Identifying critical limits for soil quality indicators in agro-ecosystems

doi:10.1016/S0167-8809(01)00252-3

Agriculture, Ecosystems & Environment

Volume 88, Issue 2, February 2002, Pages 153-160

https://doi.org/10.1016/S0167-8809(01)00252-3 Get rights and content

Abstract

The maintenance of soil quality is critical to environmental sustainability. Although, several papers have been published on this subject, progress in soil quality monitoring has been slow. Knowledge and assessment of changes (positive or negative) in its status with time is needed to evaluate the impact of different management practices. Selection of key indicators and their critical limits (threshold values), which must be maintained for normal functioning of the soil, are required to monitor changes and determine trends in improvement or deterioration in soil quality for various agro-ecological zones for use at district, national and global levels. Many soil indicators interact with each other, and thus, the value of one is affected by one or more of the selected parameters. Interdependence of pH and nutrient availability, electrical conductivity and infiltration, etc. has been well documented by many researchers. Some researchers have proposed procedures for evaluating soil quality functions by combining and integrating specific elements into soil quality indices. These procedures allow for weighting of various functions, depending upon the user goals and socio-economic concerns.

Although, selection of soil indicators will vary with societal goals, the followings seem to be suitable indicators for crop production in most cases: organic matter, topsoil-depth, infiltration, aggregation, pH, electrical conductivity, suspected pollutants and soil respiration. Crop yield can be used as an integrator of the foregoing soil indicators. A minimum set of data on soil indicators must be identified to develop meaningful soil quality assessment. Also, monitoring soil indicators needs to set up sampling strategies allowing assessment of changes in soil quality which might be hidden by soil heterogeneity, by seasonal fluctuations or by analytical uncertainties. This paper describes the guidelines that can be followed to identify critical limits for the key indicators and the procedure for monitoring changes in soil quality trend.

Introduction

A significant decline in soil quality has occurred worldwide through adverse changes in its physical, chemical and biological properties and contamination by inorganic and organic chemicals. In the past half a century, about 2 billion of the 8.7 billion ha of agricultural land, permanent pastures, and forests and woodlands have been degraded. The rate of growth of global grain production dropped from 3% in the 1970s to 1.3% in the 1983–1993 period, and one of the key reasons of this decline is inadequate soil and water management (Steer, 1998).

Concerned by the decline in soil quality, and in an attempt to reverse this trend, Dennis Keeney, director of the Leopold Center for Sustainable Agriculture (IO, USA), calls for an enactment of a national soil quality act, similar to the water and air quality legislation with an emphasis on a strong, co-ordinated research-demonstration-incentives approach. When soils are degraded to the level that they can no longer perform their ecosystem functions, restoration is slow, expensive, and uncertain. “How many waste sites have been truly reclaimed?” “How many salt slicks made productive?” Asks Keeney. Any nation or state that supports an ecosystem that degrades soil is not sustainable (Keeney, 1999).

In 1996, a review by the Consultative Group of International Agricultural Research (CGIAR) of 14 international research centers found good progress in reorienting their research towards soil and water management, but it also found inadequate attention paid to off-site interactions at the river basin and regional levels (Steer, 1998). This was of particular concern since, off-site costs of unsustainable management practices are often greater than their impacts on on-site productivity.

The soil, like air and water, is an integral component of our environment, and together with water constitutes the most important natural resource. The wise use of this vital resource is essential for sustainable development and feeding the growing world population. In the past decade, several studies have dealt with the selection of suitable criteria for assessment of soil quality. However, monitoring of changes in soil quality, resulting from various management systems, have been slow. Selection of key indicators and their threshold values, which must be maintained for normal functioning of the soil, are required to monitor changes (direction, rate, magnitude, extent, etc.), and determine trends in improvement or deterioration in soil quality for various ecosystems.

The objectives of this paper are: (1) to review work done in the last decade on indicators for soil quality assessment; (2) to propose guidelines that can be followed to identify critical limits for the key indicators, and (3) describe a procedure for monitoring changes in soil quality trends.

Section snippets

Defining soil quality

Many definitions of soil quality have been proposed in the last 10 years (Arshad and Coen, 1992, Doran and Parkin, 1994, Karlen et al., 1997) with similar elements. The most recent, proposed by Karlen and a committee for the Soil Science Society of America is as follows: “the fitness of a specific kind of soil, to function within its capacity and within natural or managed ecosystem boundaries, to sustain plant and animal productivity, maintain or enhance water and air quality, and support human

Soil quality indices

Three basic components of a soil quality index were proposed at the International Conference on the Assessment and Monitoring of Soil Quality held at the Rodale Institute (Rodale Institute, 1991). The components were: (1) the ability of soil to enhance crop production (productivity component); (2) the ability of soil to function in attenuation of environmental contaminants, pathogens, and offsite damage (environment component); and (3) the linkage between soil quality and plant, animal and

Soil quality indicators

Soil quality indicators refer to measurable soil attributes that influence the capacity of soil to perform crop production or environmental functions. Attributes that are most sensitive to management are most desirable as indicators. In a given agro-climatic region, the measurable soil attributes that are primarily influenced are: soil-depth, organic matter, respiration, aggregation, texture, bulk density, infiltration, nutrient availability and retention capacity. A minimum number of

Assessment of soil quality

Changes in soil quality can be assessed by measuring appropriate indicators and comparing them with desired values (critical limits or threshold level), at different time intervals, for a specific use in a selected agro-ecosystem. Such a monitoring system will provide information on the effectiveness of the selected farming system, land use practices, technologies and policies. A farming system or policies that contribute negatively to any of the selected indicators could be considered

Models to assess soil quality

The development of relationships between soil attributes and soil functions may be a monumental task. However, algorithms in existing simulation models (e.g. NLEAP, EPIC, CREAMS, WEPP) may serve a useful starting point (Doran and Parkin, 1994). The models provide a predictive tool about the process such that given what we know, if we change one of the parameters that affect the process, we can predict the change in outcome caused by the change in the parameter. We can often measure the things

Requirements for monitoring soil indicators

The following details in measuring changes in each selected indicator within a defined ecological zone required are as follows.

•
Direction of change –– positive or negative, increase or decrease, etc.
•
Magnitude of change — percent change over the baseline values.
•
Rate of change — duration: months, years.
•
Extent of change — percentage of the area being monitored i.e. what percentage of the farm or district has changed with respect to the selected indicator during a specified period.

Monitoring soil

Guidelines for monitoring soil quality

The following guidelines and steps are suggested to monitor soil quality.

1.
Divide the region or the country into different ecological zones.
2.
Select the ecological zone, farms or watershed with similar soil types.
3.
Define the goal or requirements for sustainability; the goal could be production of a crop or a group of crops, environmental protection or any other use.
4.
Select a set of indicators for the ecological zone, farms or watershed. Although, selection of soil indicators will vary with the

Concluding remarks

In order to quantify and evaluate changes in soil quality, various combinations of management practices and their interactions with different soil indicators must be understood. Case studies in different agro-ecological zones should be conducted, with emphasis on the quality of the data.

A minimum set of data on soil indicators and relevant sampling strategies must be identified to develop meaningful soil quality assessment and monitoring program.

Long-term experiments (10–30 years) should be

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