fe and Mn are present in the upper 30 cm of the soil, or gray colours of reducing conditions are present. A permanent water table usually exists at a depth greater than 75 cm. In some of these soils the ground water may reach to the surface during the wet period of the year. Water is removed from the soil so slowly that the soils are wet at shallow depth for long periods.

Parent material: It is defined using the geological map of the study area. The various types of parent materials are grouped into the following classes according to their petrology and mineralogical composition.

Rock fragments: Unattached pieces of rocks of 2 mm diameter or larger that are strongly cemented or more resistant to rupture are called rock fragments. They are present on the soil surface or distributed in various quantities into the soil body. Rock fragments are classified according to their diameter to the following categories: pebbles (diameter 2-75 mm), cobbles (diameter 75-250 mm), stones (diameter 250-600 mm), and boulders (diameter >600 mm). Rock fragments in the soil surface are defined according to the percentage cover in three classes: >40%, 15-40%, and <15%.

Slope aspect: can be defined by using a compass or by assessing the relative earth's surface with respect to the magnetic north. The following classes are defined: (a) NW, NE, (b) SW, SE, and (c) plain areas.

Soil texture: Soils are classified according to their texture in classes, and each textural class has a given range of sand, silt and clay. For practical value in agriculture 12 classes were designated: Sand (S), loamy sand (LS), sandy loam (SL), loam, (L), silt loam (SiL), silt (Si), clay loam (CL), sandy clay loam (SCL), silty clay loam (SiCL), clay (C), silty clay (SiC), and sandy clay (SC) (see attached figure). Four broad groups of classes are recognized: Sands, Silts, Clays and Loams. Sands contain at least 80% sand particles and 15% or less clay particles by weight. Silts contain at least 80% silt and 12% clay particles, respectively. Clays contain at least 35% of clay particles. Loams are mixtures of sand, silt and clay particles that exhibit the properties of those particles in equal proportions. Loam soils have the best combination of physical and chemical properties in terms of cultivation and crop growth.

The USDA textural triangle showing the limits of sand, silt and clay contents of the various textural classes.

Soil texture can be estimated in the field through the feel of a moist soil moulded between fingers and thumb. The soil texture by this method is found in a qualitative way. The standard analysis of sand, silt and clay content involves dispersion of mineral particles after destroying the organic matter. The size classes for sand are separated using sieves and the silt and clay classes by sedimentation. Finally the mass in each particle class is determined.

Water storage capacity: Soil water storage capacity corresponds to the amount of water which can be stored into the soil. It refers to the amount of water which is available for the plant growth. If this water content becomes too low, plants become stressed. The plant available moisture storage capacity of a soil provides a buffer which determines the plant's capacity to withstand dry conditions.

The terms saturation, field capacity, wilting point, available water, evapotranspiration, rooting depth, irrigation, plant's ability to extract water, soil organic matter content, previous irrigation pattern, stage of growth and water demand of a plant, have a direct influence on the water holding capacity and irrigation demands. Saturation refers to the situation when the soil's pore spaces are filled with water. Field capacity refers to the situation when excess water has drained out due to gravitational pull. Permanent wilting point refers to the situation when a plant wilts beyond recovery due to a lack of water in the soil. Available water is the amount of the water held in a soil between field capacity and permanent wilting point. The following classes have been defined: <50 mm, 50-100 mm, 100-200 mm, 200-300 mm, and >300 mm.

Exposure of rock outcrop: Exposure of rock outcrops refers to a miscellaneous area which consists of spots of exposures of bedrock and soil. A kind of area having little or no soil supporting spare or no vegetation. Rock outcrops are defined according to the percentage cover in four classes: >60%, 30-60, 10-30% , <10%.

Organic matter in surface horizon:The primary source of soil organic matter is plant material. Soil organic matter can be determined by chemical analysis of soil samples of fine earth (materials pass through a 2 mm sieve). The dichromate method is recommended for organic carbon determination. The following classes are defined: (a) high organic matter content (6% OM), (b) medium (2.1-6.0% OM), (c) low (2.0-1.1% OM), and (d) very low (<1.0% OM).

Degree of soil erosion: The present situation of degree of erosion can be characterised, according to: (i) the presence or not of the A-horizon, (ii) the existence and percentage of eroded spots, (iii) the degree of exposure of the parent material on the soil surface, and (iv) the presence of erosional gullies. Five classes of degree of erosion can be distinguished, very severe, severe, moderate, slight, and no erosion. The degree of soil erosion is described using the following table.

Electrical conductivity: Salt content of the soil can be assessed by measuring the electrical conductivity in a saturated soil paste. Salinization is a major process that causes salt accumulation in soils and it is related to climate and irrigation. The climate acts directly through high evaporation rates, and indirectly as the driving force behind soil salinization associated with irrigation. A distinction can be made between primary and secondary Salinization processes. Primary salinization involves accumulation of salts through natural processes such as physical and chemical weathering and transport processes from salty geological deposits or groundwater. Secondary Salinization is caused by human interventions such as inappropriate irrigation practices, use of salt-rich irrigation water and/or poor drainage conditions. Electrical conductivity can be measured directly in the field using an electrical conductivity-probe set for salinity measurements or in the laboratory on a saturated paste. The following classes of soil electrical conductivity are used: free of salts (EC< 2 dS m^-1), slightly affected soils (EC = 2 to 4 dS m^-1), moderately affected soils (EC = 4 to 8 dS m^-1), highly affected soils (EC = 8-15 dS m^-1), and very highly affected soils (EC>15 dS m^-1).