Tropical soils

1. Tropical soils

4. General comments • tend to have more oxides of iron and aluminium due to weathering bringing about desilication of clay fraction to form laterite (see box) • because of intensive weathering (and so leaching), less fertile • P deficiency is a very common problem - P fixation because of large amounts of Fe and Al • Al and Mn toxicity can occur in low pH soils

5. Fertility : 1 = low

6. Oxisols • Properties • depth > 30 cm thick • NH4 < 10 meq / 100 g • CEC[1] < 16 meq / 100 g (but only partly saturated) • old – few tracesa of weatherable primary material • dominently sandy loam & > 15% clay • water dispersible clay • gradual boundaries • rocks < 5% • clay – kaolinitic + Fe & Al oxides • exchangeable Al+++ relatively high • stone lines common

8. Problems Physical Very hard when dry at the beginning of the dry season or after a dry spell – cutivation difficult at some times Chemical N & P particular problems

9. Typic Torrox in Hawaii

10. Oxisols in Africa • dominant soils of equatorial zone where R is high • low CEC • low base saturation (<40%) • relatively infertile • poorest of the African soils but suitable for some perenniel crops • yields of annual crops low with traditional agriculture • rooting restricted if hardened plinthite

11. Ultisols • Properties • thin A1 > leached (lighter) A2 > finer (illuviation) red to yellow B > lighter C • where small fluctuations in T (25 to 28°C); R > 2000 mm but seasonal deficit • younger than oxisols • higher CEC than oxisols - > 12 meq/100 g in B • may also have plinthite • base saturaiton< 35%

12. Florida

13. Australia

14. Alfisols • Properties • sub-humid and semi-arid areas • argillic B horizons • B has high base saturation > 40% • moisture less than wilting point for more than 3 months • course structured • small (not negligible) amounts of weatherable material

15. Alfisols in Africa • dominent soils (60%) of W Africa savannahs, some on E of L Victoria and in S. Africa • derived from crystalline acid rocks and basement complex • gravelly horizon • course textured A • low in silt • low CEC • relatively high base saturation • dominent soils in dry sub-humid and subhumid areas

17. Vertisols • black or dark, clay soils mainly in semi-arid or sub-humid areas • high shrink-swell properties • develop cracks to >50 cm • high BD between cracks • high in montmorillonitic clays • crumb structure at surface – prismatic below • alternate shrinking & drying so surface soils falls down the cracks – so-called self-mulching • form gilgais • very difficult to work when wet (avoid driving) – hard when dry – narrow window • otherwise very fertile – high CEC

19. Vertisols in Africa • widespread – classically in S. Sudan – R < 1000 mm

20. Mollisols • also cracking clay - higher in OM and have more favourable moisture regimes • Aridisols • moisture unavaliable for > 9 months • surface light • soft when dry • Aridisols in Africa • common in Sahel region & S Africa • usually too dry for crops unless irrigated

22. Andosols • soils derived from volcanic ash • very porous, friable, non-plastic and non-sticky • well structured in A horizon

23. Inceptisols and Entisols Entisols (recent) are soils of recent origin. The central concept is soils developed in unconsolidated parent material with usually no genetic horizons except an A horizon. All soils that do not fit into one of the other eleven orders are Entisols so order is characterized by great diversity.

24. Inceptisols (inception) represent the earliest horizon development. • Small amounts of organic matter that darken the topmost horizon (sometimes forming an ochric epipedon [Ochrept]) may be all that characterizes the inceptisol. • Weathering is minimal, and amounts of weatherable minerals in the profile are high. • Inceptisols and Entisols are most fertile in Africa • African wetlands are underutilised • Water management is a problem.

25. Lithosols (FAO), Regosols (FAO), Yermosols (FAO) Lithosols Lithosols are thin soils, limited in thickness by the presence of a continuous hard rock substrate within a depth of 10 centimeters. Azonal soils characterized by an incomplete solum or no clearly expressed soil morphology and consisting of freshly and imperfectly weathered rock or rock fragments.

26. Regosols Regosols, one of the azonal soils, are characterized by well to imperfectly drained mineral soils which lack horizon development or have minimal A and B horizon development (Soil Science Glossary, 1976), which indicate relatively unstable conditions that destroy soil profile zonation (McGregor, 1984). Regosols are common over a wide range of ecological conditions; there is low to moderate development of regosols in shrubs, and low development under trees, dryland grasses and wetland grasses (Soil Science Glossary, 1976). The parent materials are commonly moraine and colluvium, underlain by bedrock at different depths. Can occur on relatively young surficial deposits at higher elevations, such as colluvial slopes and active fluvial landforms.

27. Yermosols Have a very weak ochric A horizon and an aridic moisture regime (see box for description of different types of soil horizon (epipedon))

28. See the handout for more details. Please look for these soil types on the web