Tropical Soils Flashcards
What is soil?
- organic matter, minerals, atmosphere, solution(water)
- rooting medium for plants
- interfance between mineral and organic world
Primary minerals
- mineralso inheateted directly form parent rock, no chemical alterations
- quartz, feldspar, mica
Secondary minerals
- weathered products of primary minerals
- influence soil chem and phusical properties
- small size but large SA
- AL & Fe oxides, hematite, gibbsite
Clay
- active mineral portion of soil
- kind of clay depends on proportion of iond, silica and al in soil
- leaching changes clay type
tropical soil vs temperate soils
- tropical soils have lots of leaching due to rain so have different clay soils
- tropical soils have more kaolinite , temperate has more Smecitites
Kaolinite
- 1:1 clay mineral ratio between Si:Al
- heavily weahered soils
- cohesive, impermiable and sticky
- doesnt expand apon wetting
Smectites
- 2:1 Si:Al
- sweelling appon wetting and cracking when drying
- difficult ot cultivate
Illite (Hydrous Mica)
- Minimum swelling and shrinking
- Strongly attached to K
- occurs in less weathered soils
Allophane
- common in volcanic soils
- have + or - charge depending on pH
- strong adsorption of P
- Amourphous
Sesquioxide Clay
- leached silica difficent soils in warm climates
- only AL and Fe left
- not sticky & do not swell
- can absorb alot
- high adsorption of p making it less avaiable for plants
Soil air
- oxic conditions O2,N2,CO2,N2
- anoxic conditions CH4,H2S,NH3
- root trspiration and micro/macro fauna respiration effect the air
Soil water/ solution
- mobile/ liquid fractiom of soil
- transports nutrients bwtween soils and plant roots
- facilitates ion exchange
- soil quality depends on, hydrology and soil nutrient concentration
What are soil Colloids?
- any solid substance measured in microns
- large SA used fir chem exchange, and RXNs
- helps substances stick
- mostly clay and humus collids
- overall negative charge
Clay collids
morphous structure
Humus collids
- intermediate product of highly decomposed orgo mat
- amorphous structure
- most stable form of orgo mat in soils
soil ion exchange
- ions adsorbed to siol collids resist leaching
- some ions have stronger attraction depending on thier charge
- determined by soil fertility
- location of exhange: collids or plant roots
- dominated by cation or anion exchange depending on pH
Anion Exchange
- low ph
- ph below 5
- soil is positvle charged
- H+ exchanged for Fe and Al, soil accumulates Fe,Al
- red soil, tropical
Cation Exchange
- ph above 5
Soil structure
- arrangment of soil particles into sturctures that clump together
- granular, blocky, platey, massive, single grain
Soil orgo mat (SOM)
- plant, animal, microbrial residue
- soc= SOM x 1.724
- soil backbone, glue, mositure maintaining sturcture
- decreases erosion
- SOM depends on orgo input
SOM decomp rates
Fast decomp
- sugar, amino acids protiens
slow decomp
- lignin
- foration of humus stable for 1000’s of years
SOM content in tropics \
humid tropics 2%
subhumid tropics 1%
semiarid tropics 0.5%
Cental Amazon basin
- old soils , due to lack of disturbance(glaciation volcanizm)
- highly leached nutient poor
- high Al and Fe []
- high acididty
- low cation exchange
Central America
- Young soils, vocanic origin rich in nutirents
- high p but low availability
- low pH
- low cation exchange but better than old soils
what is Laterization?
- the progressive leaching of soils mobilizng Fe, Si and Al from soils to agragate lower in bhe soil coloumb
- leaving behind a residual concentration of sesquioxides (iron and aluminum oxides) and kaolinite clay.
How does laterization occur?
- removal of vegitation
- increases ersosion
- repeat wetting and drying
- Fe, AL Si leached into lower columb forming a brick like
characterisitics of Laterite soils
- found in humid tropical area old/ stable landscapes
- low pH. Highly leached
- extream peatheing of primary minerals
- low in fertility
- cannot hold water
- high p fixation
- low in humus
- low cation exchange
low rainfall and humidity
l
low rates of decomposition, slowing soil formation
parent matieral high in quartz
difficult to break down into secondary, slowing soil formation
High clay content
Aeration and water movment are poor, slowing soil formation
steep slopes
high erosion and constant exposure of bedrock, sows soil formation
High water table
Continued deposition of new matieral
no chanve for horizon developement, solwing down soil formation
O,A,B,C
- Organic horizon
- A mineral horizon, (roots)
- B Transitional Zone
- C unconsolidated rock
Oxisols and Ultisols
(ferralsol)
- deep,reddish, acidic, highly weathered
- high Fe Al oxides, low fert, low OM
- 36% of topics
- humid and subtropics along with some savannas
Alfisol(luvisol)
- not unique to the tropics
- prolonged dry seaon in tropical biomes
- predominatly under forests
- can be fertile but have curstin, erosion and rought strees problems
- 13% of tropical land area
- high in K, MG and Ca
Entisols (regosol)
- recently transported by water, young, without destinct horizon
- highly fertile, best tropical soil for plant growth
- dev on inert parent matieral
- 13% of tropical lands
Inceptisol (cambisol)
- young soils
- ditincct horizon
- very fertile
- dev on land covered by volcanic rock
- occupy 12% of tropical lands
Terra Preta do Indo
- black earth
- anthropogenicly made via charcoal, manure, bones, pants and unwanted garbage
- dark, fertile high in charcoal content
- 450-950 AD can be upto 2m deep
- high levels of microbial activity, low leaching
undistured soils in the tropics
are productive
- loss little nutirents due to plant cover, no leaching
- nutirent accumulation through rainfall, lighting, dust , nitrogen fixers, root nutient pumps
Conversion of virgin tropical rainforest to agricultural land
- year 1: high yeild , high SOM and nutirents in soil
- year 2: lower yeild, depletion of SOM
- year 3: uneconomical to produce crops ,soil depelated of som and nutrients, secondary forest succession happens
