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Selecting Indicators to Evaluate Soil Quality
Zueng-Sang Chen
Department of Agricultural Chemistry
National Taiwan University
Taipei, 10617, Taiwan ROC, 1999-08-01

Abstract

The main degraded and contaminated soils in Taiwan include highly acid soils, soils deficient in micro-elements, eroded and poorly drained sandy soils, soils suffering from water stress, compacted soils, and rural soils contaminated by trace elements and organic pollutants. Reliable and practical indicators of soil quality are needed to evaluate the condition of degraded and polluted soils. In this Bulletin, several indicators are selected for soil quality assessment in sustainable soil management systems, based on the concept of the control chart. The critical level (threshold level, an upper control limit (UCL), and a lower control limit (LCL)) represent the values within which soil quality must be kept for sustainable soil management.

Abstracts in Other Languages: 中文, 日本語, 한국어

Introduction

Definition of Sustainable Agriculture

The FAO/Netherlands conference on Agriculture and the Environment (FAO 1991) revised the original definition of "Sustainable Agricultural Development" defined by FAO in 1990 and translated it into several basic criteria to measure the sustainability of present agriculture and future trends. These criteria can be listed as follows:

  • Meeting the food needs of present and future generations in terms of quantity and quality and the demand for other agricultural products.
  • Providing enough jobs, securing income and creating human living and working conditions for all those engaged in agricultural production.
  • Maintaining, and where possible enhancing, the productive capacity of the natural resources base as a whole and the regenerative capacity of renewable resources, without impairing the function of basic natural cycles and ecological balance, destroying the socio-cultural identity of rural communities or contaminating the environment.
  • Making the agricultural sector more resilient against adverse natural and socio-economic factors and other risks, and strengthening the self-confidence of rural populations.

According to these criteria, the sustainable management of agricultural soils maintains the soil productivity for future generations in an ecologically, economically, and culturally sustainable system of soil management.

Multidisciplinary Aspects of Sustainable Soil Management

Sustainable soil management (SSM) must take a multidisciplinary approach. It is not limited only to soil science. Basically, we can consider three aspects of this management system (Steiner 1996):

  • Bio-physical aspects: Sustainable soil management must maintain and improve the physical and biological soil conditions for plant production and biodiversity.
  • Socio-cultural aspects: Sustainable soil management must satisfy the needs of human beings in a socially and culturally appropriate manner at a regional or national level.
  • Economic aspects: Sustainable soil management must cover all the costs of individual land users and society.

The concept of sustainable land management (SLM) can be applied on different scales to resolve different issues, while still providing guidance on the scientific standards and protocols to be followed in the evaluation for sustainable development in the future (Dumanski 1997). Based on this, sustainable soil management is the basis of sustainable land management, and sustainable land management is the basis of sustainable development (Dumanki 1997) ( Fig. 1).

Land Quality Indicators (LQIs) are being developed as a means of improving coordination when taking action on land-related issues such as land degradation. Indicators are already in regular use to support decision-making at a national or higher level, but few such indicators are available to monitor changes in the quality of land resources. We need more research into LQIs, including:

  • How to integrate socio-economic (land management) data with biophysical information in the definition and development of LQIs.
  • How to scale data for application at various hierarchical levels.

The quality of Taiwan's soil is Taiwan's future. The objectives of this Bulletin are to discuss the causes of soil degradation and polluted soils, especially in Asian countries, to select indicators of soil quality for degraded or polluted soils, and to discuss how Taiwan can achieve sustainable soil management.

Causes of Soil Degradation and Pollution

Causes of Soil Degradation

The most important challenge in the next century is nutrient depletion, deficiency, and erosion of soils (IBSRAM 1994). Major soil-related problems for sustainable soil management include:

  • Nutrient depletion and deficiency;
  • Soil erosion and degradation;
  • Socioeconomic prices and marketing;
  • Inefficient water use;
  • Faulty research methods;
  • Unsustainable farming;
  • Soil acidity;
  • Non-adoption by farmers of improved technology;
  • Competing uses for water;
  • Lack of organic matter;
  • Inadequate fertilizer use and management;
  • High compaction;
  • Seasonal drought; and
  • Water stress, waterlogging and poor drainage.

Causes of Soil Degradation in the Topics

The speed of soil degradation depends on different environmental factors, such as soil type, relief, climate and farming system. The UNEP (United Nations Environment Program) Project and GLASOD (Global Assessment of Soil Degradation) Project distinguishes four human-induced processes of soil degradation: water and wind erosion, plus chemical and physical degradation (Oldeman et al. 1990).

Soil erosion caused by water and wind is the most important form of degradation.

  • Soil loss due to wind erosion (28%);
  • Soil loss due to water erosion (56%);
  • Nutrient depletion due to inadequate fertilizer applications;
  • Soil acidification;
  • Salinization due to inadequate irrigation and drainage (12%);
  • Depletion of organic matter due to fast decomposition and insufficient organic fertilizer; and
  • Compaction, aggravated by the use of heavy machinery (4%).

The most important causes of water erosion are deforestation (43%), overgrazing (29%) and agricultural mismanagement (24%). The most important causes of wind erosion are overgrazing (60%), agricultural mismanagement (16%), over-exploitation of natural vegetation (16%) and deforestation (8%). The most important forms of chemical soil degradation are loss of nutrients and organic matter in South America, and salinization in Asian countries (Oldeman et al. 1990).

Physical Degradation

Compaction, hardpans and crusting are three major causes of physical degradation (Steiner 1996). Soil compaction is an increase in bulk density caused by external loading, leading to a deterioration in root penetration, hydraulic conductivity, and aeration. There are many ways of reducing soil compaction. Hardpans are common in alluvial plains in semi-arid areas with a pronounced rainy season. Crusting is due to the destruction of aggregates in the topsoils by rain, and is closely linked to soil erosion. Crusting reduces infiltration and promotes water run-off.

Chemical Degradation

About 36% of tropical soils are low in nutrient reserves. Acidification produces aluminum and ferrous oxides. This in turn results in the fixation of phosphorus, which is no longer available for plants. A ferrous oxide/clay ratio of > 0.2 is considered to be the threshold for P fixation, and affects 22% of all tropical soils. This problem also occurs in Andisols, Ultisols, and Oxisols in the humid tropics and tropical highlands.

About 30% of tropical land problems occur in highly acidic soils which contain phyto-toxic aluminum (Al) in the soil solution. This is particularly marked where the Al saturation percentage of total cation exchange capacity (CEC) exceeds 60% in the upper 50 cm of the soil pedon. About 25% of tropical soils are acidic soils with pH values below 5.5 in the upper horizons but without aluminum phyto-toxicity.

Salinization can be regarded as a specific form of soil degradation. Salinization is caused by improper irrigation, a high evapo-transportation rate, or changes in hydrological conditions.

Maintaining a sufficient level of soil organic matter is very important in tropical countries. The decomposition rate of tropical organic matter is about five times faster in the tropics than in temperate regions.

Biological Degradation

Biological degradation is related to the depletion of vegetation cover and organic matter content in the soils, but also denotes a reduction in beneficial soil organisms and soil fauna. Biological degradation is the direct result of inappropriate soil management. Soil organisms and soil organic matter content can influence and improve the physical structure of the soils, especially with regard to transportation within the soils, mixing mineral and organic materials, and changes in soil micropore volume.

Social-economic Factors

A big problem in most developing countries is high population growth. This increases the demand on natural resources, especially on soil and water resources. In many countries, population growth increases the pressure on land.

Degraded and Contaminated Soils in Taiwan

The area and classification of degraded soils in Taiwan are shown in Table 1. The total area of cultivated soils in Taiwan is about 880,000 ha (24% of the total area). The main degraded and contaminated soils can be listed as follows (Chen et al. 1996; Chen 1998).

  • Strongly acidic soils (pH<5.6) (30% of total rural soils);
  • Microelement nutrient deficiency of Zn, B, Fe, and Mn. The alluvial soils of eastern Taiwan are derived from schist mixed with limestone.
  • Salt-affected soils on the western coast of Taiwan;
  • Soil erosion along the Central Ridge of Taiwan, on steep slopes used for high-value fruits (e.g. apple and peach), vegetables and tea;
  • Poorly drained soils;
  • Water stress in deep sandy soils derived from coastal sediments of sandstone and slate;
  • Compact clay soil in Southern Taiwan; and
  • The soils contaminated by trace elements, (mainly soils in northern and central Taiwan contaminated by Cd, Pb, Cu and Zn).

Most degraded soils have been reclaimed since the 1990s. The reclamation techniques are as follows:

  • Liming on strongly acid soils;
  • Application of Zn, B, Fe, and Mn elements for nutrient deficiency soils;
  • Reclamation of salt-affected soils by natural leaching processes and underground drainage;
  • Cover cropping and mulching with Bahia grass on slopeland soils;
  • Regional improvement of drainage canals for poorly drained rice-growing soils;
  • Sprinkler irrigation for upland crops and for deep sandy soils; and
  • Deep plowing for compact soils.

However, it has not yet been possible to remedy most of the contaminated soils. Only about 10 ha of rural soils contaminated by Cd and Pb have been rehabilitated. Mechanical dilution or chemical stabilization techniques were used on these 10 ha to reduce the total concentration of Cd and Pb in surface soil in 1998.

Selecting Indicators of Soil Quality for Degraded or Polluted Soils

Indicators for Sustainable Soil Management

There are six basic ecological criteria of sustainable soil management. They should be used frequently to evaluate the sustainability of soil use. These indicators are:

  • Soil mass should be conserved long-term in each small land unit.
  • Soil fertility and biology should be conserved long-term, and damage by toxic substances from outside minimized.
  • Soil use should be stepped up when the marginal return has significantly increased.
  • All forms of degradation (erosion, biological, physical, and chemical degradation) should be prevented. In degraded soils, soil formation should be enhanced to improve soil biology and soil fertility.
  • Natural biodiversity and the other natural resources of a region should be conserved or restored, to ensure that the extinction of individual species does not endanger the biological community.
  • Local land use should not hamper the sustainable development of a zone, especially in social, institutional and economic respects.

Until now, the basic problem of developing and implementing measures for sustainable soil management is that results cannot be transferred and reproduced, because of the multiple factors involved.

Definition of Soil Quality

Soil quality can be assessed in terms of the health of the whole soil biological system (Warkentin 1995). Many scientists feel that any definition of soil quality should consider its function in the ecosystem (Acton and Gregorich 1995; Kennedy and Papendick 1995; Warkentin 1995; Doran et al. 1996; Johnson et al. 1997). These definitions are based on monitoring of soil quality (Doran and Parkin 1994), in terms of:

  • Productivity: The ability of soil to enhance plant and biological productivity.
  • Environmental quality: The ability of soil to attenuate environmental contaminants, pathogens, and offsite damage.
  • Animal health: The interrelationship between soil quality and plant, animal and human health.

Therefore, soil quality can be regarded as soil health (Doran et al. 1996).

Criteria to Evaluate Soil Quality

Just as we can assess human health, we can evaluate soil quality and health. Larson and Pierce (1994) proposed that a minimum data set (MDS) of soil parameters should be adopted for assessing the health of world soils, and that standardized methodologies and procedures be established to assess changes in the quality of these factors. These indicators should be useful across a range of ecological and socio-economic situations (Lal 1994, Doran and Parkin 1996).

Indicators should:

  • Correlate well with natural processes in the ecosystem (this also increases their utility in process-oriented modeling).
  • Integrate soil physical, chemical, and biological properties and processes, and serve as basic inputs needed for estimation of soil properties or functions which are more difficult to measure directly.
  • Be relatively easy to use under field conditions, so that both specialists and producers can use them to assess soil quality.
  • Be sensitive to variations in management and climate. The indicators should be sensitive enough to reflect the influence of management and climate on long-term changes in soil quality, but not be so sensitive that they are influenced by short-term weather patterns.
  • Be the components of existing soil databases where possible.

Cameron et al. (1998) suggested the use of a simple scoring approach, to help users decide whether to accept or reject a potential soil quality indicator for degraded or polluted soils:

  • A = sum of (S, U, M, I, R)
  • where A: Acceptance score for indicator.
  • S: Sensitivity of indicator to
  • degra-dation or remediation process.
  • U: Ease of understanding of indicator value.
  • M: Ease and/or cost effectiveness of measurement of soil indicator.
  • I: Predictable influence of properties on soil, plant and animal health, and productivity.
  • R: Relationship to ecosystem processes (especially those reflecting wider aspects of environmental quality and sustainability).

Each parameter in the equation is given a score (1 to 5) based on the user's knowledge and experience of it. The sum of the individual scores gives the level of acceptance (A) score which can be ranked in comparison to other potential indicators, thus aiding the selection of indicators for a site. For example, soil bulk density may receive the following score (S=4, U=4, M=5, I=3, and R=2) giving A values of 18/25 (72%). Particle size, on the other hand, may only get an A value of 10/25 (40%) (S=1, U=3, M=2, I=2, and R=2). In this case, we should select soil bulk density to be one of the indicators for soil quality assessment.

Indicators of Soil Quality

Assessment of soil quality is the basis for assessing sustainable soil management in the next century. It is particularly difficult to select factors of soil quality for degraded or polluted soils.

Dumanski (1994) indicated that appropriate sustainable management would require that a technology have five major pillars of sustainability, namely, it should: (1) be ecological protective, (2) be socially acceptable, (3) be economically productive, (4) be economically viable, and (5) reduce risk. Appropriate indicators are needed to show whether those requirements are being met. Some possible soil variables which may define resource management domains are soil texture, drainage, slope and land form, effective soil depth, water holding capacity, cation exchange capacity, organic carbon, soil pH, salinity or alkalinity, surface stoniness, fertility parameters, and other limited properties (Eswaran et al. 1998). The utility of each variable is determined by several factors, including whether changes can be measured over time, sensitivity of the data to the changes being monitored, relevance of information to the local situation, and statistical techniques which can be employed for processing information.

Doran and Parkin (1994) have developed a list of basic soil properties or indicators for screening soil quality and health ( Table 2). They are as follow:

  • Physical indicators including (1) soil texture, (2) depth of soils, topsoil or rooting, (3) infiltration, (4) soil bulk density, and (5) water holding capacity.
  • Chemical indicators including (1) soil organic matter (OM), or organic carbon and nitrogen, (2) soil pH, (3) electric conductivity (EC), and (4) extractable N, P, and K.
  • Biological indicators including (1) microbial carbon and nitrogen (2) potential mineralizable nitrogen (anaerobic incubation) and (3) soil respiration, water content, and soil temperature.

Harris and Bezdicek (1994) indicated that soil quality indicators might be divided into two major groups, analytical and descriptive. Experts often prefer analytical indicators, while farmers and the public often use descriptive descriptions. Soil contaminants selected as indicators may be those which have an impact on plant, animal and human health, or soil function.

Soil quality can be viewed from two perspectives: the degree to which soil function is impaired by contaminants, and the ability of the soil to bind, detoxify and degrade contaminants.

Soil Physical Indicators

Doran and Parkin (1994) have selected some physical indicators for the assessment of soil quality. These indicators include (1) soil texture, (2) depth of soils, topsoil or rooting, (3) infiltration, (4) soil bulk density, and (5) water holding capacity. Hseu et al. (1999) also selected some indicators for the evaluation of the quality of Taiwan's soils. The physical indicators he selected included (1) depth of the A horizon, (2) soil texture classes or contents of clay, silt, and sand %, (3) bulk density, (4) available water content (%), and (5) aggregate stability at a depth of 30 cm ( Table 3).

It is easy to understand that measuring the bulk density, soil texture, and penetration of resistance (or infiltration) can provide useful indices of the state of compactness, and the translocation of water and air and root transmission. Measurements of infiltration rate and hydraulic conductivity are also very useful data, but are often limited because of the wide natural variation that occurs in field soils, and the difficulty and expense of making enough measurements to obtain a reliable average value (Cameron et al. 1998). Measuring the aggregate stability gives valuable data about soil structural degradation, which is often affected by pollution (e.g. sodium) and soil degradation (loss of organic matter).

This shows that visual assessment of the soil profile is a very valuable way of assessing the physical condition of the soil, and whether there is a need for soil reclamation or remediation. These physical indicators should include:

  • Soil texture: related to porosity, infiltration, and available water content.
  • Bulk density: related to infiltration rate and hydraulic conductivity.
  • Aggregate stability: related to soil erosion resistance and organic matter content

Beare et al. (1997) have proposed a quantitative method to show the decline and restoration of soil structure conditions in a typical mixed-cropping rotation system over eight years ( Fig. 2).

Soil Chemical Indicators

Doran and Parkin (1994) have also selected chemical indicators for the assessment of soil quality. These indicators include (1) soil organic matter (OM), or organic carbon and nitrogen, (2) soil pH, (3) electric conductivity (EC), and (4) extractable available N, P, and K. Hseu et al. (1999) selected some chemical indicators for evaluating the quality of Taiwan soils. The chemical indicators include (1) soil pH, (2) electric conductivity (EC), (3) organic carbon, (4) extractable available N, P, and K, (5) extractable available trace elements (Cu, Zn, Cd, and Pb) ( Table 4).

Standard soil fertility attributes (soil pH, organic carbon, available N, P, and K) are the most important factors in terms of plant growth, crop production and microbial diversity and function. As we know, these parameters are generally sensitive to soil management. For polluted or degraded soils, these soil fertility indicators are regarded as part of a minimum data set of soil chemical indicators.

Soil Biological Indicators

Doran and Parkin (1994) have selected a number of biological indicators for the assessment of soil quality. These include: (1) microbial carbon and nitrogen, (2) potential mineralizable nitrogen (anaerobic incubation) and (3) soil respiration, water content, and soil temperature. Hseu et al. (1999) also selected some chemical indicators for the evaluation of the quality of Taiwan soils. The chemical indicators include (1) potential mineralization of N, (2) C, N, and P present in the microbial biomass (3) soil respiration, (4) the number of earthworms, and (5) crop yield ( Table 5).

Soil biological parameters are potentially early, sensitive indicators of soil degradation and contamination. It follows, then, that the minimum data sets for assessing key soil processes are composed of a number of biological (e.g. microbial biomass, fungal hyphae) and biochemical (e.g. carbohydrate) properties (Cameron et al. 1998). Two of the most useful indicators are microbial biomass and microbial activity. Microbial biomass is a sensitive indicator of a long-term decline in total soil organic matter, but does not seem to be a sensitive indicator of the effects of organic pollutants applied to fields.

Assessment of Soil Quality

There are no reliable, practical methods of assessing or evaluating soil quality/health, although some research reports have established a conceptual framework for assessing this (Karlen et al. 1997). In this Bulletin, we use the concept of threshold values to evaluate quality of Taiwan rural soils (Cameron et al. 1998).

Threshold of Soil Chemical Pollutants Used in Developed Countries

Various criteria for the assessment and remediation of contaminated soils have been developed, especially in industrialized countries, including the United States, Germany, United Kingdom, Australia, Canada, Netherlands, Japan and Taiwan (ICRCL 1987; USEPA 1989; Alloway 1990; Jacobs 1990, Tiller 1992; Ministry of Housing Netherlands 1994; Chen et al. 1996; Adriano et al. 1997; Chen 1998). Many national governments and local authorities who lack their own formal guidelines have used the Dutch standard in assessing contaminated sites, or monitoring sites.

Some have also made modifications to develop their own regulations based on soil qualities they feel are most important. However, the Dutch authorities are continually upgrading their soil quality criteria in the light of new scientific work, especially the ecotoxicology of listed substances and their impact on species in the ecosystem. Two values are considered in making decisions on regulating the level of heavy metals in soils, a target value (upper value of the normal or natural level) and the intervention value (i.e. values which mean that soil needs cleaning up) (Ministry of Housing, Netherlands 1994). The Dutch standards for assessing soil contamination on the basis of the total concentration of heavy metals in the soil are listed in Table 6, and Table 7.

Cameron et al. (1998) suggested that the dynamics of a soil quality value (Q) can be quantified by measuring the changes in soil quality parameters value over time (dQ/dt). This can be done using a quality control chart in which the soil attribute values are plotted as a time series. The control chart may have a critical limit (or threshold level, or an upper control limit (UCL) and a lower control limit (LCL)) which represents the tolerances beyond which soil quality or other measures of sustainable soil management should not go ( Fig. 3). For example, the UCL and LCL of the total soil copper concentration in Fig. 3 was proposed as 140 mg/kg as a soil quality guideline and 5 mg/kg for the minimum crops requirement. Many industrialized countries have developed regulated threshold values ( Table 8).

Threshold Values of Soil Chemical Pollutants Developed in Taiwan

Some contaminated sites were announced by Taiwan's Environmental Protection Agency (EPA) in 1983 and 1988. These sites were used to test different soil remedial techniques (Chen 1991, 1992, and 1994, Lee and Chen 1994, Chen and Lee 1997, Chen 1998, Liu et al. 1998). The EPA organized a working group to develop guidelines for assessing sites polluted with heavy metals, and has used these guidelines to monitor these sites since 1990. The guidelines primarily follow the basic soil properties of Taiwan, and the effects of heavy metals on:

  • Water quality;
  • Activity of soil microorganisms;
  • Human health, and
  • Crop productivity and quality;

Final guidelines for soil quality were proposed by this working group over the past few years (Wang et al. 1994, Chen et al. 1996, Chen 1998). They were primarily based on the effects of heavy metal concentrations on human health, on plant productivity and crop quality, and on guideline values established in other countries. The intervention value for trace elements and threshhold phytotoxicity of heavy metals extracted from soil with 0.1 M HC1 are listed in Table 9 and Table 10.

Approaching a National Level of Sustainable Soil Management

Action Level of Sustainable Soil Management

Steiner (1996) indicated that general conclusions drawn from particular projects can be transferred to other sites only under specific conditions. The solution to problems of degraded soils must be geared to local needs. Programs should coordinate activity at different levels. Sustainable soil management is part of the effort to achieve sustainable agriculture.

Depending on the problems, combined action must be taken at different levels of intervention at the same time (Steiner 1996). These levels can be listed as follows, and are shown in a simplified form in Fig. 4.

  • Plot level
  • Rural household or farm level
  • - Technical solution, economically viable
  • - Participatory approach
  • - Accounting for specific needs of target group
  • Village community or watershed level
  • - Technical solution
  • - Participatory approach
  • - Organizational options
  • Regional level
  • - Organizational development (e.g. extension service)
  • - Land use planning
  • National level
  • - National strategies for sustainable soil management
  • - Agricultural policy (including structure, input supply and marketing)
  • - Research, training and extension
  • - Approaches and technical options for sustainable soil management
  • Supra-regional level
  • - Cooperation between research institutes
  • - Networks for technology transfer and communication
  • Global level
  • - Donor coordination
  • - Trade policy (WTO)
  • - International research cooperation

Agricultural Policy

Maydell (1994) pointed out that policies to promote sustainable soil management must begin by identifying which aspects should be assisted or can be influenced. Fig. 5 depicts the relationships between soil degradation, land use and agricultural policy. The major factors in these relations are cropland area, cropping patterns and cropping techniques.

Cropping Area

Low productivity per unit area in less industrialized countries is the main reason why so much new arable land has been cleared in the past, and is still being cleared today. The only land now left is marginal, with low natural soil productivity, unstable soils and a high risk of soil erosion. In the past, prices and subsidies have encouraged people to increase the area under crops. In order to solve these problems in the long term, the most effective way is to raise land productivity by promoting agricultural research, improving agricultural services, and developing high-value special crops for farmers.

Cropping Patterns

Pricing and active promotion of certain crops can encourage farmers to plant crops that conserve soil (Maydell 1994).

Cropping Techniques

Most methods that conserve soil resources involve higher costs. For conserving our soil resources, we need more research about cropping techniques to protect soil resources.

Approaches in Research, Training, and Extension

Some important questions need to be answered in each country when projects are developed.

  • Who defines the needs and aims in research?
  • Should the research institute or university do the research, or should it be the task of extension services or farmers' associations?
  • Who assesses the efficiency of research institutes, and what yardsticks are applied?

Approaches and Technical Options for Sustainable Soil Management

A national strategy for sustainable soil management should be based on the following process.

  • Analyze the background and basic data of degraded and polluted soils.
  • Assemble the components for an effective solution.
  • Produce a set of tools at a national level to meet the needs of farmers and policy makers.

An approach to sustainable soil management at a national level is shown in Fig. 6. Available data on the background to the problem include the causes of soil degradation, current status of soil quality, numbers and needs of farmers, and agricultural policy. An effective solution for soil problems must include early warning by soil indicators, prevention of soil degradation, rapid assessment of problems, assessment of the economics of production, risk assessment for soil pollutants. In sum, it must propose a sustainable way of managing the soil.

Many technical options can be used as components in sustainable soil management systems ( Fig. 7). All of them must achieve at least one of the following goals:

  • Minimize soil erosion (erosion control)
  • Conserve, or if necessary restore, the physical, biological, and chemical properties of the soil (soil fertility and soil structure).
  • Enable the soil to retain water (water balance) and regulate surface run-off.
  • Regulate soil temperatures; so that they become higher in uplands and lower in lowlands (temperature control).
Erosion Control

Vigorous ground cover is strongly recommended to avoid soil loss in water run-off. Cropping methods include early sowing, cover crops, mixed cropping, higher seed density, inter-row cropping and planted fallows. Splash erosion can be controlled by mulching, or by leaving the residues of harvested crops on the soil surface. Rill and gully erosion can be controlled by terracing, or by placing other barriers parallel to the slope such as contour strips planted with different species of grass. Contour plowing and minimum tillage are also effective against soil erosion. These methods and technologies are not widespread. They need further development, and they need more extension to farmers.

Conserving Soil Fertility and Soil Structure

As we all know, adding crop residues, manure, and compost to the soil is a good way of maintaining soil fertility and maintaining soil structure. Another successful method is mulching, in which people gather organic substances (grass, leaves, litter, branches) from non-agricultural areas and spread it on fields to avoid soil erosion and to increase the fertility of poor soils.

The most effective way to maintain soil fertility, soil structure and biological activity is to provide enough soil organic matter, or soil organic carbon pools, in the soil (Chen and Hseu 1997). In Taiwan, the mean organic carbon content of surface soil is about 1.9 - 2.8%. The mean organic carbon pool in Taiwan's rural soils is less than 8 kg/m 2/m (or less than 5 kg/m 2/50 cm) (Chen and Hseu 1997). This organic carbon pool is not enough to maintain good soil structure and crop production. An annual application of 20 mt/ha of organic manure or compost is needed to meet the demands of crop production and provide good soil structure and biodiversity in the soil (Gregorish et al. 1995, Studdert et al. 1997, Chen et al. 1998) ( Fig. 8).

Soil Water Balance

In order to use a limited quantity of irrigation water and precipitation effectively, appropriate soil management is needed. Suitable technology includes:

  • Improving ground cover;
  • Conserving the organic matter;
  • Breaking up (plowing) the soil;
  • Harrowing or roughening the soil surface;
  • Building dams, furrows, and contour ditches;
  • Terracing steep slopelands.

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Fig. 5. Relationships between soil erosion, land use and agricultural policy

Source: Maydell 1994

Source: Chen et al. 1998

Index of Images

Figure 1 The Relationships among Sustainable Development, Sustainable Land Management, Sustainable Agriculture, and Sustainable Soil Management. (Redrawn from Dumanski 1997)

Figure 1 The Relationships among Sustainable Development, Sustainable Land Management, Sustainable Agriculture, and Sustainable Soil Management. (Redrawn from Dumanski 1997)

Figure 2 The Control Chart Shows the Decline and Restoration of Soil Structure Condition in a Typical Mixed-Cropping Rotation (Based on Beare <I>Et Al. </I>1997)

Figure 2 The Control Chart Shows the Decline and Restoration of Soil Structure Condition in a Typical Mixed-Cropping Rotation (Based on Beare Et Al. 1997)

Figure 3 A Control Chart Showing the Hypothetical Change in Soil Copper Concentrations Over Time Following the Land Application of Waste. (Ucl: Upper Control Limit; LCL: Low Control Limit). (Based on Cameron <I>Et Al.</I> 1998)

Figure 3 A Control Chart Showing the Hypothetical Change in Soil Copper Concentrations Over Time Following the Land Application of Waste. (Ucl: Upper Control Limit; LCL: Low Control Limit). (Based on Cameron Et Al. 1998)

Figure 4 People Involved at Different Levels of Operations for Sustainable Soil Management

Figure 4 People Involved at Different Levels of Operations for Sustainable Soil Management

Figure 5

Figure 5

Figure 6 A National Strategy for Sustainable Soil Management Will Assemble the Available Data, Give the Background to the Problem and the Components of an Effective Solution, Then Produce a Set of Tools for Use across the Nation

Figure 6 A National Strategy for Sustainable Soil Management Will Assemble the Available Data, Give the Background to the Problem and the Components of an Effective Solution, Then Produce a Set of Tools for Use across the Nation

Figure 7 Components of a Sustainable Soil Management System

Figure 7 Components of a Sustainable Soil Management System

Figure 8 Changes in Soil Organic Matter Content (MT/Ha) Calculated in Taiwan under Different Soil Management Systems with Long-Term Application of Composts or Fertilizers

Figure 8 Changes in Soil Organic Matter Content (MT/Ha) Calculated in Taiwan under Different Soil Management Systems with Long-Term Application of Composts or Fertilizers

Table 1 The Main Degraded and Polluted Soils in Taiwan

Table 1 The Main Degraded and Polluted Soils in Taiwan

Table 2 Proposed Minimum Data Set (MDS) of Physical, Chemical, and Biological Indicators for Screening the Condition, Quality, and Health of Soils

Table 2 Proposed Minimum Data Set (MDS) of Physical, Chemical, and Biological Indicators for Screening the Condition, Quality, and Health of Soils

Table 3 Soil Physical Indicators Selected for Assessing the Quality of Taiwan Soils

Table 3 Soil Physical Indicators Selected for Assessing the Quality of Taiwan Soils

Table 4 Soil Chemical Indicators Selected for Assessing the Soil Quality of Taiwan Soils

Table 4 Soil Chemical Indicators Selected for Assessing the Soil Quality of Taiwan Soils

Table 5 Soil Biological Indicators Selected for Assessing the Soil Quality of Taiwan Soils.

Table 5 Soil Biological Indicators Selected for Assessing the Soil Quality of Taiwan Soils.

Table 6 Dutch Standards for Soil Contamination Assessment in Total Concentration of Heavy Metals in Soils

Table 6 Dutch Standards for Soil Contamination Assessment in Total Concentration of Heavy Metals in Soils

Table 7 Compound Related Constants for Metals in Soils

Table 7 Compound Related Constants for Metals in Soils

Table 8 The Threshold Total Concentration of Trace Elements in Contaminated Soils Proposed by Some Industrialized Countries.

Table 8 The Threshold Total Concentration of Trace Elements in Contaminated Soils Proposed by Some Industrialized Countries.

Table 9 The Threshold Total Concentration of Heavy Metals Proposed for Taiwan's Rural Soils

Table 9 The Threshold Total Concentration of Heavy Metals Proposed for Taiwan's Rural Soils

Table 10 The Phyto-Toxic Threshold of Heavy Metals Proposed for Taiwan's Rural Soils

Table 10 The Phyto-Toxic Threshold of Heavy Metals Proposed for Taiwan's Rural Soils

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