2 What is meant by physical properties? Characteristics of soil which can be measured by physical means and expressed in physical terms, such as colour, density, porosity, hydraulic conductivity, structure, texture and depth.
3 Soil colour…Colour does not have a great effect on the behaviour of soilsException is the fact that darker soils absorb more solar radiation.Colours give us clues about other soil properties.Standard system for the description of soil colour = the Munsell colour chart system.
4 Soil colour described by referring to 3 components: Chroma (intensity/brightness)0 = neutral greyHue (redness or yellowness)Value (lightness or darkness)0 = black
5 What causes soil colour? Different coatings of individual soil particlesOrganic coatings = darker coloursIron oxides = yellow, red, brownManganese oxide = blackGlauconite = greenCarbonates = whitish colour
6 Why do we need to interpret soil colour? ClassificationInfo. about hydrologic regime/drainageBright colours (high chroma) = well-drainedAnaerobic conditions = greys
7 Soil Texture…Soil texture = proportions of different sized particles in a soil.We can draw many conclusions about a soil from its textureSoil texture is not readily subject to change
8 What is meant by the term “soil separates”? Division into sand, silt and clay fractions.SAND:Diameter smaller than 2mm, larger than 0.05mmMay be rounded or angularUsually consist of a single mineral (often quartz)Large particles = large pore spaces = free drainage & entry of airLow specific surface areaProne to drought
9 SILT: Particle diameter smaller than 0.05mm, larger than 0.002mm Particles not visible to naked eyeQuartz generally the dominant mineralReleases significant amount of plant nutrientsPores between particles are smaller and more numerousHigher water retentionLow “stickiness” means silt is washed away easily
10 CLAY: Particles smaller than 0.002 in diameter Very large specific surface area = high water retentionA spoonful of clay might have the same surface area as a FOOTBALL FIELD!!!Generally shaped like tiny flakes or plateletsWhen wet, clay is sticky, when dry, it forms a hard massAmount and type of clay largely dictates shrink-swell behaviour, plasticity, water-holding capacity, soil strength
11 Why is surface area important? Water retentionCapacity to retain nutrients and other chemicalsWeathering and release of plant nutrientsAbility of soil particles to combine with one anotherMicrobial reactions
12 Classification of soils into “textural classes” requires the use of the textural triangle…
14 Soil structure… Definition:- “The combination or arrangement of primary soil particles into secondary particles, units or peds. Secondary units are characterized and classified on the basis of size, shape and degree of distinctiveness” – Brady & Weil (1999)
15 How do we define soil structure? Size of secondary unitsDivided into classes from very fine to very coarseShape (types)Granular, blocky, columnar, prismatic, platyDegree of distinctness (grade)Refers to both inter- & intra-aggregate adhesion0 = structureless1 = weakly durable2 = moderately durable3 = strongly durable