Download Soil Composition

Soil 3 – Soil Composition
The ideal composition of soil,
25% Air,
25 % H2O,
45% Mineral Matter
5% Organic Matter.
Particles
 The texture of a soil depends on the relative
mixture of sand, silt and clay particles.
 The most common method of classifying soils is
based on the percentage clay in the oil.
 E.g. Soils that contain 0 – 5 % clay are known as
sandy soils.
 The
particles in the soil are classed on their size.
 Anything over 2mm in diameter is referred to as
gravel, pebbles or stones.
 Particles from 2 mm to 0.5 mm are called sand
particles.
 From 0.5 mm to 0.002 mm are called silt
particles.
 Any particle under 0.002 mm is referred to as
clay.
 Sand
and Silt are similar in composition and are
formed by physical breakdown of rocks.
 Clay particles are formed by both physical and
chemical breakdown of rocks.
 As
mentioned before soils are classified by the
amount of clay in the soil.
• 0 – 5 % Clay Sandy Soil
• 5 – 10 % Clay Sandy Loam
• 10 – 20 % Loam
• 20 – 30 % Clay Loam
• 30 – 40 % Clay Soil
• 40 % Up Heavy Clay Soil
A
more common and accurate way at looking at
the type of soil is by using a soil triangle.
Mineral matter is composed of inert solids, gravel,
coarse and fine sand, and silt and clay particles.
These different names come about because of the
size of each is different.
 Soil
Texture
Gravel
> 2mm
Coarse Sand
~ 2mm
Fine Sand
< 0.2mm
Silt
< 0.02mm
Clay
<0.002mm
 The
% of each particle in a soil can be estimated
using sieves.
1.
2.
3.
4.
A known weight of a dry soil particle is placed
in the top sieve.
Shake vigorously
Weigh the clay fraction
Result:
Clay
Total
X
100 =
1
% Clay
 The
clay particle is the most important soil
particle as it is the only particle, which is
charged.
 It holds a negative charge and is called an anion.
 It is able to attract positively charged ions
towards it (cations)
 When lime is spread on land it replenishes the
Ca2+ (calcium) in the soil and flocculation occurs.
 This
is the gathering together of clay particles
forming aggregates
 It is responsible for giving soil its structure.
 A well-flocculated soil will have a desirable
crumb structure.
 It will be friable.
 An
undesirable structure is referred to as being
plastic or blocky.
 Lime promotes flocculation.
 It is recommended that it be spread on average
every 5 years.
 Flocculation in soil promotes aeration.
When a soil sample is rubbed between you fingers
it breaks up into crumbs.
This is the ability of clay articles to swap positively
charged ions (cations) from one to another.
Sandy Soils
 Have large air holes.
 Free Draining soils
 Is easy to work with (light)
 Dries out quickly
 Minerals are easily leached.
 Poor soil with little or no nutrients.
 Is a warm soil.
Clay Soils
 Holds water easily
 This protects from leaching of minerals.
 Is naturally fertile soil.
 Very poor drainage, which can lead to water
logging
 Is a cold soil.
Loam Soils
 Intermediate characteristics of both clay and
sandy soils.
 More advantages and fewer disadvantages than
sandy or clay soils.
 A good mixture is 40 % Sand, 40 % Silt and 20 %
clay.
 While the nature of soil depends on the particle
composition, the amount of humus in the soil is
also a major factor.