Tropical Agro-Ecosystem Function

Transcription

1 Tropical Agro-Ecosystem Function Soil factors affecting agriculture in the tropics Soil quality indicators Dr. Ronald F. Kühne; Georg-August-University Göttingen Department for Crop Sciences Tropical Agronomy Grisebachstr. 6, Göttingen, Germany

2 Soil map of the world

3 Soil horizons (road cut) (Brady &Weil, 1996)

4 An area of land with its soil shown as a pedon (left) and as a profile (right). Source: Bridges (1997).

5 The position of soil in the environment (soil forming factors)

6 Zonal, intrazonal and azonal soils

7 Age and position of soils on the geomorphic surface

8 The soil as an open system

9 Concepts of soil quality-1 1) SUITABILITY FOR DIFFERENT USES rating soils for crop growth, soil fertility ( rich, poor soils) deductive ratings based on crop yield data, and inductive ratings based on inferences about the effects of soil properties on crop yield. (properties such as soil texture, profile morphology, soil depth, and drainage are used) draft requirements for tillage ( heavy, light soils) cost of inputs required to change soils (drainage of clay soils, liming of acid soils, etc.) suitability of soils for disposal of various "wastes" (effective recycling, long-term storage and inactivation of "pollutants".

10 Concepts of soil quality-2 2) SOIL HEALTH for human health (concept of organic farming) soil health and the health of animals and humans eating the crops produced by the soil, components: balance of major and minor nutrients, trace organics that have enzymatic functions, freedom from plant diseases and from various pests that attack unhealthy crops growing in unhealthy soils

11 Concepts of soil quality-3 (3) BIOLOGICAL PARAMETERS Measurements of biomass and specific components of biomass, biological activity measured by respiration, enzymatic activity, and diversity of organisms. Related to soil management -- to determine the effects, for example, of different tillage practices on soil biota. (4) FUNCTIONS OF SOIL IN ECOSYSTEMS. (5) INTRINSIC VALUE Uniqueness and irreplaceability

12 Soil functions recycling of organic materials in soils to release nutrients for further synthesis into new organic materials; partitioning of rainfall at the soil surface into runoff and infiltration; (see water harvesting) maintaining habitat diversity of pore sizes, surfaces, and water and gas relative pressures; maintaining habitat stability, including a stable structure, resistance to wind and water erosion, and buffering of habitat against rapid changes of temperature, moisture and concentration of potentially toxic materials; storage and gradual release of nutrients and water; and the partitioning of energy at the surface, which is important in global circulation processes

13 The process of leaching

14 The process of clay eluviation

15 The process of ferralitization

16 The process of salinization

17 Distribution of soil-forming processes and diagnostic features (Soil Taxonomy) in a hypothetical landscape

18 Soil fertility

19 Cation (Ca, Mg, K) stock of typical soils in temperate (Iowa) and tropical regions (Zambia). Source: Van Wambeke (1992)

20 Major soil types of the tropics

21 Agronomic characteristics

22 Alluvial soils-1 Considerable extent and agricultural importance in the tropics. Occurrence in floodplains, deltas, estuaries of rivers, alluvial fans, former lake beds, coastal plains. Class features: formed from materials transported and deposited by water; occur on flat land in the lowest part of landscapes; recent origin; lack of well-developed characteristic profiles; but very diverse soils.

23 Alluvial soils-2 Reasons for diversity: nature of the deposited material varies greatly depending on the geology and soils of the catchment area and the degree of weathering of the material. Soils of lower floodplains and deltas of larger rivers are formed from mixtures of diverse materials (from areas of different climate, geology and soils) Texture and depth varies greatly (even small scale) according to differences in the nature of sediments and the sedimentation pattern (velocity and turbulence of the water, particle size). Differences in natural vegetation. Water quality: fresh, brackish, saline.

24 Alluvial soils-3 Prospects: Alluvial soils of the floodplains and deltas of many great rivers are more fertile than most other tropical soils. Reason: annual enrichment with silt brought down by floodwaters. Areas of immense importance for food production, intensively farmed for many centuries. Limitations: Poor drainage and flooding may restrict crop production to only short periods, or risk of crop destruction in years of high floods.

25 Minimum data set for soil quality

26 Management for soil quality How can these soil quality concepts be used? Soil quality index To compare different soils or To determine for a soil whether quality is increasing or decreasing (sensitivity) Difficulty Drawing conclusions from comparisons across broad climatic regions. Lack of knowledge how to relate biological diversity to soil quality? critical limits (calibration)

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28 Concept of using soil quality indicators to evaluate land management

29 Maize yield, selected soil chemical and microbiological properties at Westbank1 (Nigeria) between April and October 1994 at 0-5 cm depth as affected by improved fallow management systems and natural regrowth (modified from Wick,Kühne & Vlek 1998)

30 Principal components (PC) and component loadings extracted from 17 original soil chemical, physical and microbiological variables, sampled in 1994 at 0-5 cm depth; original variables with underlined component loadings were used to interpret the PC

31 Phosphorus component (PC 2) degrading Colour code of sites: WB-3, D-2, WB-1 (increasing degradation) Fallow management treatments: P = Pueraria (simultaneous fallow - herbaceous) S = Senna (S = rotational fallow) L = Leucaena alley cropping C P C L C C L S P N S SOM-related nutrient status (PC 1) N = Natural regrowth, C = Control (continuous cropping), F = Forest reference S S L L N L C C N P N Restoration: tree fallow rotations Maintenance: simultaneous fallows (herbaceous,shrubs) L P F F