General Guidelines for Soil ImprovementsGeneral Guidelines for Soil Improvements
Factor 01 of Soil Improvements: Drainage
For most plants, the soil should be well drained, but should not lose water too rapidly. This is especially important for plants which need an early start for their season, e.g. Onions. The ideal soil is one that spends as much of its time at what the boffins call field capacity that is, when water is held as a thin film around each mineral particle, and the organic matter sponges are filled with water, but the rest of the mineral particles are filled with air because excess water has drained away. Plant roots have fine hairs that encircle the mineral grains and penetrate the organic matter sponges and are then able to both breathe the air in the pore spaces and absorb the water they need.
Provided there is adequate subdrainage, the drainage of topsoil can be improved by adding organic matter, or by adding materials such as lime or gypsum that cause fine clay particles to flocculate or stick together into sand-sized multi-particles. Drainage can also be improved by the careful working of a heavy soil using the weather to advantage, and in some cases, physical aeration can help. This involves inserting spikes into the soil which, when withdrawn leave fissures and channels that help water to move in the soil. Ideally, this sort of work should be done during dry weather on heavy soil, so that the tines going in don’t smear and seal the clay particles. Dry weather also helps the tines to cause a shattering effect that further produces drainage fissures. The downside of this is that it is harder to work if you are using a garden fork or similar tool!
It is possible to improve drainage in the obvious way by changing the physical composition of the soil by adding sand or grit. Unless you are looking at just local changes (say for a small area in the garden) this is a jolly expensive job, because to do it properly it will often need vast quantities of sand or gravel (and be careful of the sand you use.
There are some, like concreting sand, that has a high proportion of fines that are almost as bad as clay for drainage). If you are contemplating adding sand or gravel on any scale, do a quick estimate of the quantity you will need with a soil composition analysis. Then choose where you want to be on the triangular chart. You can work out what sort of amount you will need if you measure the area of ground you need to cover and assume that your soil is 12 inches (300 mm) deep for volume purposes. The number of cubic meters you will need will surprise you.
Factor 02 of Soil Improvements: Aeration
This is necessary for lots of reasons, to ensure full and proper decomposition of organic matter to humus and plant foods, to allow roots and other soil organisms to breathe, and to assist drainage. Aeration can be improved by most of the techniques used to improve drainage.
Factor 03 of Soil Improvements: Workability
Improving the workability of the soil generally means making a heavy clay soil easier to manage, although in some instances it might mean that you want to reduce the regard you need to have for the weather when deciding whether to garden or not.
Improvements in workability are therefore generally as for drainage, but with the emphasis on adding the organic matter of the right type.
Factor 04 of Soil Improvement: Colour
At first sight, the color of the soil might seem unimportant. It will generally be the same as the rock the soil was derived from or the organic matter content of the soil. It is useful sometimes to help decide how long the soil has been in cultivation, (dark soils may have been cultivated for many generations) but the main purpose of wanting to change the color of the soil is usually to darken it so that (as a dark color) it absorbs more heat and warms up slightly more quickly in spring and is therefore earlier. Traditionally weathered soot (which used to be readily available) was used to darken a soil, but these days darkening tends to be a by-product of adding organic matter except for a few dedicated (or obsessed?) exhibition growers.
Factor 05 of Soil Improvement: Water Retention
In sandy type soils, the capacity of your soil to hold water will probably be the growth limiting factor. The answer is generally to add as much of the right sort of organic matter as you can get your hands on so that you increase the number of little organic sponges in the soil that can absorb water in times of plenty and give it up to root hairs in times of dryness.