Understanding Soil – The Lifeblood Of The Garden

No garden, however beautifully designed, can thrive without the owner understanding the basic ‘lifeblood’ of the garden – the soil.

Soil is divided into two layers, subsoil and topsoil. Subsoil is the lower heavily compacted structure, with topsoil being the upper layer that is more open structured and contains water, air, minerals and (if healthy) earthworms, insects and bacteria.

The structure of soil is generally made up of three main particles, sand, silt and clay. Clay is the smallest at less than 0.002mm, next silt at between 0.002 and 0.05 mm, and sand the largest at between 0.05 and 2mm.

The ideal ‘friable’ loam soil for plants to thrive has 40% sand particles, 40% silt particles and 20% clay particles. Unfortunately, many gardens see one particular type dominating and once a particular particle type is over 50% then different characteristics of the soil become evident.

Clay Soil

Whilst this soil retains nutrients extremely well, it has poor drainage, is impossible to work when damp, slow to warm in Spring and bakes hard in hot sun. To bring down the percentage of clay particles gritty sand, well-rotted compost, manure and perlite can be added to break up the heavy structure.

Sandy Soil

This soil type is free draining and warms quickly in Spring, however, due to its very open structure nutrients tend to wash downwards by rain to the below root level. This soil structure can also dry out quickly in hot weather. To adjust the overall structure, add well-rotted compost, manure and/or vermiculite to add smaller particles for water and air to cling to.

Silty soil

While this soil type is quite fertile and better than heavy clay or sandy soils it is also slow to warm up in Spring and can get waterlogged. This soil structure can always be improved by adding well-rotted manure, compost and/or coco peat.

Air and water

With the addition of a small percentage of minerals, the two other components of a healthy soil structure are water and air. The diagram below also demonstrates the ideal loam soil make up of 50% soil, 25% water and 25% air.

Some very good soils are rendered unworkable if drainage in the garden is poor and leaves the soil constantly waterlogged. In these circumstances extra physical drainage methods need to be employed below the soil.

French drains (or curtain drains) are ditches dug approx. 1m deep and approx. 30cm wide and filled with large washed stone. The excess water drains into this channel where it can slowly dissipate into the surrounding subsoil.

The other drainage option is to install piped drains in gravel filled ditches below the soil. These pipes are perforated to allow excess water to enter and then run into a public storm drain or natural drainage area such as a pond.

So before any plans are put into place and any plants are chosen, it is essential to understand the soil makeup of your particular garden to ensure that your new garden will last a lifetime!