Introduction

1. Humid tropical American is one of the areas which offers mankind perhaps his best hope in the war between food supply and population growth.

2. Largest continuous block of unused and potentially arable land in the world lies in humid tropical American.

3. Soils, acid, highly weathered, low base saturation, infertile oxisols and ultisols.

4. Potentially arable 391 million hectare in humid tropical America alone.  (241 million more hectares than entire harvested cropland in U. S.)

5. Mounting population pressures will force settlement of the acid, infertile soils of humid tropical America.

6. With proper management, these soils can be among the most productive in the world.

Environment of the Humid Tropics

1. Geographic area.

• 7 or more humid months
• More than 1500 mm precipitation annually

• Do not suffer from moisture limitations
• Potential evapotranspiration exceeded by rainfall

• 5 degrees C or less variation/warmest to coldest month
• Mean annual temperature 26 degrees C
• Mean annual soil temperature more than 22 degrees C

• Average solar radiation is 400 langleys/da (2 x temperate zone)

• Variation ranges from zero at equator to 2 hr and 50 min. At 23.5 degrees latitude

• Oxisols and ultisols
• Oxisols oldest land surface in Latin America
• Oxisols, deep, well drained, low fertility, and red or yellow in color
• Ultisols most widespread soils of tropical world
• Ultisols deep, low in base saturation, low fertility, red or yellow in color

8. Soils most productive in world when proper soil-water management and conservation techniques used.

9. Vegetation.

• Savannas and forests
• Savannas/oxisols
• Forests/ultisols

• Tall grass - low tree
• Acacia - tall grass (dominant type)
• Acacia - desert grass
• Acacia - tall grass
• Tussock grass " m/in. Rainy season/interspersed with deciduous or evergreen trees
• Savannas, low nutrient availability and high levels of exchangeable aluminum

• Both semi-deciduous and deciduous forests.
• Almost closed nutrient cycle of rain forest and its soil is maintained by constant litter fall and decomposition

1. Slash and burn.

2. Crop production systems that are established on these cleared savannas and forested soils are often as varied as vegetation which proceed them.

Oxisol Savannas

1. Most widespread and traditional use of these areas is pasture for extensive livestock production.

2. Primarily beef and some dairy cattle.

3. Some native legumes in pasture but are not usually prominent.
 
4. Periodic burning of savannas is a universal management practice for pastures.

5. Animal and pasture pests reduced.

6. Most of N, S, and C lost to atmosphere.

7. High rainfall areas/2-3 year burn.

8. Lower rainfall areas/2-3 year burn.

9. Native grasses well adapted to low native fertility of soils (inefficient in their response to added N and P).

10. Annual beef live weight grains from 10-50 kg/ha.

11. Stocking rates of 0.05 to 0.30/ha.

12. Calving rates 40-50%.

13. Beef cattle requires 5 to 8 years to reach marketable weight.
 
14. Crops - wide variety.

• Corn
• Soybeans
• Beans
• Upland rice
• Peanuts
• Wheat
• Cowpeas
• Cotton
• Cassava
• Sweet potato

16. Constraints to high crop and pasture yields.

• Feed supply for livestock
• Environment stress and disease
• Quantity and quality of forages

• Crops produced in rainy season (Oct.-April)
• Low water-holding capacities of soils
• Occasional short term droughts
• 7-day drought at corn silking can reduce corn yields more than 50%

• Aluminum toxicity
• Deficiencies of N and P
• Al toxicity pronounced in subsoil (inhibits rooting depth)
• Native P levels from 0-3 ppm (60% clay in surface soil - require 730-1070 kg P/ha to maintain 0.05 ppm P in soil solution)

• Availability and costs of fertilizer and lime
• Transportation costs
• Availability of markets

• Judicious use of lime and fertilize to overcome soil fertility barriers
• Use of crop species and varieties tolerant to high Al and low P
• Gravity irrigation where possible
• Deep liming
• Use of grass mulch to reduce effect of short term droughts
• Adapt crop to the soil instead of modifying the soil to fit the crop
• Use plants developed in the cerrado of Brazil exhibiting joint tolerance to Al toxicity and low available P
• Broadcast and ban P fertilizer
• ime deeply into subsoil (30 cm)
• Use N to get increased yields
• Zn increased corn yields
• K cost efficient on corn
• Use less expensive P source
• Under native acid conditions use rock phosphate
• Establish legumes with native grass structures
• Plant pastures after row crop to make use of residual fertilizer nutrients

1. Land clearing is generally more difficult due to the large amount of vegetation.

2. Higher rainfall with no marked dry season.

3. Relatively higher native fertility of newly cleared forests.

4. Less developed infrastructure of supplies, transportation and markets in jungle areas compared to savannas.

5. "Shifting cultivation"
 
6. Crops - most common:

• Rice
• Beans
• Corn
• Cassava
• Sweet potatoes

8. Fallow period of 4-20 years.
 
9. Pastures.

• After burning, plant Guinea grass
• Neither legumes nor fertilizers are used in most pastures

• Fertilizer value of ash
• Severe soil compaction and soil removal by bulldozer clearing
• More rapid soil fertility decline on bulldozed land

• Occurs on both burned and bulldozed land
• Decrease in soil levels of nitrogen, phosphorus, potassium, calcium, and magnesium
• Exchangeable aluminum content is increasing

• Adequate year-round rainfall, temperature and solar radiation
• Ultisol areas some of most attractive in world for crop or pasture

• Continue traditional burning

• 3 and 4 crops can be produced each year in these soils
• Intercropping is tradition in these soils/can be intensified

• Beneficial effects of mulching and green manuring with legume adapted to the area (kudzu)
• Monetary expenses for fertilizer are replaced by labor expenditures
• Kudzu reduced bulk density
• Kudzu decreased day soil temperature by 2 degrees C, reduced soil water loss and inhibited weed competition

• Intercrops with modest, but sufficient fertilization/provides higher income per unit land/unit time than monoculture
• Marketable value of this intercropped system exceeded that of the combined monocultures by about 25% though individual crop yields were lower

• Establishing an improved grass/legume pasture with adequate fertilizer and mineral supplements to cattle proven beneficial
• Development and use of legumes resistant to insect and disease attack and tolerant of overgrazing is a large gap that needs to be filled

1. Enormous challenge and hope.

2. Food production in the humid tropics can be from 150 to 200% greater than that produced in the temperate zone on a hectare per year basis.