Soil Composition

SOIL S N O I T I S O P M O C

Soil Composition A general definition of Soil: A multi layered, loose covering of fine rock particles on the Earth’s surface.

Actual soil composition results from the effects of climate, topography and organisms on parent material, over geological time.

In Horticulture, soil is defined as the layer containing organic matter, in which plant roots grow.

The terms Organic Matter and Humus are often used interchangeably although there is a subtle difference between the two.

Organic matter describes the relatively undecomposed manure, animal and plant remains which are visible in the soil.

Humus is formed by decomposition of organic material by soil organisms. It cannot be identified but its presence is determined by the effect it has on soil structure. Humus rich soil is dark in colour, of a friable, crumbly structure, holds water and nutrients yet is free draining.

The top 10 – 12 inches we call Topsoil.

The Sub soil, below, varies in depth from a few centimetres to many metres.

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The main constituents of soil are;  Sand particles – largest (2.0 – 0.05 mm) 40% (by weight) determine drainage and aeration - Resist compaction  Clay particles – smaller (less than 0.002 mm)

20%

 Silt – smallest (0.05 – 0.002 mm) 40% Particles pack close together limiting air and water movement  Organic Matter 5% variable. Leaves, roots, remains of dead animals and other organisms  Water (variable)

25%

 Air - Aeration influences health of soil flora & fauna and movement of gases in and out of soil.  Nutrients - Soil fertility is determined by humus content, store of nutrients, and moisture. Natural nutrient content and level depends on the parent rock type and its solubility.  Micro Organisms (1Billion cells & 50,000 species per gram of soil) Most of these organisms are involved in the production and maintenance of soil fertility. Soil improvement methods help maintain a healthy population, leading to vigorous growth.

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 Invertebrates – Earthworms, woodlice snails, slugs, millipedes, centipedes.

Soil Preparation Digging brings soil into a condition which allows rapid establishment of young plants and seeds.

Advantages  Removes weeds which compete with crop plants for nutrients  Breaks up soil to a fine tilth, assisting root penetration  Improves drainage  Aerates the soil  Allows incorporation of organic matter  Encourages development of beneficial soil organisms Autumn is generally the best time of year for digging as the winter rain and frost assist in forming a crumb structure.

This is especially important on heavy clay soils, less so on lighter loam or sandy soil.

Do not dig when the soil is very wet or frozen.

Single Digging The top spit (spade depth) of soil is inverted, burying annual weeds, allowing removal of perennial weeds and incorporation of organic material.

At one end of the plot, remove any turf from the surface of the first strip. Take out a trench, the width and depth of the

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spade. Deposit the soil and turf at the opposite end of the plot.

Slice any turf off the next strip, lay it grass side down in the bottom of the trench and chop it into pieces.

The next row of soil is lifted out, inverted and placed in the trench.

As work proceeds one trench is opened out and filled as the next one is produced. The final trench is filled using the soil from the first trench.

If organic material is being added, spread it evenly, on the sloping face of the newly turned soil and cover it with the next layer of turned soil.

Leave the soil rough at this stage for weathering to assist in producing a crumb structure.

Double Digging This technique involves digging two spits deep. It is suitable for heavily compacted soils or to accommodate deep rooted plants. Do not mix or reverse the positions of top and sub soil.

Begin by taking out a trench one spade deep and three wide.

Removed topsoil is placed at the opposite end of the plot, as previously.

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The subsoil, at the bottom of the trench, is now broken up with a fork to a depth of one spit and covered by topsoil from the next trench. Work continues in this way to the end of the plot. Incorporate organic material and turf as for single digging.

Spring preparation of Soil When preparing for planting or sowing, it is essential to use soil conditions as a guide, not the date. Patience will pay dividends.

If the ground is too wet it will prove impossible to create a good tilth and in addition, wet, cold soils will inhibit germination and growth.

As a general guide, cultivate when the soil is dry enough not to stick to tools, but breaks into small pieces when struck with the spade.

If the original clearing was incomplete, or the soil very heavy, a garden fork can be used to break up large lumps and remove stones or rubbish. Try not to disturb the soil too deeply.

Following this a rake is used to break the surface into a fine tilth and level the site.

The ground should be firm enough for most crops if several months have passed between initial digging and seed bed preparation.

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However, Brassicas and Onions will not produce a satisfactory crop unless the soil is well firmed. (Brassicas tend to produce loose hearts and Onions run to seed).

Firming, at the same time as raking, will also assist production of a crumb structure. However, don’t overdo it, especially on heavy soil.

Keep your feet together, weight mainly on the heels and using a shuffling action, walk in straight lines, across the plot. This will assist levelling, prevent settlement after sowing or planting and gives better results than use of roller. Follow this with shallow raking to remove footmarks and to produce a fine tilth.

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