soiknursery notses

1. Texture refers tothe ‘feel’ ofthe soil. This is affectedby the constituent materials foundwithinit, specificallysand, silt andclay particles. A
coarse sandwill feel gritty but a wet clay will feel heavy andsticky.The texture of a soil has a direct impact onthe way the soil reacts to certain
environmental conditions – for example,towards drought orheavyrain (withsandysoils morefreely draining).
There is a bigdifference in the size of thedifferent particles.
Coarse sand = diameter2-0.2mm
Fine sand = diameter0.2-0.02mm
Silt= diameter 0.02-0.002mm
Clay= diameter less than 0.002mm
Note howtheclay particles aremuch smaller thanthe sandparticles – this is important as it means the total surface area of a claysoil is much
greater andso the capacity to holdwater is also much greater.
Between the sand, silt andclay particles there are lots ofpores. Infact a soil as a whole is generally 45% mineral, 5%organic matter (depending
on the soil) and50% pore spacethrough which air andwatercan pass.
Sand –
 Made up of weatheredprimaryrock minerals.
 The particles are irregular inoutline.
 They are large andso do not packtogether easily.
 Large pore spaces in between.
 Air gets in very easily andwater flows rapidly through it.
Silt–
 Particles aresmaller than sand.
 Smaller pore spaces.
 More difficult todrain.
 Unlike clay particles, silt does not flocculate.
Clay–
 Very small indeed.
 Secondary minerals (chemicallyaltered by weathering) – chemical weathering(hydration,hydrolysis anddissolution)causes the
original mineral mattertodegrade intoits chemical constituents – mainlysilicon, aluminium,iron, magnesium,potassium and
calcium. Once released, theyrecrystallize to form‘secondary minerals’.
 Large surface area (theycan holdmore water) although the porespaces are tiny.
 Negatively charged(which means they attract positively chargedcations – such as calcium, magnesium,hydrogen, aluminium,
potassium).
 Clay has a capacity for waterretention andwill expandon beingwettedandshrinkingon drying. Anyonefamiliar with claylands will
have seen deep andwide cracks in grass fields duringtimes of drought andprobablygot claystuckon their boots
Sandy andclayey soils differ fromeach otherin various parameters such as -
1. Grain sizes - Sand is coarse comparedtoclay which has veryfineparticles
2. Plasticity - Sandis non plastic material whereas clayeysoil is identifiedfor its plasticity
3. Shear Strength parameters - Sandy soils possess shear strengthdue to internal frictional angle betweenparticles where as clayeysoils
have its shear strength(cohesion) basedon the cohesionbetween particles.
4. Compressiblity - Clayeysoils are more compressible (basedon soil properties) comparedtosands.

2. saturated