ABSTRACT - Food and Fertilizer Technology ?· countries (Oldeman et al. 1990). Physical Degradation…

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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 Taiwans soil is Taiwans

    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 northe

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