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balance, destroying the socio-culturalidentity of rural communities or contami-nating the environment.

• Making the agricultural sector more re-silient against adverse natural and socio-economic factors and other risks, andstrengthening the self-confidence of ruralpopulations.

According to these criteria, the sustainablemanagement of agricultural soils maintains the soilproductivity for future generations in an ecologi-cally, economically, and culturally sustainable sys-tem of soil management.

Multidisciplinary Aspects of SustainableSoil Management

Sustainable soil management (SSM) musttake a multidisciplinary approach. It is not limitedonly to soil science. Basically, we can consider threeaspects of this management system (Steiner 1996):

• Bio-physical aspects: Sustainable soilmanagement must maintain and improvethe physical and biological soil conditionsfor plant production and biodiversity.

SELECTING INDICATORS TO EVALUATE SOIL QUALITY

Zueng-Sang ChenDepartment of Agricultural Chemistry

National Taiwan UniversityTaipei, 10617, Taiwan ROC

ABSTRACT

The main degraded and contaminated soils in Taiwan include highly acid soils, soils de-ficient in micro-elements, eroded and poorly drained sandy soils, soils suffering from waterstress, 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 degradedand polluted soils. In this Bulletin, several indicators are selected for soil quality assessment insustainable soil management systems, based on the concept of the control chart. The criticallevel (threshold level, an upper control limit (UCL), and a lower control limit (LCL)) representthe values within which soil quality must be kept for sustainable soil management.

INTRODUCTION

Definition of Sustainable Agriculture

The FAO/Netherlands conference on Agri-culture and the Environment (FAO 1991) revised theoriginal definition of “Sustainable Agricultural De-velopment” defined by FAO in 1990 and translatedit into several basic criteria to measure thesustainability of present agriculture and future trends.These criteria can be listed as follows:

• Meeting the food needs of present andfuture generations in terms of quantityand quality and the demand for otheragricultural products.

• Providing enough jobs, securing incomeand creating human living and workingconditions for all those engaged in agri-cultural production.

• Maintaining, and where possible enhanc-ing, the productive capacity of the natu-ral resources base as a whole and theregenerative capacity of renewable re-sources, without impairing the functionof basic natural cycles and ecological

Keywords: agricultural policy, soil quality, soil quality criteria, soil quality indicators, erosioncontrol, land use, polluted soils, soil fertility, soil degradation, soil structure, sustainable soilmanagement, threshold level, water balance

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• Socio-cultural aspects: Sustainable soilmanagement must satisfy the needs ofhuman beings in a socially and culturallyappropriate manner at a regional or na-tional level.

• Economic aspects: Sustainable soil man-agement must cover all the costs of in-dividual land users and society.

The concept of sustainable land manage-ment (SLM) can be applied on different scales toresolve different issues, while still providing guid-ance on the scientific standards and protocols to befollowed in the evaluation for sustainable develop-ment in the future (Dumanski 1997). Based on this,sustainable soil management is the basis of sustain-able land management, and sustainable land manage-ment is the basis of sustainable development (Dumanki1997) (Fig. 1).

Land Quality Indicators (LQIs) are beingdeveloped as a means of improving coordinationwhen taking action on land-related issues such asland degradation. Indicators are already in regularuse to support decision-making at a national orhigher level, but few such indicators are available tomonitor changes in the quality of land resources. Weneed more research into LQIs, including:

• How to integrate socio-economic (landmanagement) data with biophysical infor-mation in the definition and developmentof LQIs.

• How to scale data for application atvarious hierarchical levels.

The quality of Taiwan’s soil is Taiwan’s

Fig. 1. The relationships among sustainable development, sustainable landmanagement, sustainable agriculture, and sustainable soil management.(Redrawn from Dumanski 1997)

future. The objectives of this Bulletin are to discussthe causes of soil degradation and polluted soils,especially in Asian countries, to select indicators ofsoil quality for degraded or polluted soils, and todiscuss how Taiwan can achieve sustainable soilmanagement.

CAUSES OF SOIL DEGRADATIONAND POLLUTION

Causes of Soil Degradation

The most important challenge in the nextcentury is nutrient depletion, deficiency, and erosionof soils (IBSRAM 1994). Major soil-related prob-lems 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.

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Causes of Soil Degradation in theTopics

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

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

• Soil loss due to wind erosion (28%);• Soil loss due to water erosion (56%);• Nutrient depletion due to inadequate fer-

tilizer applications;• Soil acidification;• Salinization due to inadequate irrigation

and drainage (12%);• Depletion of organic matter due to fast

decomposition and insufficient organicfertilizer; and

• Compaction, aggravated by the use ofheavy machinery (4%).

The most important causes of water ero-sion are deforestation (43%), overgrazing (29%)and agricultural mismanagement (24%). The mostimportant causes of wind erosion are overgrazing(60%), agricultural mismanagement (16%), over-exploitation of natural vegetation (16%) and defor-estation (8%). The most important forms of chemi-cal soil degradation are loss of nutrients and organicmatter in South America, and salinization in Asiancountries (Oldeman et al. 1990).

Physical Degradation

Compaction, hardpans and crusting are threemajor causes of physical degradation (Steiner 1996).Soil compaction is an increase in bulk density causedby external loading, leading to a deterioration in rootpenetration, hydraulic conductivity, and aeration.There are many ways of reducing soil compaction.Hardpans are common in alluvial plains in semi-aridareas with a pronounced rainy season. Crusting isdue to the destruction of aggregates in the topsoilsby rain, and is closely linked to soil erosion. Crustingreduces infiltration and promotes water run-off.

Chemical Degradation

About 36% of tropical soils are low in

nutrient reserves. Acidification produces aluminumand ferrous oxides. This in turn results in the fixationof phosphorus, which is no longer available forplants. A ferrous oxide/clay ratio of > 0.2 is consid-ered to be the threshold for P fixation, and affects22% of all tropical soils. This problem also occurs inAndisols, Ultisols, and Oxisols in the humid tropicsand tropical highlands.

About 30% of tropical land problems occurin highly acidic soils which contain phyto-toxic alu-minum (Al) in the soil solution. This is particularlymarked where the Al saturation percentage of totalcation exchange capacity (CEC) exceeds 60% in theupper 50 cm of the soil pedon. About 25% oftropical soils are acidic soils with pH values below5.5 in the upper horizons but without aluminumphyto-toxicity.

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

Maintaining a sufficient level of soil organicmatter is very important in tropical countries. Thedecomposition rate of tropical organic matter isabout five times faster in the tropics than in temper-ate regions.

Biological Degradation

Biological degradation is related to thedepletion of vegetation cover and organic mattercontent in the soils, but also denotes a reduction inbeneficial soil organisms and soil fauna. Biologicaldegradation is the direct result of inappropriate soilmanagement. Soil organisms and soil organic mattercontent can influence and improve the physical struc-ture of the soils, especially with regard to transpor-tation within the soils, mixing mineral and organicmaterials, and changes in soil micropore volume.

Social-economic Factors

A big problem in most developing countriesis high population growth. This increases the de-mand on natural resources, especially on soil andwater resources. In many countries, populationgrowth increases the pressure on land.

Degraded and Contaminated Soils inTaiwan

The area and classification of degradedsoils in Taiwan are shown in Table 1. The total areaof cultivated soils in Taiwan is about 880,000 ha

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ments, (mainly soils in northern and cen-tral Taiwan contaminated by Cd, Pb, Cuand Zn).

Most degraded soils have been reclaimedsince the 1990s. The reclamation techniques are asfollows:

• Liming on strongly acid soils;• Application of Zn, B, Fe, and Mn ele-

ments for nutrient deficiency soils;• Reclamation of salt-affected soils by

natural leaching processes and under-ground drainage;

• Cover cropping and mulching with Bahiagrass on slopeland soils;

• Regional improvement of drainage canalsfor poorly drained rice-growing soils;

• Sprinkler irrigation for upland crops andfor deep sandy soils; and

• Deep plowing for compact soils.However, it has not yet been possible to

remedy most of the contaminated soils. Only about10 ha of rural soils contaminated by Cd and Pb have

(24% of the total area). The main degraded andcontaminated soils can be listed as follows (Chen etal. 1996; Chen 1998).

• Strongly acidic soils (pH<5.6) (30% oftotal rural soils);

• Microelement nutrient deficiency of Zn,B, Fe, and Mn. The alluvial soils ofeastern Taiwan are derived from schistmixed with limestone.

• Salt-affected soils on the western coastof Taiwan;

• Soil erosion along the Central Ridge ofTaiwan, on steep slopes used for high-value fruits (e.g. apple and peach), veg-etables and tea;

• Poorly drained soils;• Water stress in deep sandy soils derived

from coastal sediments of sandstone andslate;

• Compact clay soil in Southern Taiwan;and

• The soils contaminated by trace ele-

Table 1. The main degraded and polluted soils in Taiwan

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been rehabilitated. Mechanical dilution or chemicalstabilization techniques were used on these 10 ha toreduce the total concentration of Cd and Pb insurface soil in 1998.

SELECTING INDICATORS OF SOIL QUALITYFOR DEGRADED OR POLLUTED SOILS

Indicators for Sustainable Soil Manage-ment

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

• Soil mass should be conserved long-termin each small land unit.

• Soil fertility and biology should be con-served long-term, and damage by toxicsubstances from outside minimized.

• Soil use should be stepped up when themarginal return has significantly in-creased.

• All forms of degradation (erosion, bio-logical, physical, and chemical degrada-tion) should be prevented. In degradedsoils, soil formation should be enhancedto improve soil biology and soil fertility.

• Natural biodiversity and the other naturalresources of a region should be con-served or restored, to ensure that theextinction of individual species does notendanger the biological community.

• Local land use should not hamper thesustainable development of a zone, espe-cially in social, institutional and eco-nomic respects.

Until now, the basic problem of developingand implementing measures for sustainable soil man-agement is that results cannot be transferred andreproduced, because of the multiple factors involved.

Definition of Soil Quality

Soil quality can be assessed in terms of thehealth of the whole soil biological system (Warkentin1995). Many scientists feel that any definition of soilquality should consider its function in the ecosystem(Acton and Gregorich 1995; Kennedy and Papendick1995; Warkentin 1995; Doran et al. 1996; Johnsonet al. 1997). These definitions are based on monitor-ing of soil quality (Doran and Parkin 1994), in termsof:

• Productivity: The ability of soil to en-hance plant and biological productivity.

• Environmental quality: The ability of soilto attenuate environmental contaminants,pathogens, and offsite damage.

• Animal health: The interrelationship be-tween soil quality and plant, animal andhuman health.

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

Criteria to Evaluate Soil Quality

Just as we can assess human health, we canevaluate soil quality and health. Larson and Pierce(1994) proposed that a minimum data set (MDS) ofsoil parameters should be adopted for assessing thehealth of world soils, and that standardized method-ologies and procedures be established to assesschanges in the quality of these factors. These indica-tors should be useful across a range of ecological andsocio-economic situations (Lal 1994, Doran andParkin 1996).

Indicators should:• Correlate well with natural processes in

the ecosystem (this also increases theirutility in process-oriented modeling).

• Integrate soil physical, chemical, andbiological properties and processes, andserve as basic inputs needed for estima-tion of soil properties or functions whichare more difficult to measure directly.

• Be relatively easy to use under fieldconditions, so that both specialists andproducers can use them to assess soilquality.

• Be sensitive to variations in managementand climate. The indicators should besensitive enough to reflect the influenceof management and climate on long-termchanges in soil quality, but not be sosensitive that they are influenced byshort-term weather patterns.

• Be the components of existing soil data-bases where possible.

Cameron et al. (1998) suggested the use ofa simple scoring approach, to help users decidewhether to accept or reject a potential soil qualityindicator for degraded or polluted soils:

A = sum of (S, U, M, I, R)

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where A: Acceptance score for indicator.S: Sensitivity of indicator to degra-dation or remediation pro-

cess.U: Ease of understanding of indicator value.M: Ease and/or cost effectiveness of mea-

surement of soil indicator.I: Predictable influence of properties on

soil, plant and animal health, and pro-ductivity.

R: Relationship to ecosystem processes (es-pecially those reflecting wider aspects ofenvironmental quality and sustainability).

Each parameter in the equation is given ascore (1 to 5) based on the user’s knowledge andexperience of it. The sum of the individual scoresgives the level of acceptance (A) score which can beranked in comparison to other potential indicators,thus aiding the selection of indicators for a site. Forexample, soil bulk density may receive the followingscore (S=4, U=4, M=5, I=3, and R=2) giving Avalues of 18/25 (72%). Particle size, on the otherhand, 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 shouldselect soil bulk density to be one of the indicators forsoil quality assessment.

Indicators of Soil Quality

Assessment of soil quality is the basis forassessing sustainable soil management in the nextcentury. It is particularly difficult to select factors ofsoil quality for degraded or polluted soils.

Dumanski (1994) indicated that appropri-ate sustainable management would require that atechnology have five major pillars of sustainability,namely, it should: (1) be ecological protective, (2) besocially acceptable, (3) be economically productive,(4) be economically viable, and (5) reduce risk.Appropriate indicators are needed to show whetherthose requirements are being met. Some possiblesoil variables which may define resource manage-ment domains are soil texture, drainage, slope andland form, effective soil depth, water holding capac-ity, cation exchange capacity, organic carbon, soilpH, salinity or alkalinity, surface stoniness, fertilityparameters, and other limited properties (Eswaran etal. 1998). The utility of each variable is determinedby several factors, including whether changes can bemeasured over time, sensitivity of the data to thechanges being monitored, relevance of informationto the local situation, and statistical techniques whichcan be employed for processing information.

Doran and Parkin (1994) have developed a

list of basic soil properties or indicators for screeningsoil quality and health (Table 2). They are as follow:

• Physical indicators including (1) soiltexture, (2) depth of soils, topsoil orrooting, (3) infiltration, (4) soil bulkdensity, and (5) water holding capacity.

• Chemical indicators including (1) soilorganic matter (OM), or organic carbonand nitrogen, (2) soil pH, (3) electricconductivity (EC), and (4) extractable N,P, and K.

• Biological indicators including (1) micro-bial carbon and nitrogen (2) potentialmineralizable nitrogen (anaerobic incuba-tion) and (3) soil respiration, water con-tent, and soil temperature.

Harris and Bezdicek (1994) indicated thatsoil quality indicators might be divided into twomajor groups, analytical and descriptive. Expertsoften prefer analytical indicators, while farmers andthe public often use descriptive descriptions. Soilcontaminants selected as indicators may be thosewhich have an impact on plant, animal and humanhealth, or soil function.

Soil quality can be viewed from two per-spectives: the degree to which soil function is im-paired by contaminants, and the ability of the soil tobind, detoxify and degrade contaminants.

Soil Physical Indicators

Doran and Parkin (1994) have selectedsome physical indicators for the assessment of soilquality. 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 forthe evaluation of the quality of Taiwan’s soils. Thephysical indicators he selected included (1) depth ofthe A horizon, (2) soil texture classes or contents ofclay, silt, and sand %, (3) bulk density, (4) availablewater content (%), and (5) aggregate stability at adepth of 30 cm (Table 3).

It is easy to understand that measuring thebulk density, soil texture, and penetration of resis-tance (or infiltration) can provide useful indices ofthe state of compactness, and the translocation ofwater and air and root transmission. Measurementsof infiltration rate and hydraulic conductivity arealso very useful data, but are often limited because ofthe wide natural variation that occurs in field soils,and the difficulty and expense of making enoughmeasurements to obtain a reliable average value(Cameron et al. 1998). Measuring the aggregate

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Table 2. Proposed minimum data set (MDS) of physical, chemical, and biological indicatorsfor screening the condition, quality, and health of soils

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stability gives valuable data about soil structuraldegradation, which is often affected by pollution(e.g. sodium) and soil degradation (loss of organicmatter).

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

• Soil texture: related to porosity, infiltra-tion, and available water content.

• Bulk density: related to infiltration rateand hydraulic conductivity.

• Aggregate stability: related to soil ero-sion resistance and organic matter con-tent

Beare et al. (1997) have proposed a quan-titative method to show the decline and restorationof soil structure conditions in a typical mixed-crop-ping rotation system over eight years (Fig. 2).

Soil Chemical Indicators

Doran and Parkin (1994) have also selectedchemical 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) extractableavailable N, P, and K. Hseu et al. (1999) selectedsome chemical indicators for evaluating the qualityof Taiwan soils. The chemical indicators include (1)

soil pH, (2) electric conductivity (EC), (3) organiccarbon, (4) extractable available N, P, and K, (5)extractable available trace elements (Cu, Zn, Cd, andPb) (Table 4).

Standard soil fertility attributes (soil pH,organic carbon, available N, P, and K) are the mostimportant factors in terms of plant growth, cropproduction and microbial diversity and function. Aswe know, these parameters are generally sensitive tosoil management. For polluted or degraded soils,these soil fertility indicators are regarded as part ofa minimum data set of soil chemical indicators.

Soil Biological Indicators

Doran and Parkin (1994) have selected anumber of biological indicators for the assessment ofsoil quality. These include: (1) microbial carbon andnitrogen, (2) potential mineralizable nitrogen (anaero-bic incubation) and (3) soil respiration, water con-tent, and soil temperature. Hseu et al. (1999) alsoselected some chemical indicators for the evaluationof the quality of Taiwan soils. The chemical indica-tors include (1) potential mineralization of N, (2) C,N, and P present in the microbial biomass (3) soilrespiration, (4) the number of earthworms, and (5)crop yield (Table 5).

Soil biological parameters are potentiallyearly, sensitive indicators of soil degradation andcontamination. It follows, then, that the minimumdata sets for assessing key soil processes are com-

Table 3. Soil physical indicators selected for assessing the quality of Taiwan soils

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posed of a number of biological (e.g. microbialbiomass, fungal hyphae) and biochemical (e.g. car-bohydrate) properties (Cameron et al. 1998). Twoof the most useful indicators are microbial biomassand microbial activity. Microbial biomass is a sensi-tive indicator of a long-term decline in total soilorganic matter, but does not seem to be a sensitiveindicator of the effects of organic pollutants appliedto fields.

ASSESSMENT OF SOIL QUALITY

There are no reliable, practical methods ofassessing or evaluating soil quality/health, althoughsome research reports have established a conceptualframework for assessing this (Karlen et al. 1997). Inthis Bulletin, we use the concept of threshold valuesto evaluate quality of Taiwan rural soils (Cameron etal. 1998).

Threshold of Soil Chemical PollutantsUsed in Developed Countries

Various criteria for the assessment andremediation of contaminated soils have been devel-oped, especially in industrialized countries, includ-ing the United States, Germany, United Kingdom,Australia, Canada, Netherlands, Japan and Taiwan(ICRCL 1987; USEPA 1989; Alloway 1990; Jacobs1990, Tiller 1992; Ministry of Housing Netherlands1994; Chen et al. 1996; Adriano et al. 1997; Chen1998). Many national governments and local au-thorities who lack their own formal guidelines have

used the Dutch standard in assessing contaminatedsites, or monitoring sites.

Some have also made modifications to de-velop their own regulations based on soil qualitiesthey feel are most important. However, the Dutchauthorities are continually upgrading their soil qual-ity criteria in the light of new scientific work, espe-cially the ecotoxicology of listed substances andtheir impact on species in the ecosystem. Two valuesare considered in making decisions on regulating thelevel of heavy metals in soils, a target value (uppervalue of the normal or natural level) and the interven-tion value (i.e. values which mean that soil needscleaning up) (Ministry of Housing, Netherlands 1994).The Dutch standards for assessing soil contamina-tion on the basis of the total concentration of heavymetals in the soil are listed in Table 6, and Table 7.

Cameron et al. (1998) suggested that thedynamics of a soil quality value (Q) can be quantifiedby measuring the changes in soil quality parametersvalue over time (dQ/dt). This can be done using aquality control chart in which the soil attribute valuesare plotted as a time series. The control chart mayhave a critical limit (or threshold level, or an uppercontrol limit (UCL) and a lower control limit (LCL))which represents the tolerances beyond which soilquality or other measures of sustainable soil manage-ment should not go (Fig. 3). For example, the UCLand LCL of the total soil copper concentration in Fig.3 was proposed as 140 mg/kg as a soil qualityguideline and 5 mg/kg for the minimum crops re-quirement. Many industrialized countries have de-veloped regulated threshold values (Table 8).

Fig. 2. The control chart shows the decline and restoration of soil structure condition in atypical mixed­cropping rotation (based on Beare et al. 1997)

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Table 4. Soil chemical indicators selected for assessing the soil quality of Taiwan soils

Threshold Values of Soil ChemicalPollutants Developed in Taiwan

Some contaminated sites were announcedby Taiwan’s Environmental Protection Agency (EPA)in 1983 and 1988. These sites were used to testdifferent 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 aworking group to develop guidelines for assessingsites polluted with heavy metals, and has used theseguidelines to monitor these sites since 1990. Theguidelines primarily follow the basic soil propertiesof 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 pro-posed 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 heavymetal concentrations on human health, on plantproductivity and crop quality, and on guideline val-ues established in other countries. The interventionvalue for trace elements and threshhold phytotoxic-ity of heavy metals extracted from soil with 0.1 MHC1 are listed in Table 9 and Table 10.

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Table 5. Soil biological indicators selected for assessing the soil quality of Taiwan soils.

APPROACHING A NATIONAL LEVEL OFSUSTAINABLE SOIL MANAGEMENT

Action Level of Sustainable Soil Man-agement

Steiner (1996) indicated that general con-clusions drawn from particular projects can be trans-ferred to other sites only under specific conditions.The solution to problems of degraded soils must begeared to local needs. Programs should coordinateactivity at different levels. Sustainable soil manage-ment is part of the effort to achieve sustainableagriculture.

Depending on the problems, combined ac-tion must be taken at different levels of interventionat the same time (Steiner 1996). These levels can belisted as follows, and are shown in a simplified formin Fig. 4.

• Plot level• Rural household or farm level

- Technical solution, economically viable- Participatory approach- Accounting for specific needs of tar-get group

• Village community or watershed level- Technical solution- Participatory approach- Organizational options

• Regional level- Organizational development (e.g. ex-tension service)- Land use planning

• National level- National strategies for sustainable soilmanagement- Agricultural policy (including structure,input supply and marketing)

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Table 6. Dutch standards for soil contamination assessment in total concentration of heavymetals in soils

- Research, training and extension- Approaches and technical options forsustainable soil management

• Supra-regional level- Cooperation between research insti-tutes- Networks for technology transfer andcommunication

• Global level- Donor coordination- Trade policy (WTO)- International research cooperation

Agricultural Policy

Maydell (1994) pointed out that policies to

Table 7. Compound related constants for metals in soils

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Fig. 3. A control chart showing the hypothetical change in soil copperconcentrations over time following the land application of waste. (UCL:upper control limit; LCL: Low control limit). (Based on Cameron et al.1998)

Table 8. The threshold total concentration of trace elements in contaminated soilsproposed by some industrialized countries.

promote sustainable soil management must begin byidentifying which aspects should be assisted or canbe influenced. Fig. 5 depicts the relationships be-tween soil degradation, land use and agriculturalpolicy. The major factors in these relations arecropland area, cropping patterns and cropping tech-niques.

Cropping Area

Low productivity per unit area in less in-dustrialized countries is the main reason why somuch 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 thepast, prices and subsidies have encouraged people toincrease the area under crops. In order to solve theseproblems in the long term, the most effective way isto raise land productivity by promoting agriculturalresearch, improving agricultural services, and devel-oping high-value special crops for farmers.

Cropping Patterns

Pricing and active promotion of certaincrops can encourage farmers to plant crops thatconserve soil (Maydell 1994).

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Table 9. The threshold total concentration of heavy metals proposed for Taiwan’s ruralsoils

Table 10. The phyto­toxic threshold of heavy metals proposed for Taiwan’s rural soils

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Cropping Techniques

Most methods that conserve soil resourcesinvolve higher costs. For conserving our soil re-sources, we need more research about croppingtechniques to protect soil resources.

Approaches in Research, Training, andExtension

Some important questions need to be an-swered in each country when projects are developed.

• Who defines the needs and aims in re-search?

• Should the research institute or univer-sity do the research, or should it be thetask of extension services or farmers’ as-sociations?

• Who assesses the efficiency of researchinstitutes, and what yardsticks are ap-plied?

Approaches and Technical Options forSustainable Soil Management

A national strategy for sustainable soil man-agement should be based on the following process.

• Analyze the background and basic dataof degraded and polluted soils.

• Assemble the components for an effec-tive solution.

• Produce a set of tools at a nationallevel to meet the needs of farmers and

policy makers.An approach to sustainable soil manage-

ment at a national level is shown in Fig. 6. Availabledata on the background to the problem include thecauses of soil degradation, current status of soilquality, numbers and needs of farmers, and agricul-tural policy. An effective solution for soil problemsmust include early warning by soil indicators, pre-vention of soil degradation, rapid assessment ofproblems, assessment of the economics of produc-tion, risk assessment for soil pollutants. In sum, itmust propose a sustainable way of managing the soil.

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

• Minimize soil erosion (erosion control)• Conserve, or if necessary restore, the

physical, biological, and chemical proper-ties of the soil (soil fertility and soilstructure).

• Enable the soil to retain water (waterbalance) and regulate surface run-off.

• Regulate soil temperatures; so that theybecome higher in uplands and lower inlowlands (temperature control).

Erosion Control

Vigorous ground cover is strongly recom-mended to avoid soil loss in water run-off. Croppingmethods include early sowing, cover crops, mixedcropping, higher seed density, inter-row croppingand planted fallows. Splash erosion can be con-

Fig. 4. People involved at different levels of operations forsustainable soil management

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Fig. 5. Relationships between soil erosion, land useand agricultural policy

Source: Maydell 1994

trolled by mulching, or by leaving the residues ofharvested crops on the soil surface. Rill and gullyerosion can be controlled by terracing, or by placingother barriers parallel to the slope such as contourstrips planted with different species of grass. Con-tour plowing and minimum tillage are also effectiveagainst soil erosion. These methods and technolo-gies are not widespread. They need further develop-ment, 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 ofmaintaining soil fertility and maintaining soil struc-ture. Another successful method is mulching, inwhich people gather organic substances (grass, leaves,litter, branches) from non-agricultural areas andspread it on fields to avoid soil erosion and toincrease the fertility of poor soils.

The most effective way to maintain soilfertility, soil structure and biological activity is toprovide enough soil organic matter, or soil organiccarbon pools, in the soil (Chen and Hseu 1997). InTaiwan, the mean organic carbon content of surfacesoil is about 1.9 - 2.8%. The mean organic carbonpool in Taiwan’s rural soils is less than 8 kg/m2/m (orless than 5 kg/m2/50 cm) (Chen and Hseu 1997).This organic carbon pool is not enough to maintaingood soil structure and crop production. An annualapplication of 20 mt/ha of organic manure or com-post is needed to meet the demands of crop produc-tion and provide good soil structure and biodiversityin 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 irriga-tion water and precipitation effectively, appropriatesoil management is needed. Suitable technologyincludes:

• Improving ground cover;• Conserving the organic matter;• Breaking up (plowing) the soil;• Harrowing or roughening the soil sur-

face;• Building dams, furrows, and contour

ditches;• Terracing steep slopelands.

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Fig. 6. A national strategy for sustainable soil management will assemble the availabledata, give the background to the problem and the components of an effectivesolution, then produce a set of tools for use across the nation

Fig. 7. Components of a sustainable soil management system

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Fig. 8. Changes in soil organic matter content (mt/ha) calculated in Taiwan underdifferent soil management systems with long­term application ofcomposts or fertilizers

Source: Chen et al. 1998

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