How to Recognize the Fragility of Soil Structure in your Garden
Inexperienced gardeners often fail to recognize the fragility of soil structure. While some soils are less stable than others, all are liable to damage. The main problem is compaction. This results from mechanical pressure which forces the particles together, driving out air and spoiling the environment for micro-organ¬isms. In extreme cases roots will be unable to pene¬trate and water will not drain away. Light, sandy soils may not suffer but heavier soils with a high clay con¬tent can be ruined and are particularly prone to com¬paction if walked on when wet. It is like stepping on, and sinking into, semi-hard concrete, and does plant roots no good whatsoever. The best way to avoid this problem involves creating beds never more than one footstep wide, so there is no need to walk on the soil, with stone paths running between. The ideal is rarely practical in the flower garden, but you can minimize difficulties when undertaking winter projects by lay¬ing planks on the soil surface. These reduce the pres¬sure on the ground, distributing weight more evenly, and are more pleasant to tread on than mud.
A hard compacted layer below the surface is known as a “pan”. It can be created by the weight of a mechanical cultivator, although you may not be aware of the problem because the surface soil can still look friable or crumbly. One of the worst instances occurs when a garden has previously been used as a building site. The contractors will have churned up the subsoil and afterwards, as a cosmetic exercise, spread a load or two of imported topsoil over the plot, hiding faults and problems. Standing water after rain is a sure sign, but even without it, investigate the state of the soil sooner rather than later.
Once the pan forms it impedes drainage and needs to be broken up. Various tools are available, those particularly worth considering being mechanical cul¬tivators which have an extension at the back designed to crack through the pan as the machine drives for¬ward. Otherwise compaction is best repaired by deep digging and by incorporating bulky material, particu¬larly in heavy soils, to open up the structure and let in oxygen. In cold areas, deep digging can be under¬taken in autumn and the ground left rough through¬out the winter, allowing the action of frost and thawing to convert clotted land into a more manageable tilth.
To conclude this, three examples of prob¬lem soils are given with suggested treatments.
Heavy, sticky clay – The problem with clay is that it does not forgive abuse, being easily damaged and dif¬ficult to repair. Any means of making it more porous and getting more air into the mix will improve grow¬ing conditions. The prime objective is to build up the humus by digging in bulky compost. As the garden develops, be assiduous in your composting, hoarding every scrap of organic refuse from rose prunings to kitchen waste. Farmyard manure, if you can obtain it, is beneficial, adding humus as well as nutrients. Further additions of coarse grit, shingle and even ash will also open up the texture and give a crumbly, more manageable texture in which plants thrive.
In many respects the plants themselves help to im¬prove heavy soils. The roots penetrate and open the structure while falling foliage increases the organic content so that gradually, over the years, the surface soil becomes easier to work.
To establish new plants in the heaviest soils you must improve conditions around their roots. When digging planting holes incorporate extra quantities of leaf mould enriched with bone meal or a slow release fertilizer at the bottom to ensure rapid establishment. The one huge compensation for having heavy clay is that once the plants have settled they succeed much better than in less fertile ground.
Fine, blowing sand – In some areas, particularly near the coast, shifting sands or sandy soil may cause prob¬lems. Unlike clay which stays put, sand at its worst can literally blow away, leaving roots exposed, or it can blow in, depositing a desert-like dune over the en¬tire surface of the garden.
The main advantage of sandy soil is that it is easy to work and difficult to damage. The drawback is that water runs straight through it, flushing away dis¬solved mineral salts, leaving plants dry and under¬nourished. It may seem contradictory to suggest that organic matter is the best way of adding body when it also lightens heavy clay, but it works. The addition of humus in the form of leaf mould or rotted manure im-proves moisture retention. In the case of existing beds which need bulking up, it may be necessary to remove all the plants, transferring them to a temporary bed, while the soil is treated. Most sands tend to be acid, so if you want to grow food, mix lime into the soil.
Thin topsoil over chalk or bare rock – In such con¬ditions, moisture retention will be impaired since there is very little soil to hold the water (made worse with chalk as it is so porous); as a result, plants are likely to suffer from summer drought. Also, pure chalk or pure rock are poor sources of plant nutrients. As an additional problem, soil over chalk is likely to be strongly alkaline, restricting the choice of plants to the most lime-tolerant.
The solution to these difficulties is simpler than you might think. Wise plant choice is the first consider¬ation. Species which thrive in the wild on chalk downs or rocky outcrops will be natural choices and many have superb garden cultivars. Blossom trees, such as crab apples and hawthorn, do well, and among chalk-loving herbaceous plants are pinks, carnations and many campanulas.
It also makes sense to build up the topsoil as much as possible with imported material. Extra loam will help, as will generous additions of leaf mould and organic matter or manure. As for moisture retention, building up the soil’s humus content will enable it to hold more water, and a thick mulch either of com¬post, tree bark chippings or similar material spread generously over the surface helps reduce the eva¬poration rate from the ground.
Tags: fertilizer, flower garden, gardener, light sandy soils, planks, plant roots, plants, soil structure, soil surface, soils, stone paths, surface soil, topsoil, winter projects