Growing Green: Animal Free Organic Techniques

By Jenny Hall & Iain Tolhurst

2.2 Soil structure and physical components

An optimum soil structure has been described as:

a water-stable, organically enriched, granular structure where all the water reserves

within aggregates can be fully exploited by root hairs and the space between aggregates

is large enough to allow rapid drainage, to admit air and to facilitate the deep

penetration of roots.

The mineral components of soil are derived from rocks, which are weathered

into all sorts of sizes from the largest boulders through to stones, sand, silt

and the tiniest particles of clay. Soils vary widely in their relative contents of

sand, silt and clay, but a good proportion is around 20% clay, 50% sand and

30% silt. Such mixtures are known as loams.

All soils need careful management, especially the timing of cultivations, which

should be carried out when the soil moisture content is just right – not too

wet and not too dry.

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Sandy soil – is the easiest to work, warming quickly in spring and draining

easily. However, it tends to be slightly acidic and can dry out easily in the

summer. It has large pore spaces and, if the soil is not carefully managed, rapid

water movement through the sand will leach nutrients, leading to fertility

deficiencies. Also, the larger air spaces mean that organic matter is more likely to

be oxidised and lost. Sandy soils need organic matter to bind particles together.

Silty soil – is a fragile soil that can cap at the surface.The pores are small and

can remain completely waterlogged in wet conditions or can become dusty in

dry conditions.

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Even if some form of structure with large air pores can be

achieved, it can disintegrate easily with a packing down effect leading to

compaction where the soil becomes airless. Silts benefit from the addition of

organic matter to open up pore spaces.

Clay soil – is late to warm in spring, heavy to work and has poor drainage. It

is sticky when wet and tends to bake hard in summer. As the soil is wetted

and dried the clay particles can expand and shrink, causing cracking. Clays

benefit from the addition of organic matter, which opens up the air pores and

makes the soil less dense.

The characteristic behaviour of clay particles is very different from that of sand

or silt.The latter are chemically inert and only affect water retention and

drainage. In contrast, clay particles, the smallest particles of rock, are

electrically charged and can attract, hold and make nutrients available to plants.

Peaty soil – is organic, as opposed to mineral, in origin.

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An example of its

formation is when, millennia ago, seas flooded established forests and in recent

times such land has been reclaimed from the sea. Such soil can look and feel

just like compost or peat.These soils tend to be too acidic for earthworms.

They can also be boggy in places and need careful attention to drainage.

Organic matter – The average content of organic matter in arable land is

around 2%. A lower organic matter content will give rise to greater structural

stability and hence greater susceptibility to erosion. High organic matter levels

e.g. over 10% in non-peaty soils will generally indicate low levels of biological

activity.This may be due to acidic pH levels and / or poor drainage.

2.3 Recommended practice – adding plant-based compost to soil

Topsoil is a mixture of disintegrating mineral rock and organic matter.

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Organic matter is material of once living origin: plant debris, manures and

dead bodies of all animals and microscopic creatures. Organic matter is

continually decaying, feeding the soil biota (billions of soil bacteria, fungi,

microscopic soil animals and larger animals like worms). Replenishing organic

matter lost through oxidation (accelerated by tillage) will improve soil

structure. Organic matter retains moisture, binds sands and opens up clay

soils, making all soils more easily worked. Earthworms are the most

significant species for soil structure, as their burrows provide air and drainage

channels. Earthworms require plenty of organic matter and do not like acidic

conditions, poor drainage or frequent tillage.

If you follow the guidelines for making good compost in chapter 4, then you

should be left with friable dark compost that crumbles in your hands and

smells pleasantly earthy.

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