Garden Cultivation Techniques

Here I want to consider the way in which soil should be cultivated. The word cultivation describes all the physical operations on the soil: digging, forking, raking, trenching and hoeing. You may wonder why any cultivation is necessary; no-one cultivates the soil for wild plants and yet they still grow perfectly well. It is important to realise, however, that when we grow plants in a garden we may be using natural principles but many of the ways that we treat them and our expectations of them are distinctly unnatural.

We grow plants that are alien to the habitat, they may be removed before the end of their life span, they are grown in artificial spatial arrangements and we expect much more from them than nature can manage: bigger flowers, larger fruiting heads, more (eaves and so forth. In the course of all this unnatural behaviour the soil is subject to a great deal of interference. We walk over it and compact it with machinery, we neglect the natural processes of organic matter recycling, and we use powerful hoses and sprinklers that may fall with much greater force than rain droplets. Our abuse of the soil is mainly, therefore, a matter of physical compaction. The additional weight causes the soil crumbs to be pressed together, resulting in loss of pore spaces for water and air. Loss of pore spaces adversely affects root development and soon this in turn leads to poor plant growth overall.

At its most severe, the lack of air can limit the growth of the aerobic bacteria and fungi that are needed to break down organic matter into humus. These organisms may be replaced with populations of anaerobic bacteria, which bring about a different chemical degradation of organic matter resulting in the formation of a dark unpleasant material and the liberation of gases such as hydrogen sulphide (with its characteristic smell of bad eggs), hydrogen and methane. Available nitrogen is lost more quickly when there is insufficient oxygen and there may also be an accumulation of chemicals that at best are useless as plant nutrients and, at worst, may be toxic to plant growth.

When water is unable to drain freely through the soil, it accumulates in puddles on the surface, so further restricting air from entering. When the soil crumbs disintegrate and block pore spaces, this causes an effect known as capping. It is a special problem on clay soil as the minute particles pack together very tightly making them particularly impenetrable. Most compaction occurs near the surface of the soil but a breakdown of physical structure can also occur at some depth, even when the soil above is unaffected. The result of this is called a pan or hard pan, a compacted layer at about 20cm (8in) below the surface.

The presence of a hard pan prevents water from moving downwards, although it may initially appear to drain away from the surface. Pan formation can arise naturally in regions of very high rainfall when soluble minerals are precipitated from solution or when small insoluble particles gradually accumulate. But pans can also form if the soil is not properly managed. For example, if the soil is dug repeatedly and frequently to the same shallow depth or if a mechanical cultivator is used regularly.

As the soil structure deteriorates in the way I’ve described, so the conditions become less favourable for earthworms and other soil life and the end result is that plant growth can decline. All these adverse physical effects on soil can be corrected but it is, of course, better to avoid or minimise them happening in the first place. The way to do this is to cultivate your soil using one or more of a variety of techniques that I describe below.

Digging

Of all cultivation techniques, digging is the most important as it breaks through compacted soil and allows water and air to penetrate. As the soil is turned over, lumps or clods of soil are exposed to the weather (especially frost) which then breaks them down into smaller particles. Digging also provides the opportunity for organic matter to be added to the soil. To dig the soil thoroughly, the plot must be free of plants, so most digging takes place in early spring (before planting or sowing) or autumn (after the area is cleared). However, you should take into account the weather conditions; ideally, frozen soil should not be dug nor should soil that is heavily waterlogged.

The most familiar type of digging is called single digging – the cultivation is limited to one spade’s depth. From time to time, however, the lower levels of the soil should be broken up too and this is referred to as double digging. It offers the best way of preventing pan development and ensuring that deep rooted plants have the best growing conditions.

Before explaining the detailed differences between single and double digging, let me stress that proper digging, and double digging especially, is strenuous exercise and so it is an activity that should be undertaken at a steady pace, not in a rush. Divide up the area to be dug and aim to tackle a section each week, from say, early autumn to just before Christmas. Ideally, you should warm up your muscles beforehand by gently bending and stretching. Wear suitable clothing and footwear; leather boots are preferable to rubber Wellington boots. Take regular breaks by doing other lighter tasks around the garden. And when digging, try to keep your back as straight as possible; a spade or fork with an appropriate handle length for your height will help. Avoid making awkward jerking movements; aim to build up a steady rhythm.

It is worth reminding ourselves of the correct tools to use. A spade is used for moving soil from one place to another but a fork is better for breaking down the soil into smaller pieces. It is also easier to lift, spread and mix in soil improvers such as organic matter with a fork rather than a spade. Some soils are hard to penetrate with a spade and a fork may be easier to push in. Further breaking up of clods in spring is often easier with the back of a fork.

Single Digging

With single digging, the cultivation or disturbance of the soil is limited to the depth of the spade and the penetration of organic matter below this level is solely by the activities of earthworms and other soil animals. Because soil can only be dug by spade when it is fallow (free from plants) and not fro/en, you have a choice between autumn and early spring. Both times are important. In autumn, soil should be rough dug; that is, the spade inserted approximately to its own depth (usually called a spit) and the soil turned over but then scarcely broken down. During this autumn digging, manure or compost should be forked over the surface and roughly mixed with the soil. The water within the clods will be frozen during the winter and the soil will thus break down naturally into smaller pieces as a result of the expansion of the ice.

Even after this breakdown, however, the soil will still be in too coarse a condition for sowing seeds or planting plants when spring arrives. A second digging, either with a spade or fork, should be performed then, making liberal use of the back of the tool to break down the clods into finer pieces. If the soil is broken down in this way in the autumn, effort is being expended wastefully (the frost will do the task for nothing) but more importantly, the heavy winter rains beating onto the line surface will bring about capping once again.

Double Digging

Double digging is digging to two spade’s depth. It is also known as bastard trenching (using, I’ve always believed, ‘bastard’ in the sense of ’counterfeit’ or ‘spurious’: it looks like proper trenching but isn’t). Work your way in a line across the plot to form a trench, removing the soil by barrow to the end of the plot. Fork organic matter into the base of the trench and then dig another trench, parallel to the first. Using the soil from the second to re-fill the first, gradually work your way along the plot, making parallel trenches and filling the final trench with the soil that you harrowed away at the start. You should only double dig when the ground has not previously been dug for some time; and also when it won’t be dug again for some time. The two most obvious occasions when there will be no digging for some while are when planting a new herbaceous or shrub border and when preparing a new deep bed for vegetables (see below).

double digging

Use of Trenches for Crops

Trenching (real trenching, not bastard trenching) is a traditional technique used lot-providing certain types of crops, such as runner beans and sweet peas, with sustained moisture through their growing season. The principle is to incorporate organic matter up to a depth of 30-50cm (12-20in) in order to hold the moisture. Care must be taken, however, not to add layers of manure continually to the base of the trench. This can led to a sump forming which provides ideal conditions for root-rotting organisms to which the pea and bean family is especially prone. Rather than put the organic matter in layers at the bottom of the trench, incorporate it uniformly, therefore, throughout the depth of the trench as you refill it.

Trenching is also used to produce traditional trenched celery. Celery is a plant that grows naturally in wet, boggy places and therefore grows well in a moist trench. Blanching or whitening of the stems is enhanced by back filling the trench with soil as the plants grow.

The Deep Bed System

Sometimes called a no dig system – although this can also reler to blanket mulching — this approach seeks to minimise compaction by feet and machinery so that routine annual digging is no longer necessary to improve the soils physical structure. However, the facility for incorporating organic matter at some depth is also lost too. To replace this, there is a need to dig very thoroughly by double digging when the bed system is set up and every four to six years thereafter.

The beds should be constructed so that all parts can be reached from the pathways without the need to tread on the beds – a width of about 1.2m (4ft) is ideal — and if access is required for tasks such as planting or thinning, then planks can be placed over the beds sup-ported on bricks or blocks.

Hoeing

There are two main types of hoe: the draw hoe, used to earth up or cover vegetables with soil to protect them from frost or to exclude light, and the Dutch hoe which is used to sever annual weeds and also to loosen the soil surface. In dry weather, pulling and pushing a Dutch hoe over the soil surface exposes darker, more moist soil underneath.

Two contrasting beliefs have grown up around this type of hoeing. One is that moisture release is encouraged as damp soil is brought to the surface, the other is that moisture loss is minimised by keeping the surface friable. There is some truth in the first although in drought conditions the soil is best left not hoed as the dry layer does act as a mulch. Hoeing is a very valuable practice but always remember its function in weed control and take care not to disturb shallow- or brittle-rooted crops such as onions.

Raking

Even after the soil has been forked over in spring, the soil surface is still too coarse for seed germination. The pieces of soil need to be further broken down and if the soil is very stony, then some should be removed if seeds are to be sown direct (some stones are valuable nonetheless as they retain warmth and act as a positive aid to early growth). A fork will help initially but a soil rake is needed to create a seed-bed. The aim is to produce a level area where the soil is friable and the soil particles form small, uniform crumbs. Such a soil is said to have a good tilth although this is a strange word and you will seldom hear gardeners talk of a bad tilth; all tilth is good tilth. A friable soil enables the surface to warm up more evenly and provide more uniform conditions of moisture and light for the seeds. There is no merit in raking and creating a fine tilth in autumn as the heavy winter rain will cause capping on the surface and a hard crusty layer will form.

Use of Powered Cultivators

I have already discussed powered cultivators as tools and have pointed out their drawbacks, most notably encouraging the formation of an impervious pan within the soil. They also cause smearing of clay soils by breaking down the crumbs. If a powered cultivator must be used, because the area is too large to dig, it is important to cultivate more deeply than usual (this is called sub-soiling) every two or three years so any pan is broken up.

Mulches and Organic Matter

Mulching is a technique that can be used instead of hoeing to control annual weeds; it also helps retain moisture within the soil. A mulch is simply a layer of material placed over the surface of the soil. Traditionally, it is some form of organic matter such as garden compost or manure but there are now other materials often called sheet mulches, made from various forms of plastic or paper. It is worth mentioning that it is only the organic mulches that contribute to improving soil structure. The material works as a mulch while it is on the soil surface but in time.earthworms and other animals drag it down into the soil where it is broken up.

The need for well-rotted as opposed to fresh organic matter to add to the soil is often emphasised and it is worth understanding why this distinction is important. If fresh organic matter, such as plant debris, is placed on or mixed into the soil it will begin to decompose as it comes into contact with the soil organisms. The organic matter provides the bacteria and fungi with nutrients. Fresh plant remains are rich in cellulose; the ratio of the two important elements of carbon and nitrogen in them is about 33:1, whereas in partly-decomposed organic matter the ratio is 10:1. The explanation for this is that as fungal and bacterial activity increase on the plant remains, some of the carbon is used by them (some is lost as carbon dioxide) but there is a shortage of nitrogen. A convenient source of nitrogen is the soil itself and in time as the decomposition process proceeds, the carbon to nitrogen ratio rails to 10:1 and the nitrogen becomes mineralised so there is a time when soil nitrogen is depleted or unavailable to plants. This is why applying fresh manure or plant remains directly to the soil is not recommended; the material should be composted in a compost bin first.

29. July 2013 by admin
Categories: Garden Management, Gardening Techniques, Soil Cultivation | Tags: , , | Comments Off on Garden Cultivation Techniques

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