Soils of the Tropics PDF

Earth’s Soil [email protected] Alfred Hartemink Department of Soil Science Earth’s Soil 1 Earth’s Soil [email protected] Earth’s soil What are soils? Soils & the grand environmental challenges Introduction Soils and people Soils of the tropics Soils of the drier regions Soils of temperate regions...

Earth’s Soil [email protected] Alfred Hartemink Department of Soil Science Earth’s Soil 1 Earth’s Soil [email protected] Earth’s soil What are soils? Soils & the grand environmental challenges Introduction Soils and people Soils of the tropics Soils of the drier regions Soils of temperate regions Across the globe Soils of the cold regions Soils of the USA Soils of Wisconsin How to obtain soil information Wrap up Important dates Mid-term exam March 4 (first 5 lectures) Essay April 1 (printed, in my mailbox, corridor) Final exam April 22 (cumulative 10 lectures) Multiple choice. No re-exam Grading based on 2 exams, 1 essay Earth’s Soil [email protected] Alfred Hartemink Department of Soil Science Soils of the Tropics 4 Earth’s Soil [email protected] …so little was known… Paul Gauguin 1848 - 1903 Oxisol Parau Parau II 1892 5 Earth’s Soil [email protected] …in those early years 6 Earth’s Soil [email protected] Myths on tropical soils… Tropical soils are uniformly rich Soils of the humid tropics are rich in humus because of the abundant vegetation supplying plant material 7 Earth’s Soil [email protected] Myths on tropical soils… When the forest was cut, crops planted, yields were disappointingly low. Then it was emphasized that soil fertility in the tropics is uniformly low and easily lost by cultivation (Jacks and Whyte, 1939). 8 Earth’s Soil [email protected] Tropical soils - inherently different? § Lack of difference between summer and winter temperatures § Range of conditions under which soils are formed is as diverse as in temperate regions § Similar rock types occur § Erosional and depositional patterns are similar 9 Earth’s Soil [email protected] Tropical soils - inherently different? § Arid and humid as well as warm and cold climates occur in both regions § Extent of recent volcanic ash deposits is greater in the tropics § Larger proportion of young soils in the temperate regions 10 Earth’s Soil [email protected] Tropical soils - inherently different? § Recent glaciation and erosion by wind had great impact in the temperate regions § Soils in the tropics are often intensely weathered and derived from very old materials § Extent of soil types is very different 11 Earth’s Soil [email protected] The soils 12 Earth’s Soil [email protected] Oxisols § very weathered § low pH - acid soils § 8% globally § 23% of soils in the tropics are Oxisols § Low soil fertility (phosphorus) § Good physical properties 13 Earth’s Soil [email protected] Rain and evapotranspiration Rainfall Evaporation From the soil surface Result: Large percolation Leaching Transpiration From the vegetation Capillary rise Earth’s Soil [email protected] Global Distribution of Oxisols 15 Earth’s Soil [email protected] 16 Earth’s Soil [email protected] Oxisols in the USA 17 Earth’s Soil [email protected] 18 Earth’s Soil [email protected] 19 Earth’s Soil [email protected] Origin Most extensive areas in Africa and South America Very deep soils: § Formed under warm and humid conditions over long periods § Material that has been weathered, eroded, transported and re-deposited 20 Earth’s Soil [email protected] 21 Earth’s Soil [email protected] 22 Earth’s Soil [email protected] 23 Earth’s Soil [email protected] Maintaining soil fertility § Slash and burn agriculture Shifting cultivation § Slash the forest, burn, plant crop § Ash is fertilizer (lime, potassium) § One or two crops, then move, leave the land fallow § Soil fertility is restored during fallow period 24 Earth’s Soil [email protected] 25 Earth’s Soil [email protected] Permanent agriculture Fertilized Unfertilized Unfertilized Fertilized Inputs are needed: Manure Inorganic fertilizers Compost Lime 26 Earth’s Soil [email protected] The soil as a bank t In Out In Out In Out In Out Steady state In Out Build-up In Out Decline Less simple Earth’s Soil [email protected] Natural ecosystems Agricultural ecosystems Manure and inorganic fertilizers Atmospheric Gaseous deposition losses Atmospheric deposition Symbiotic and non-symbiotic fixation (BNF) Run on Run off Symbiotic and non-symbiotic fixation (BNF) Gaseous losses Run on Run off Erosion Deep uptake Erosion Mineral weathering Turnover of o.m. Fixation Immobilization Mineral weathering Turnover of o.m. Fixation Immobilization Leaching Crop removal Deep uptake Leaching Earth’s Soil [email protected] Nutrient depletion Millions hectares Light Moderate Strong Total 20 19 6 45 5 9 1 15 South America 25 34 13 72 Other Regions 3 1 0 4 53 63 20 136 Africa Asia World 29 Earth’s Soil [email protected] Fertilizer use 30 Earth’s Soil [email protected] Nutrient depletion 31 Earth’s Soil [email protected] The scale of the problem (3X US) Inherently low fertility soils Considerable soil degradation Poverty Size of the continent, infrastructure 32