Tropical Agroecosystems

Tropical Soils

Most of the tropics are oxisols and ultisols, which are characterized by:

• Low pH [81% of tropical soils]
• High Al⁺³ [69% of tropical soils]
• Low cations; low CEC; low % base saturation [72% of tropical soils]
• Low silicate clays
• Low SOM? Where SOM is high in the tropics, it tends to consist of highly stable humus.
• Low P [50% of tropical soils]

Nutrient Cycling processes differ somewhat in the tropics:

• Most of the nutrients are found in the biomass, rather than the soil.
• Mineralization rates are high,
• Nutrient cycling of tropical forests is very efficient with low losses; ectomycorrhizal hyphae may capture nutrients directly from litter/residues.
• Aluminum and Manganese inhibit BNF,

Cropping Systems in the Tropics

Shifting Cultivation

Shifting cultivation ("slash and burn") systems are still common. Small plots are cleared, burned, cropped for 1-4 years, then "fallowed"--allowed to regenerate to forest through secondary succession.

The viability of such systems depends on the length of fallow. In many parts of the tropics population pressures lead to reduced fallows and lower yields. For example, Silva-Forsberg and Fearnside (1997) in Pará, Brazil, found:

• Maize yields following clearing of young secondary forest (0-10 y): 2.25 Mg ha^-1
• Maize yields following clearing of mature secondary forest (11-30 y): 3.24 Mg ha^-1
• Maize yields following clearing of virgin forest: 1.66 Mg ha^-1. [The lower yields from cleared virgin forest are not seen in other studies, may have been due to incomplete burning of residues or allelochemicals.]

Soil fertility recovers during the fallow; reducing the length of the fallow period prevents the soils from rebuilding sufficient SOM and nutrients to attain adequate yields.

Effects of burning the forest biomass after clearing: The pool of available nutrients increases temporarily (in ash and by mineralization of root biomass-80%-90% in first year), but there is a net loss of nutrients from the ecosystem. Nutrient losses occur by:

• Volatilization - N & S
• Erosion - P, Ca, Mg, K
• Leaching - Ca, Mg, K

Cations added in ash increase soil pH; burning also tends to destroy weed seed banks. Positive effects of burning--mainly increase in pH and killing of weeds--outweigh negative effects of N & S volatilization in the short term.

Available nutrients are depleted after 1-5 crops; a plot requires a fallow of 10-50 years to restore fertility.

Experimental Intensive Systems

It has been argued that the development of intensive systems-mechanization, fertilization, pesticides-could boost yields in the tropics and decrease deforestation rates. A key question is-can fertilization overcome the fertility limitations of tropical soils?

Experiments conducted by North Carolina State at Yurimaguas, Peru, on ultisols:

• Inputs include lime, NPK, micronutrients
• High yields maintained for 41 crops planted over 17 years

Problems with implementation of "Yurimaguas Technology":

• Lack of soil testing labs and extension services + need for site-specific analyses
• Lack of infrastructure (roads, fertilizer plants, markets, storage facility)
• Lack of capital for machinery
• Lack of suitable land (topography limitations for machinery)

Yurimaguas station began investigating lower input systems in 1987:

• Greater return of crop residues
• No tillage
• Al-tolerant varieties
• Yield generally declined after 7 crops due to weed problems

Agroforestry Systems (including "alley cropping")

Agroforestry refers to any agroecosystem that includes a mixture of trees and annual crops. Presumed advantages of such systems include:

• Decreased soil erosion [e.g., < 1 Mg ha^-1 y^-1 in Yurimaguas alley crops vs 10-30 Mg ha^-1 y^-1 on contoured slopes with row crops].
• Addition of N through BNF (when using legume trees).
• More efficient use of light and water resources.
• Improved nutrient cycling--depends on tree roots extending to deeper soil depths than roots of annual crops.
• Better weed control.
• Beneficial modification of the microclimate.
• Conservation of biodiversity.
• Multiple harvests.

The primary disadvantages of agroforestry systems are:

• They are very labor-intensive (trees need to be pruned; fields mulched).
• They are suited best to the more fertile alfisols than to ultisols and oxisols; appropriate legume trees for the more acid soils are lacking.
• Competition between crops and trees may reduce crop yield (especially for C4 cereals).
• They are knowledge- and management-intensive.
• Economic returns are often long-term--there are no perceived short-term benefits. This is particularly important where the grower does not hold secure tenure (ownership or other legal guarantee of access) to the land. Investments in long-term strategies to improve agroecosystems are absolutely dependent on land tenure. This is a major obstacle to increased production and sustainablity of agriculture in much of the developing world.

Types of agroforestry include:

• Alley cropping--growing crops between rows of trees.
• Rotational fallows--use of managed, very short fallows using selected trees having tolerance to acid soils, fast growth, and good competitive ability against weeds. Examples of such trees include Inga edulis (Fabaceae), Pterocarpus santalinoides (Fabaceae), and Grewia mollis (Tiliaceae) (Kanmegne et al., 2000).
• Home gardens.

Agroforestry research originally concentrated on tree-crop interactions, particularly effects of trees on crop yields and resource-use complementarity between trees and crops; the focus is now shifting to more diverse systems with trees selected for their production of non-wood products for market (fruits, fodder, extracts).

Leakey et al. (1999) suggest that future increased productivity in the tropics will require a "Woody Plant Revolution" similar to the earlier "Green Revolution"

URL: http://ag.arizona.edu/~spmcl/lecturenotes/tropics.htm