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Karl Ramjohn
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Role of Soil Properties in Sustainable Agriculture
« Thread started on: Aug 11th, 2008, 3:46pm » |
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The Role of Innate Properties of Soils in Sustainable Agriculture Land Management Strategies: Dominance of Soil Organic Matter
INTRODUCTION
Soil formation results from a number of dynamic processes at the interface of the atmosphere and the earth. This involves several factors, including: parent material, climate, biota, topography and time. Interactions between these features (largely inter-dependent) are the fundamental determinants of the properties, and hence the type of soil, which develops at a particular location. In turn, the type of soil is a major forcing function in determining the nature of ecosystems (natural or man-made) which can be supported. In any ecosystem, soils have a number of crucial ecological roles, including:
• Supporting plant growth,
• Regulating water supply,
• Recycling raw materials,
• Habitat for soil organisms, and
• Engineering medium
The soil, in association with the large number and diversity of organisms supported and the functions thus established, can be regarded as an ecosystem. Activities of organisms in the soil enhance organic matter accumulation, profile mixing, nutrient cycling, and structural stability. These factors contribute to the ecological character of any given soil type. Monitoring and managing potentially adverse changes in the ecological character of a soil are a useful tool in an overall management plan for any land use system, particularly agriculture.
The purpose of this article is to discuss the role of innate properties of soils, and the particularly dominant effect of soil organic matter in influencing the development of sustainable strategies for agricultural land.
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Karl Ramjohn
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Re: Role of Soil Properties in Sustainable Agricul
« Reply #1 on: Aug 11th, 2008, 5:01pm » |
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AGRICULTURAL LAND USE: IMPACTS ON SOILS
Agro-ecosystems
Agro-ecosystems are restructured and simplified biomass production systems, intensively managed to sustain the major function of component extraction (food production). They are artificially subsidized solar-powered ecosystems, characterized by a (net) unidirectional flow in energy and materials (as opposed to the cyclic, more closed feature of natural systems). The unidirectional flow derives from the specific component extraction, which is the function of agro-ecosystems. This has significant negative impacts on the soil quality, and hence the sustainability of the food production system.
Land Quality: Soil Impacts
The useful components are extracted from agro-ecosystems, frequently for use at an external location. The harvesting of material removes large amounts of organic matter from the system. This reduced productivity, and the material removed must be compensated by inputs to the system, to attempt to maintain the ecological processes which maintain the viability and functioning of the system. This is done by placing fertilizers on the land surface: either naturally-derived organic matter or artificial (chemically-designed).
The unidirectional flow also has impacts on soil mineralogy. Continuous monocropping depletes the soil of specific mineral resources, leading to its deterioration. This can be addressed by fallowing or crop rotation. Leaving the soil exposed to direct solar radiation, wind and water during the cropping cycle can also affect its physical characteristics.
Sustainable Agricultural Management
The goal of sustainable agriculture is to attain profitable yields (biomass productivity), while maintaining the soil’s responsive stability (i.e., not irreversibly affecting its ecological resilience), using socially acceptable methods, for extended periods. The management of agro-ecosystems requires the maintenance of a specific type of vegetation (attempting to keep the system at a particular level of succession). As such, an agro-ecosystem resembles an ecosystem in its developmental stages, as opposed to one at a more mature successional stage.
Table-01 summarises a model of ecological successions, showing trends to be expected in the development of ecosystems. This can be used as a guideline in evaluating the status of an agro-ecosystem, for developing criteria for sustainable soil ecosystem management. The use of this model has its limitations; agro-ecosystems are not true successional systems, they are subjected to frequent, regular and intensive disturbance (Swift & Woomer 1993). However, Table-01 can be used to comparatively assess the general conditions of soil under agricultural production, particularly total organic matter, inorganic nutrients, mineral cycles, nutrient exchange, the role of detritus and nutrient conservation.
Table-01: Model of Ecological Successions: Expected Trends in the development of ecosystems
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Karl Ramjohn
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Re: Role of Soil Properties in Sustainable Agricul
« Reply #2 on: Aug 11th, 2008, 5:59pm » |
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ROLE OF INNATE PROPERTIES IN SUSTAINABLE MANAGEMENT
The characteristic properties of soils, important to their ecological processes and functions are mineralogy and organic matter content, and the manner in which these attributes affect the overall determinants of “soil quality”.
Soil Mineralogy
The mineralogical features of soils are primarily dependent on parent material, and the subsequent weathering processes, resulting in their disintegration (physical) and decomposition, recombination and solution (chemical), which have an important role in the formation of clay minerals. In the humid tropics, where there is heavy rainfall and high temperatures, biochemical reactions are enhanced, and as such, decomposition and recombination are the processes of primary importance, resulting in distinctive (deep-weathered) profile establishment. However, physical weathering largely determines the possible routes that are available to subsequent biochemical processes.
The major importance of mineralogy to soil productivity is related to the effect on surface soil (dynamic) and subsoil (static) attributes, including texture, strength, bulk density, pH and electrical conductivity. These are determinant factors of soil quality, particularly in the case of nutrient availability and cycling. The mineralogical characteristics of a soil can be regarded as more “enduring features” than soil organic matter (particularly in the humid tropics). However, they cannot be regarded as completely static (including subsoil).
For sustainable agriculture on a temporal scale appropriate to the goals of management (i.e., good biomass yields), changes in these features should be periodically assessed, as they provide good indicators of the likelihood of the continued productivity of a particular site, and can be used to mitigate trends which appear to compromise the overall sustainability of a given agro-ecological landscape. Soil mineralogy will also have an important role in determining the scale and context of possible attempts to restore agriculturally degraded land or otherwise change land use patterns, as the type of replacement ecosystem will not only depend on how the land was used, but also the inherent and long-term character of the original soil ecosystem.
Soil Quality: Determinant Factors
Table-02 summarises soil attributes that can be used to determine soil quality. In this context, soil quality is related to its fertility, i.e., its capacity to produce adequate biomass yields. This can assist in assessing the sustainability of soil management systems. As shown on Table-02, soil organic matter is crucial to maintaining a number of key productivity factors, including nutrient and water availability, capacity to sustain plant growth and support biological activity, and sensitivity to management and disturbance. However, Table-02 also indicates that physical and chemical characteristics in the surface and subsurface soil also have an important role, particularly in the promotion of plant growth and the soil’s responsive stability. As noted by Swift & Woomer (1993), soil organic matter is also an important modifier of some of the properties listed on Table-02.
Table-02: Soil attributes used to determine soil quality
Soil Organic Matter
Soil organic matter is generally found within the top 10 cm of the soil profile. The non-living soil organic matter component comprises mainly humus, most of which is adsorbed by clay. The humus component consists of fulvic acid, humic acid and humin. The other component of soil organic matter is the light fraction of slowly decomposing material. The humus component forms as a result of decomposition of plant material by soil organisms and other chemical processes. In the tropics, climatic factors tend to increase the rate of decomposition of soil organic matter; however, this is usually balanced by a higher rate of addition. The level of soil organic matter is determined by the equilibrium between factors which cause its formation and those which are responsible for its breakdown.
Figure 1 summarizes the role of the various fractions of soil organic matter in maintaining soil fertility. This figure clearly demonstrates that soil organic matter is a key resource to agricultural productivity, and the dominant role of soil organic matter in maintaining sustainability in soil use is underlined by the variety of soil properties and processes influenced by soil organic matter.
Some of the roles fulfilled by soil organic matter can generally be achieved by other means, such as chemical additives, irrigation and liming. However, this represents high-input (energy, labour, materials and water) and intensive management, which in itself is of questionable value to long-term sustainability. Soil organic matter can be regarded as a renewable resource (Swift & Woomer 1993). This renewability depends on the equilibrium established between the organic matter formation and decomposition processes. The type of land-cover ecosystem has a major role in this equilibrium and the rate of soil organic matter renewal. Clearly, the dynamics of this process will be very different in a rainforest than in adjacent cleared agricultural land.
Therefore sustainability of agricultural land will require appropriate management to ensure the renewability of soil organic matter, allowing for a higher degree of internal self-regulation of carbon, nitrogen and other nutrients within the soil component of the agro-ecosystem.
Indicators of Sustainability
The beneficial effects of soil organic matter on productivity are generally recognized as: plant nutrient supply, enhancement of cation exchange capacity (CEC), improvement of soil aggregation (water retention) and support of biological activity (Dudal & Deckers 1993). In order to appreciate the role of soil organic matter in sustainable management strategies, it is useful to consider some indicators based on this fraction of the soil component. These have been investigated and recommended by a number of authors and have been summarized by Swift & Woomer (1993) as:
1. Total soil organic matter (TSOM)
2. Microbial biomass (MBSOM)
3. MBSOM/TSOM ratio
4. Labile soil organic matter (LSOM)
5. N mineralization capacity
6. Charge (cation exchange capacity) contribution to soil
7. Quantity and diversity of organic inputs
8. Indicator species or groups of soil fauna and microflora
These criteria can be used to assist in quantifying the beneficial effects of organic matter on soils, and adverse changes can be used to assess the overall sustainability of cropping systems. Note that these criteria should not be applied in isolation as individual aspects are generally interrelated. Indicators 2. & 8. are further discussed below.
Microbial Biomass
Soil microorganisms (microbial biomass) are responsible for mineralization of nutrients and mitigation or assimilation of toxic compounds. The activities of this component of the organic matter (labile) are the significant feature of organic matter turnover, and depending on the successional stage of a given agro-ecosystem, can act as either a source or a sink of plant nutrients. As discussed by Sparling and Ross (1993), quantification of microbial biomass can assist in estimating nutrient fluxes, especially with regard to temporal availability in the cropping cycle. MBSOM rapidly responds to organic matter additions and environmental stresses. As such, soil microbial biomass characterizations can provide important inputs for planning a sustainable management strategy for agricultural land.
Soil Fauna
Soil fauna have an important role in nutrient dynamics and organic matter retention/turnover rates, and thus represent a significant consideration in agricultural land management. This is particularly important in the humid tropics, where there are no significant seasonal climatic constraints to soil faunal activity. In order to maximise the benefits that the interactions of soil fauna can potentially offer agricultural systems, it is important that management strategies include sound ecological modelling and understanding of soil food webs.
Maintenance of the ecological processes which allow interactions between soil communities is important to perpetuate their functions thus established which may be crucial to the ability of soils to support agricultural land use in a sustainable manner. There are various options available to manipulate soil fauna in tropical cropping systems (Brussaard et al. 1993); however, it must be noted that all environmental implications of such a management strategy must be carefully considered, especially from the perspective of intergenerational sustainability.
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Karl Ramjohn
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Re: Role of Soil Properties in Sustainable Agricul
« Reply #3 on: Aug 11th, 2008, 6:00pm » |
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BASED ON:
Ramjohn, Karl 1999. The Role of Innate Properties of Soils in Sustainable Agriculture Land Management Strategies: Dominance of Soil Organic Matter. Tropical Environment Research & Management Center, Trinidad & Tobago. April 1999.
REFERENCES & BIBLIOGRAPHY
Brussard, L., Hauser, S. & Tian, G. 1993. “Soil Faunal Activity in Relation to the Sustainability of Agricultural Systems in the Humid Tropics”. In Mulongoy, K. & Merckx, R. (eds). Soil Organic Matter Dynamics and the Sustainability of Tropical Agriculture. John Wiley & Sons; pp. 241-256.
Christensen, N.L. (Chair) et al. 1996. Report of the Ecological Society of America Committee on the Scientific Basis for Ecosystem Management. Ecological Applications 6(3); pp. 665-691.
Dudal, R. & Deckers, J. 1993. “Soil Organic Matter in Relation to Soil Productivity”. In Mulongoy, K. & Merckx, R. (eds). Soil Organic Matter Dynamics and the Sustainability of Tropical Agriculture. John Wiley & Sons; pp. 377-380.
Elliot, E.T., Cambardella, C.A. & Cole, C.V. 1993. “Modification of Ecosystem Processes by Management and the Mediation of Soil Organic Matter”. In Mulongoy, K. & Merckx, R. (eds). Soil Organic Matter Dynamics and the Sustainability of Tropical Agriculture. John Wiley & Sons; pp. 257-267.
Pieri, C., Dumanski, J., Hamblin, A. & Young, A. 1995. Land Quality Indicators. World Bank Discussion Paper 315; 63 pp.
Sparling, G.P. & Ross, D.J. 1993. “Biochemical Methods to Estimate Soil Microbial Biomass: Current Developments and Applications”. In Mulongoy, K. & Merckx, R. (eds). Soil Organic Matter Dynamics and the Sustainability of Tropical Agriculture. John Wiley & Sons; pp. 21-37.
Swift, M.J. & Woomer, P. 1993. “Organic Matter and the Sustainability of Agricultural Systems: Definitions and Measurement”. In Mulongoy, K. & Merckx, R. (eds). Soil Organic Matter Dynamics and the Sustainability of Tropical Agriculture. John Wiley & Sons; pp. 3-18.
Related Resources:
> http://tropicalenv.conforums.com/index.cgi?board=manmade03&action=display&num=1218399006
> http://sustainablelanduse.wordpress.com/2008/07/23/some-terminology-definitions-sustainability-land-use-impact-assessment/
> http://hydroterrestrial.pulseblog.net/The-first-blog-b1/Spatial-Ecological-Assessment-of-Land-use-Land-cover-Caparo-River-Valley-Republic-of-Trinidad-Tobago-b1-p2.htm
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