Soil Quality â€" Definition and Assessment
Authors: Rajendra Kumar Yadav, Malu Ram Yadav and Chiranjeev Kumawat
PhD. Scholar, Division of Soil Science & Agricultural chemistry, ICAR-IARI, New Delhi-110012

Soil quality has several definitions as an indefinable concept can be defined indefinitely. These fall under two categories- related to soil functions or soil use. For the former, Soil Science Society of America officially defined soil quality as “the capacity of a specific kind of soil to function, within natural or managed ecosystem boundaries, to sustain plant and animal productivity, maintain or enhance water and air quality, and support human health and habitation”. The most important functions include water flow and retention, solute transport and retention, physical stability and support, retention and cycling of nutrients, buffering and filtering of potentially toxic materials and maintenance of biodiversity and habitat. The latter one simply defines soil quality as “fitness for use” in agriculture, construction of roads, etc. So, the first definition emphasizes soil ecological services, whereas the second one specifies uses of soil according to soil’s environmental or industrial context.

After defining soil quality, the next task is to assess it. Soil quality assessment helps us to evaluate the condition of soil i.e. whether it is aggrading, sustaining or degrading under a particular land-use or management practice and based on that helps us to judge different land-use patterns and management practices. Periodic monitoring also helps us to check pollution related problems. Soil quality can be assessed by indicators which are the measurable soil properties having influence on its capacity to carry out a given function; and by indices that combine a group of properties targeting one or more functions. According to the United States Department of Agriculture, soil quality indicators can be visual, physical, chemical and biological.

Visual Indicators can be obtained through field visits, perception of farmers and local knowledge, e.g. subsoil exposure, colour, type of coverage, etc.

Physical Indicators like bulk density, texture, aggregate stability, porosity are related to particles and pores and influence root growth, emergence, water infiltration, etc.

Chemical Indicators like pH, salinity, organic matter content, heavy metals determine the nutrient cycling, mobility of contaminants, etc.

Biological Indicators include measurements related to micro and macroorganisms and biochemical properties e.g. enzyme activity. These are related to soil development and nutrient cycling.

We should seek for such indicators which are sensitive to variations of soil management, well correlated with the beneficial soil functions, helps in revealing ecosystem processes, integrative and comprehensible and cheap and easy to measure.

Apart from developing a reliable index, another problem to be solved is selection of reference criteria. For this we can either use the conditions maximizing soil productivity and environmental performance or the conditions prevailing in native soils. The first approach suffers from the drawback that soil quality determined for a single crop may not respond similarly to other crops. So, the quality of the same soil will vary from crop to crop. Whereas, the second option of taking native soil under climax vegetation offers a more widely applicable measure of soil quality with respect to sustainability as various properties of native soils have generally evolved without any disturbance towards a balanced state that ensures long-term viability of the surrounding ecosystem. But, the limitation of this concept remains in the fact that there might be no native soils left nearby for comparison.

Biological Indicators of Soil Quality

As the title of the current topic demands this manuscript will confine only into biological indicators (bio-indicators) which are thought to be more sensitive and integrative in nature than physical or chemical indicators. So far a total of 183 bio-indicators have been reported in different literatures. The common ones are the properties related to the size and activity of microorganisms, abundance and diversity of macroorganisms and biochemical properties like enzyme activities. The potential of microbial parameters as indicators of soil quality is attributed to their essentiality in organic matter decomposition and nutrient cycling and sensitivity to changes in soil environment in terms of changes in land-use, management or pollution. On the other hand, biochemical properties like enzyme activities contribute to nutrient release, reflect modifications to microbial community and are easy to determine. The emphasis on macroorganisms, mainly macrofauna is based several criteria. First, they are sensitive to environmental changes and shifts in their community structure offer an integrative measure of ecosystem impacts. Second, ecosystem engineers like ants, earthworms can have significant influence on soil and ecosystem functioning. Thus, their population reflects key ecological processes. Finally, large fauna are relatively simple to measure, ubiquitous and familiar to farmers as they are frequently encountered during field maintenance.

All these indicators can be used in three different approaches as given below-

i) Individual properties- consider a single soil biological property at a time. For example-microbial biomass C (MBC), microbial biomass N (MBN), microbial biomass P (MBP), absolute activities of several enzymes, etc.

ii) Simple indices- consider two properties at a time. For example-

a) Metabolic Quotient(qCO₂) : CO₂-C unit⁻¹ microbial biomass-C h⁻¹

b) Death rate quotient (qD) : [(Cmic)t₁ – (Cmic)t₂ / (Cmic)t₁]/t₂–t ₁, where t ₁ and t₂ = start and 35 days of incubation, respectively

c) Metabolic potential (MP) : DH-ase/WSOC

d) Biological index of fertility (BIF) : (DH-ase + k catalase)/2, where k is a proportionality coefficient

e) Hydrolysing coefficient (HC) : µmol of fluorescein diacetate hydrolysed/µmol of total fluorescein di- acetate before hydrolysis

iii) Complex indices- consider relation between more than two biological or biochemical properties at a time. For example- Geometric mean of enzyme activity (GMea), enzyme activity number (EAN), PCA based index, etc.

About Author / Additional Info:
Ph.D Scholar, Division of Soil Science and Agricultural Chemistry, IARI New Delhi ,India