28 Soil Quality Analysis

Meenakshi Nandal

29.1 Introduction

Maintaining the productivity of our agricultural soils is of principal importance not only for the present but for the future also. The analysis of soil helps in assessing the need to apply nutrients in fertilizers and/or manures to preserve soil fertility.A wide range of agricultural management practices are adopted with goal to improve soil quality, crop yield, and decrease the ecological foot print. The management practices can either progress or reduce soil quality. Soil quality can be degraded by erosion, compaction, salinization, sodification, acidification, and pollution with toxic chemicals.By increasing soil protection through crop residues and plants, adding organic matter to the soil, through crop rotations, manures, careful management of fertilizers and pesticides we can improve soil quality etc.Soil quality and soil health are interchangeable terms. Soil quality is considered as the capacity of a soil to function and health is most often used to highlight the linkage between soil and human or animal health. Soil quality is the ability of soil to provide ecosystem and society services through its carrying capacities and respond to external influences and cannot be measured directly in the field or laboratory as it is a complex functional concept. Hence,a range of soil parameters or indicators has been identified to estimate soil quality. However, it is often related to the management goal and practices as well to soil characteristics.Thus, a mathematical or statistical framework was put forward in early 1990’s to estimate soil quality index (SQI) which can be used as an indicator of the ‘goodness’ of soil with regards to functions and responses. The SQI was assessed so that the management goals are not only focused on productivity, but also on environmental issues. Further SQI may serve three important goals: environmental quality, agronomic sustainability, and socio-economic viability. Soil quality or its capacity to function can be evaluated by using inherent and dynamic soil properties. Inherent soil properties like soil texture, type of clay, bedrock form over thousands of years and result primarily from the soil forming factors: climate, topography, parent material, biota and time and are not easily affected by management practices. On the other hand dynamic or management dependent, soil properties are affected by human management and natural disturbances. Significant changes can be observed in dynamic soil properties in a single year or growing season.The functions of soil get affected by soil indicators and further depend on their physical, chemical and biological categories.Since a soil property or indicator can affect multiple soil functions or categories, however these categories are not always clearly defined.

The concept of soil quality is more complex than that of water or air quality analysis. However, despite the complexity the maintenance of soil quality is significant for ensuring the sustainability of the environment and the biosphere. The method for evaluating soil quality is designed fully flexible in order to link it with the evaluation of degradation threats.For assessing the quality of the soil system consideration of soil threats is essential. Some of the major threats faced by soils are

Soil erosion

Soil contamination

Decline in organic matter and biodiversity Soil compaction

Soil salinization

Floods and landslides Soil sealing

Sometimes these threats are place specific for example with respect to restoration of soil quality soil erosion is the most severe consequence of soil degradation in some of the place and controlling erosion is a requirement of a healthy soil. Most of the soil degradation processes are interlinked and are often related by similar contributory factors.Understanding the soil quality process means managing the soil so that it functions optimally now and is not degraded for future use.Three main indexes in the sustainable soil-use domain is calculated based on the evaluation procedure:

• 1) Soil Quality Index: It is expressed as the ability of soil to execute ecosystem and social services

• 2) Soil Threat Index: It definesthe intensity of hazard on which the soil is exposed to degradation threats.

• 3) Soil Sustainability Index: This parameter is used for the comparative measurement of soil quality across a gradient of stress or disturbance

The effects of management practices on soil function can be learnt through soil quality evaluation and the evaluation fall into three categories:

Awareness and education: An ecological approach to land management depends upon soil quality concept. In complex systems such as soil, management actions don’t have simple, single effects whereas it has multiple effects, both direct and indirect. For example, tillage is used to loosen surface soil, prepare the seedbed, and control weeds and pests.On the other hand tillage can also cause the disorders like soil structure fracturing, increase the decomposition and loss of organic matter, increase the risk of erosion, destroy the habitat of helpful micro biota and cause compaction.Adopting the better existing management options is a first step towards improved land management and public policy. Assessment as an Educational Tool includes one-on-one and field day use of in-field testing tools.

Evaluation of practice effects and trouble-shooting: Soil quality is often referred to as “Soil Health” because of objectives similar to the monitoring and maintenance of human health. Monitoring health indicators and watching for irregularities or declines in health status doctors can judge the state of individual health. During a check-up the set of health indicators measured is familiar to all of us: temperature, pulse, blood pressure, heartbeat, blood and urine sample analyses etc. By monitoring these indicators we can reveal potential problems well in time before the onset of painful symptoms; the earlier problems are detected the easier they are treated. Likewise, soil indicators reveal irregularity or decline over time and provide a signal for management aspect to be taken into account.Although soil fertility testing already serves this role in regard to plant nutrition, soil quality assessment expands this to include the wider range of soil functions and environmental outcomes.Soil quality measurement is one of the ways to investigate specific problems related with soil. The low nutrient status may only be one cause of low productivity in a specific area out of many, or indeed a symptom rather than a cause.

Over time when soil quality is assessed, it can put into picture about the sustainability of management practices.

Evaluation of alternative practices: Beyond awareness and evaluating current practices, soil quality assessment methods provide a framework for comparing management tradeoffs and deciding which management options provide the greatest good, whether for one’s farming operation or at a watershed or regional scale.

29.2 Assessment as an adaptive management tool:

Soil quality assessment tools allow one to examine the effects of making a change in management before actually committing full resources to that change. Soil quality assessments can be used to compare the effects of management practices on similar soils.

29.3 Soil Quality Indicators

Any indicator criteria used to assess soil quality should be practical and useful across a range of ecological and socio-economic situations. As per Chen, 1999, the Soil quality indicators should meet the following criteria:

1. Correlate and encompass natural processes in the environment
2. Integrate soil physical, chemical and biological characteristics and processes
3. Be easy to use and accessible for a broad range of users and field conditions
4. Be sensitive to changes in management or environmental conditions
5. Be present in existing environmental databases when possible

Cameron et al. (1998) proposed the application of these steps to a simple scoring system to decide, whether to accept a possible soil quality indicator in the assessment of polluted soils.

A = L(S, U,M,I,R)

Whereas       A = Acceptance level of the indicator

S = Sensitivity of the indicator to changes from degradation or remediation U = Ease of use and/or understanding

M = Cost effectiveness of measurement of the indicator

I = Predictable influence of properties on soil, plant and animal health R = correlation and relationship to ecosystem processes

In this equation each parameter is rated between 1 and 5.

The sum of all the parameters gives the acceptance level, which can then be compared to other indicators. By applying this system screening of the most useful and vigorous soil quality indicators for a specific assessment can be identified. A soil quality indicator scoring level of 23 would be more valuable compared to an indicatorwith a level of 15.By evaluating indicators such as physical, chemical, and biological properties of soils assessments of soil quality can be conducted (Table 29.1).Morphological or visual features as indicators of plants are considered to observe management induced changes in the soil.

Table 29.1 Minimum Data Set of Indicators for Soil Quality

(Adapted from Doran et al, 1996; Larson and Pierce, 1994; and Seybold et al, 1997)

Different chemical indicators widely used are related to the respective basic functions they measure. Those basic functions include:Promoting biodiversity activity and productivity,

Filtering, buffering, degrading, and detoxifying organic and inorganic materials, Controlling the regulation and partition of water and solute flow, Cycling carbon and nutrients, Providing physical stability for plants and animals  as well  as providing support for structures associated with human habitats (Fig 29.2)

The chemical components and properties of the soil affect many reactions and processes occurring in the soil environment. For example, soil pH plays important role as

1. It controls the solubility and mobility of heavy metals, such as Al, Fe, Mn, Cu, and Zn, and nutrients, such as phosphorus. It also controls the toxicity of many heavy metals.
2. It affects soil biological properties like microbial growth and diversity.
3. Other properties like percent saturation, soil buffering capacity, cation-exchange capacity (CEC) are influenced by pH.

Like physical and biological indicators chemical indicators are also being affected by soil management and natural disturbance like

Tillage practices (e.g. continuous till, conservation till, and organic and inorganic amendments) may change the levels of soil reaction (pH), as well as nitrate, TOC, and P content.Without the correction of pH by liming can lead to soil acidification during continuous cultivation processes.

To lowers soil pH values of alkaline soils a continuous application of acidifying fertilizers, such as ammonium nitrate NH4NO3, ammonium sulfate (NH4)2SO4, and elemental sulfur (S) is being added.

Soil can slightly acidify by leguminous crops

Soil salinizationalso increases with irrigating water having high amounts of salt concentrations (high EC values).Soil quality can be assessed at three points of the soil system shown in Figure 29.3.

Source- Accessing the soil system, Ann Lewandowski, 1999 The insight into the living component of the soil is being provided by the soil biological indicators. Biological, physical and chemical indicators have a relationship to soil functions and can evaluate soil functions to assess soil quality and these indicators are dynamic properties of soil that are very susceptible to land management, natural disturbances, and chemicalcontaminants. Doran and Safley (1997) and Pankhurst et al. (1997) suggested that an indicator, regardless of its nature, must fulfill the following criteria:

Be interpretable  Correlate well with ecosystem processes Integrate soil physical, chemical, and biological properties and processes Be accessible to many users Be sensitive to changes.

In addition, an indicator must have reproducibility, low temporal and spatial variability, and simple sampling and analytical methods. There are myriads of organisms in the thin layer of the soil surface. They play key roles in the decomposition of soil organic matter, nutrient cycling, soil pollutant degradation, and the formation and stability of soil structure. They adapt to changes in their environment, such as stress due to drought, flooding, substrate shortages (e.g. food shortages), and contaminants. To soil management and land use changes soil biota responds rapidly and are considered suitable soil quality indicators. However, measuring soil organisms directly as indicators of soil quality have their own limitations. For the biological dynamic properties of soil (respiration, particulate organic matter, potentially mineralisable nitrogen), enzymes are often selected as surrogates for measurement of processes mediated by soil biota. Biological indicators may reflect the overall number, type, and activity of microorganisms and the diversity of the living organisms in soil, particularly the microbial population. Earthworms have been considered usable biological indicators as they are not often diverse and are easy to count in the field by determination of their abundance. Earthworms are native to non-glaciated areas of North America, but non-native species from Europe and Asia also exist in this areas.Top soil that is rich in organic matter provides home to mineral soil-dwellers. Deep soil-burrowers (night crawlers) dig long, large burrows into deep soil layers. They carry with them plant residues for consumption. They burrow narrow channels and feed on a mixture of soil and plant residues. The digested material that is excreted back into the soil is known as earthworm cast. While passing through the worm’s digestive system cast is enriched with nutrients (N, P, K, and Ca) and microorganisms. Nutrient cycling and intense microbial activity occurs in fresh cast. Earthworms not only help in building of soil structure and aggregate stabilization but also contribute nutrients to the soil and improve porosity, tilt, and root development and serve to be the best visible bio indicators towards any disturbance in the soil.

Summary

Integration of soil quality measures to policy planning is essential to harmonize efforts of soil resources utilization and environmental resources conservation.Ability of soil to provide ecosystem and social services through its capacities to perform key functions and respond to externalinfluences depends upon soil quality.For soil quality soil structure is an important component; therefore, soil quality gets affected by management factors and soil structure most likely.The formation of water stable aggregates and bio pores are the two basic soil structure components which gets influence by the nutrient cycles which thereby gets influence by the processes acting on soil quality parameters like biological, chemical, and physical. Soil organic mattertransformations lead to primary process changes linking to nutrient cycling and soil structure, and therefore, influencing soil quality. By observing or measuring several different properties or processes soil quality is estimated. For indexing of soil quality no single property can be used. The selection of indicators should be based on the land use process; the connection between an indicator and the soil function can be assessed depending on the simplicity and consistency of the measurement; variation between sampling times and variation across the sampling area; the sensitivity of the measurement to changes in soil management; compatibility with regular sampling and monitoring; the skills required for use and analysis. Because of differences in parent material, climatic condition, topographic or landscape position, soil organisms, and type of vegetation; soils and their indicator values vary. For example, cation exchange capacity may relate to organic matter, but it may also relate to the kind and amount of clay. Soil quality has also been described as the balance between soil degradation and soil resilience. Preserving and improving soil quality is about sustainability. It is about maintaining the long-term function of our soils.Quality soil will produce healthy crops over the long-term without increasing levels of inputs. It will also control water flow and will filter and degrade potential environmental contaminants. Against wide swings in temperature, moisture and other environmental conditions healthy soils always remain buffered. This buffering capacity can be reflected in low levels of pest outbreaks control and comparatively achieving stable production levels. The first step in the development of systematic criteria of sustainability includes the minimum data set criteria. The setting of soil quality as the long-term goal for the management of soil has implications in national-level assessments, design of programs for conservation of soil resources and for analyses of sustainable farming systems.

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