The Global Assessment of Human
Induced Soil Degradation (GLASOD) methodology provide insight in the assessment
of soil degradation process. It is expert based assessment to develop map on
soil degradation is the GLASOD map, which used indicators of soil degradation
to assess the cause, severity, extent and trend in soil degradation through a
questionnaire (Oldeman et al., 1990). The experimental methodology frequently
used in soil degradation assessment based on soil quality parameters (Swanepoel et al.,
2014; Nezomba et al., 2015). The soil quality concept
involves measuring a set of soil properties which influenced by the soil management
practices (Karlen et al.,
1997).
Participatory approaches which based on the farmers’ local knowledge such as
perceptions of local peoples on the soil degradation processes and properties
also used to assess soil degradation (Lima et al., 2011). Participatory
Rural Appraisal (PRA) and household surveys using a house hold questionnaire
were also used as the research tools to assess soil degradation with the
integration of scientific evident (Malley et al., 2006).

2.1.3. Expert-based laboratory assessment of soil quality

Identification of soil quality
indicators

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Identification of appropriate
soil quality indicators is a first step in the assessment and evaluation of the
contribution of soils in the delivery of ecosystem goods and services (Robinson
et al., 2012). Thus, the need is to develop indicators for on-site assessment
of soil quality by farmers, researchers and extension personnel (Doran &
Parkin, 1994).

Soil structure is considered a key physical soil factor in the
functioning of soil, as its ability to support plant and animal life,
control  other soil functions (porosity,
water and air movement and retention in soil) and moderate environmental
quality (Bronick
& Lal, 2005). The soil structure assessment is divided
in to direct and indirect methods. Indirect characterization of soil structure
includes its estimation from soil properties such as infiltration rate, hydraulic
conductivity, soil aggregation, bulk
density (BD) ), available water content and pore-size distribution (Pagliai
et al., 2004). Direct methods involve observation of
morphological structural features by microscopy, analysis of images like city scans for quantification of spatial pores
arrangement, measuring soil aggregation and aggregate stability under
laboratory conditions, and visual field description of structural form (Pagliai
et al., 2004).

The
soil chemical properties (indicators of soil quality) are cation exchange
capacity (CEC) (García-Ruiz et al., 2008), available K, soil organic matter (SOM)
(Qi et al., 2009), Total organic carbon (Rojas
et al., 2016), soil pH (Zornoza et al., 2007) and
total nitrogen content are frequently used. These
properties are easy to measure in the laboratory, avoiding expensive and
time-consuming methods required to analyses other rather more complex
biochemical properties, such as microbial biomass or enzyme activity,
among others. Besides, SOM is the most used indicator and important in soil
quality assessment (Zornoza et al., 2015).

 

Evaluation of soil
quality indicators

The evaluation and identification of soil variables
that use some level of control or impact over numerous soil properties can provide
the best return on management in a soil monitoring program. Multivariate analysis is use to evaluate the relationships
between indicators, detecting main scopes of indicators and targeting on linear
relationships between variables (Husson et al., 2011).
It is used to determine minimum indicator set (MDS) through principal component
analysis (PCA) for different soils indicators (Qi et al., 2009).

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