Compare and contrast the properties of topsoil with subsoil: organic matter content (living and dead organisms, humus), colour, pore space, aeration, water content, nutrient content. Suitability for plant growth.

ORGANIC HORIZON Consisting mainly of organic matter from the vegetation which accumulates under conditions of no disturbance and free aeration; Only 1 or 2 centimetres deep; Drawn down by worms; Can be spongy or loose.

LITTER LAYER the organic-horizon in soil

TOPSOIL HORIZON Usually darker colour due to good organic matter content; Has good worm and bacterial activity, drawing down organic matter and creating drainage holes; Zone of leaching and biological activity; Levels of nutrients are highest in this zone due to the breakdown of organic matter/minerals by micro-organisms (good aeration) and the addition of fertilizer to the topsoil by gardeners. As water moves down through the zone it leaches soluble nutrients and also tends to draw down the fine particles of clay and Humus into the subsoil. The presence of a good supply of organic matter aids the build-up of a good soil structure which in itself gives good pore spaces (Macro 0.06 mm up – good for drainage; Meso 0.06 mm down – good for holding onto capillary water; Micro holds water not available to plants) so the soil has a good balance of water retention/drainage.

TOPSOIL main zone of planty growth Topsoil is the outermost layer of soil, varying in depth. It has the highest concentration of organic matter and micro-organisms and is where most of the earth’s biological soil activity occurs. Plants generally concentrate their roots in and obtain most of their nutrients from this layer. The actual depth of the topsoil layer can be measured as the depth from the surface to the first densely packed soil layer known as the subsoil.

Soil is made up of: Partials 45% (sands, silts and clay partials); Humus/organic matter 5%; Air 20/30%; Water 20/30%. In the air and water you have the living zone.

For the growth of plants its properties are:

Nature according to its texture (read notes on texture);

Has a good structure ( notes on soil structure);

Good levels of organic matter creating soil structure, providing nutrients and holding moisture;

A natural openness giving a balance of air and water;

Good supply of plant nutrients (clay particles have a buffering effect, i.e., hold onto molecules of plant foods by a chemical reaction);

Stable structure to anchor roots;

Good biological activity, bacteria to worms and moles all contributing to soil structure, nutrition and aeration.

SUBSOIL HORIZON Usually lighter in colour than the topsoil, colour change often very distinct;

Zone of accumulation where the clay partials tends to accumulate.

Zone tends to be wetter and heavier in texture, denser.

Can have a good nutrient content from minerals leaching down plus the buffering effect of the clay.

Less nitrogen as there is less oxygen for the aerobic reaction of bacteria.

Can become water-logged,

iron/clay and plough pans can form in this layer


ZONE OF ACCUMULATION Subsoil is zone of accumulation where the clay partials tends to accumulate

SOIL HORIZON O organic layer, top 1-2cm

SOIL HORIZON A top soil layer

SOIL HORIZON B subsoil layer

ROOT ZONE Horizons “A” and “B” together constitute the root zone. The top layer is the best layer to promote growth where there is the best balance of all things (nutrients, air and water available).

ORGANIC SOIL A soil type


GREY SOIL A soil type

GREY ASH SOIL A soil type

BROWN EARTH A soil type

RENDZINA A soil type

PODZOL A soil type

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