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The components of soil
The mineral skeleton of the soil
Soil is important for plant growth in both natural and agricultural systems. It provides
anchorage for roots and a medium that contains water and nutrients. The fertility of the
soil is the combination of properties and processes that permit abundant plant growth
by enabling the plants to absorb the nutrients and water.
The mineral particles that make the mineral skeleton of the soil are produced by the
weathering of rocks that are broken down to form regolith - the solid particles and
solutes that dissolve in water and provide nutrients.
Clay particles are the smallest (<0.002mm), silt in medium sized (0.002-0.02mm) and
sand particles are the largest (0.02-2.0).
The proportions of the particles of different sizes are described as soil texture and
affect the properties of the soil.
Property Effect of particle size on soil properties
Drainage Sandy soils have larger pore spaces, which aids water drainage.
Water content Clay soils have slower drainage. Water is retained by being absorbed onto the
clay particles. Water can be drawn up from deeper layers by capillary action.
Aeration Because sandy soils are better drained, they have a higher air content.
Nutrient levels Rapid drainage causes leaching of nutrients out of sandy soils. Clay particles have
negative charges so positively charged nutrient ions absorb on the surface.
Root penetration Sand particles are not held together by the adhesion of surface water. So sandy
soils are looser and roots can penetrate more easily.
Air in the soil
Air will be found in the spaces between the solid particles that are not occupied by
water. The air provides the gases for aerobic organisms to use in important processes
such as decomposition and nitrogen fixation. A lack of oxygen will give anaerobic
conditions in which decay will be incomplete, leaving organic matter such as peat
releasing methane gas and producing acidic conditions in which nutrient uptake will be
This is found in the spaces between the solid particles. Plants need water for many
physiological functions, to replace the water lost during transpiration and they can
only absorb nutrients if they are dissolved in water.
These are responsible for decomposition, nutrient recycling and aiding drainage.
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Plants Roots hold soil together. Dead vegetation provides nutrients and humus.
Detritivores e.g. worms, Break up dead organic matter, releasing nutrients and increasing the surface
millipedes, woodlicem dung area to enable decomposers to complete decomposition. The passages they
beetles open in the soil aids drainage.
Decomposers: bacteria Digest the dead organic matter left by detritivores.
Nitrogen cycle Nitrogen fixation, nitrification, denitrification.
Mycorrhizal Fungal network in soil provides plant roots with nutrients, receiving sugars in