AGRICULTURAL SCIENCE
1 © Noted Summaries [Grade 12 Agricultural Sciences]
www.notedsummaries.co.za
, GEOLOGY > SOIL SCIENCES
SOIL TEXTURE TYPES OF SOIL PARTICLES :SOIL FRACTIONS
CLAY SAND
→ relative amount of sand, silt & clay in the soil W smallest soil particles [>0.002mm W largest soil particles (coarse) [0.5
→ physical & chemical properties of soil diameter] - 2.0mm diameter]
→ determination of type of crops to plant W large surface area & negative W seen without a microscope or
ONLY inorganic solid fraction of soils charge [ ↑ water & plant magnifying glass
Z proportions of sand, silt and clay = % [soil Labouratory] nutrients] W sandy soils= < 2o% clay, > 5o%
W affects shrinkage & expansion, sand
Z Texture classes = based on the composition of the soil
plasticity, water holding capacity W [sand, loamy sand, sandy loam]
& soil strength W soils are divided according to
SOIL TEXTURE TRIANGLE [∆] W seen only with a microscope particle sizes: coarse, medium and
u classes are classified with increasing proportion of fine particles: W dry particles: smooth and fine
[sand, loamy sand, sandy loam, loam, silty loam, silt, sandy clay loam, clay powdery W contains mineral quartz (SiO₂)
loam, silty clay loam, sandy clay, silty clay, clay] W wet then dried: hard
u Sand, silt and clay soil particles = soil separation → help scientists W wet particles: smooth, tough,
understand soil behaviour [nutrients, water holding capacity] holds shape easily
W clay soil = 3o–35% clay
[sandy clay, silty clay, clay]
Loam → equal amount of sand, clay
& silt particles
W Mark the %'s
SILT
W Draw line from clay side to
Clay < silt < sand particles [0.002 –
RIGHT SIDE [PARALLEL WITH 0.5mm diameter]
BASE] Weatherable minerals
W Line from sand side up Small = weather quick / fast & lots of
[PARALLEL WITH RIGHT SIDE] plant nutrients release
W Lines cross / intersect = texture seen only with a microscope
class dry particles: smooth and floury wet
particles: smooth, not smooth and
2 © Noted Summaries [Grade 12 Agricultural Sciences] sticky www.notedsummaries.co.za
, REASONS : KNOW TEXTURE CLASS
u Determine type of crops that can be planted = maximise productivity [seed SOIL PROFILES [MORPHOLOGY]
germination]
u Type of irrigation system to be installed [sprinkler / drip irrigation] BUT
[clay soil = flood irrigation] → water holding capacity
SOIL HORIZONS
u What kind of drainage can be used [sandy = no extra; clay soil = → layer parallel to the soil surface whose physical, chemical and biological
waterlogged easily] → drainage properties properties differ from the layers above and below. Horizons are in many
u root development cases defined by distinct physical features, primarily colour and texture
u pesticide movement
u minimise environmental damage [cover crop in sandy soils = wind erosion ↓ ]
PROFILE HOLE
→ assessment, evaluation and quantification of the type and extent of soil
cover, soil health and cultivation, soil depth, soil type, soil stratification in
INFLUENCE OF SIZE : SOIL BEHAVIOUR upper, middle and lower layers
CLAY SOIL [small particles] SANDY SOIL [large particles] SOIL PROFILE
CHEMICAL Large area Small total area
→ the vertical section of soil from the soil surface down to where the soil
REACTIVITY
DRAINAGE & Poorly / limitedly drained High % macropore space = good meets the underlying rock
AERATION and aerated = waterlogged in aeration & drainage = = easy to
rainy seasons [ ↓ macropore handle in cultivation MASTER HORIZONS
space]
WORKABILITY Heavy soil = difficult to till Easy > ↓ cohesion forces & soil • organic horizon
[hard if dried out] fraction [light soil] O-HORIZON • contains undecomposed organic litter / decomposed
organic matter & humus
EROSION Decomposes faster > large Decomposes more slowly > small
• surface horizon or topsoil
POTENTIAL surface area surface area
A-HORIZON • contains inorganic material [humus & mineral
WATER HOLDING Large surface area = high Low water holding capacity =
particles]
CAPACITY water holding capacity = poor for dryland farming • underground
small micropores = difficult [micropores = drainage] B-HORIZON • illuminated zone [concentration of clay & iron,
to irrigate aluminium & manganese oxides]
GROUND FRUIT High = large surface areas & Absence of colloids = fertility ↓ C-HORIZON • slightly weathered unpaved material [substrates]
ABILITY colloids = plant nutrients [no absorption of plant matter] • solid rock / consolidated material
PH OF SOIL Alkaline Acid R-HORIZON
CROP Not suitable for deep rooted Most vegetables
White • underground horizon
crops > deep clay layers = E-HORIZON
• eluted zone containing little or no organic matter
limited roots Bluish, green / • usually wet and clayey in texture, stains may present
G-HORIZON
greyish again > reduction > anaerobic
3 © Noted Summaries [Grade 12 Agricultural Sciences] www.notedsummaries.co.za
, SOIL PROFILES SOIL CLASSIFICATION
INFLUENCE SOIL FORMATION & DEVELOPMENT : SOIL PROFILE
SOIL CLASSIFICATION
→ = grouping of soil with similar properties / characteristics
W Mother Material: unconsolidated material that provides the most soil [morphological, physical, chemical & mineralogical]
horizon character.
W Climate: rainfall and temperature affect the rate of weathering. TAXONOMIC SOIL CLASSIFICATION SYSTEM
W Topography: arrangement of physical features (e.g., slope) that influence
the vegetation and soil type of a location.
W Time: soil formation is extremely slow → younger soil profile more closely LANDFORMS LAND FAMILIES
reflects the parent material. u an upper or general level u a lower and more specific level
W Organisms: type and number of organisms are important → influenced by u defined by a unique vertical u they share characteristics of the
plants sequence of horizons. form
u 73 landforms u they are differentiated within the
u SPECIFIC COMBINATION & form based on other
SOIL HORIZONS > SOIL TYPES
SEQUENCE > diagnostic horizons = characteristics.
Mature / old soil A A inferences about land u There are 404 soil families
u significantly developed soil profile B E u = represents specific combination u Subdivision of landforms
u B-horizon = clay silicate, or oxides C B of soil formation conditions u DIFFERENCE FROM EACH OTHER
[aluminium, manganese and iron]
u yellowish, reddish or dark in colour
R C [features > diagnostic horizon & [soil texture, pH, lime content &
[parent material = high in white successions > soil profile] organic matter + water holding
calcium carbonate] capacity]
u influence = climate, vegetation & Steps involved in binomial soil classificationsystem:
drainage u delineate master horizons
Young soil A A u identify diagnostic horizons (subdivisions of master horizons)
u minimal horizon C R u name a landform
u A-horizon = directly above R-horizon u identify distinguishing family marks
u parent material = strongest influence u identify soil family
Wet / waterlogged soil A O A u determine texture class of A horizon and add it to the soil family name →
u poorly drained E G G sandy loam, loam
u rubbery consistency G
u shrinks, hardens and cracks into MASTER HORIZONS DIAGNOSTIC HORIZONS
squares when it dries out
→ ground layers = similar → landforms = formation processes
Eroded soil B
→ appearance of soil > soil profile → contains whole / parts of master
u topsoil = wind, water or gravitational C horizons
creep removed
u B-horizon = upper layer
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