What is pH?
A measure of acidity or alkalinity on a scale of 1 to 14, especially important in gardens in relation to the soil. The mid-point is called neutrality, while values below 7 are acid and above 7 alkaline.
The technical details of pH needn't worry you but you should be aware that garden soils rarely have pH values below 4.5 (very acid) and rarely above 8 (very alkaline). Most types of garden plant, and certainly most fruit and vegetables, prefer a pH just on the acid side of neutral (about 6.5), although a few lime-loving species prefer it slightly higher and a fairly large group of very attractive ornamentals that includes rhododendrons, azaleas, camellias and many heathers prefers it more acid.
How can I test the pH of my garden soil?
With a small pH testing kit
It is fairly easy to test the pH of garden soil with a small pH testing kit - usually, a test solution is added to a small sample of soil, the colour change noted and the pH value read from a chart. In theory, the desired adjustment may then be made. This is not as easy as you might think however because although it's possible to raise the pH of an acid soil by adding lime, it is very difficult to lower the pH of an already alkaline soil. You can go some way towards acidifying your soil by adding sulphur chips.
What is 'top-soil'?
The upper layers of the soil in which your plants grow. Beneath is the sub-soil, comprising the less well weathered parts of the underlying rock and, by and large, lacking any humus content.
The relative depths of top- and sub-soil vary greatly from garden to garden, depending on how easily the underlying rock breaks down, on the factors that have influenced this breakdown, the extent to which mineral matter has been transported from elsewhere and deposited on the surface and, conversely, the extent to which mineral matter has been removed. From time to time, you will see top-soil advertised for sale, sometimes by builders who have removed it from a development site. It is rarely very cheap but, if you have an eroded site, it may represent the best, and certainly the quickest way of producing a few small beds for growing plants satisfactorily. Bear in mind, however, that a cubic metre of soil weighs approximately 1 tonne and that, as you need at least 30 cm depth to garden satisfactorily, this 1 tonne will only cover an area of about 9 square metres.
What is the best way of making compost?
It is important to confine the material in some type of container. For an medium-sized garden, the best plan is to build or buy a box-like compost bin , approximately 1 or 1.25 m cube with slatted sides, no base and no lid. For a large garden, a double structure is a good idea, one half being filled while the contents of the other are decomposing. For small gardens, you might choose a plastic barrel-like compost bins which work fairly satisfactorily provided they are well ventilated. Use an accelerator like Garotta to enhance the decomposition process. Site the bin in partial shade with free air circulation all round. Place a layer of coarse rubble (broken bricks, thick branches and similar material) in the bottom to provide a means for air to circulate beneath. Then start to fill the bin with organic matter. Virtually all non-woody material can be included, although the toughest stems and roots (brassicas, for instance) are best chopped or crushed. Household waste may be added, including shredded paper, but not anything plastic or any animal remains (which will tend to attract vermin), together with garden weeds, lawn mowings, windfall fruit, straw and similar matter. It is important to keep a balance between the various types of substance; no more than one third of the contents should comprise lawn mowings for instance as it is otherwise difficult to prevent the contents from becoming too slimy. Leaves tend to rot more slowly than most other forms of organic matter and can block up a compost bin by preventing the free circulation of air. They are used much more efficiently if stacked separately and used to make leaf-mould. The roots and rhizomes of perennial weeds like couch can be included although they are best placed in the centre of the bin where the temperatures will be highest and decomposition most efficient. Although all such organic matter should rot in a well-made bin, it is wise not to include Brassica roots attacked by club root or onion roots attacked by white rot, for the risk of spreading viable fungus over the garden in the compost is not worth taking. Some gardeners like to turn the compost from the outer part of the bin inwards after a few months to ensure that all is rotted uniformly, but this shouldn't really be necessary in a well-made bin. Although I don't recommend a tight-fitting lid some form of cover is useful to prevent the compost from becoming too wet or too dry. A compost accelerator such as Garotta will add valuable nitrogen to encourage the bacteria.
How can I improve heavy clay?
Not easily or quickly. There is no substitute for organic matter dug in over a long period of time.
Most of the problems of a clay soil stem from its composition - very fine particles of mineral matter that pack together extremely closely, thus preventing the free movement of water and air. Water tends to accumulate in the soil and not drain away therefore, while the inability of air to penetrate means that the soil tends to be cold - the warmth of the sun simply cannot reach more than a few centimetres below the surface. To improve both of these features the clay requires 'opening up' - the solid, impenetrable mass must be changed into a finer, crumb-like structure. At different times, various amendments have been suggested to bring about this improvement. Lime does reduce the stickiness but its effects are short-lived. It's the view of almost all professional and experienced gardeners that the thorough incorporation of large quantities of bulky organic matter like farmyard manure or garden compost is the only reliable way to improve the structure of heavy clay soils. It should be dug in as thoroughly as possible but when the exercise is done, the resulting soil will be extremely fertile, because clay particles attract and hold the many chemical substances needed for plant growth.
What are the special problems of a chalky soil?
The biggest problem with alkaline or chalky soil is that most mineral nutrients needed by plants are taken up less readily than they are from more acid conditions. As a result, leaves turn yellow and some plants grow less effectively.
Strictly speaking, a chalky soil is one derived from chalk rock, but the word is used colloquially to mean any highly alkaline soil - chalk is simply a rather purer form of calcium carbonate than limestone. Although it's often thought that most choice trees and shrubs prefer or require acid conditions, there are, in fact, more garden plants tolerant of chalk than there are of high acidity. Our impression is simply biased by such lovely plants as rhododendrons, azaleas and camellias. Hence, although plants that are well-adapted to such conditions (and known as calcicoles) will grow very satisfactorily, those more marginally tolerant of chalk will require certain special treatment if they are to thrive. In particular, an amendment known as sequestered iron will be necessary if the foliage is not to become yellowed and thus impaired in function. This sequestrene should be applied annually at the start of the season; granular, liquid and powder formulations are now available.
Once soil is contaminated with pests and diseases, is there any way of disinfecting it?
No, the answer lies in crop rotation
There is no reliable way of disinfecting soil in the garden, although small quantities can, of course, be treated quite easily for compost preparation. The easiest way to do this is in a domestic oven for about 1 hour at 150 deg. C (gas regulo 2), or in 4 kg batches in a microwave oven at maximum setting for 8 minutes. It is sometimes claimed that beds, borders and vegetable plots can be disinfected satisfactorily by watering on various proprietary solutions. The claims shouldn't be taken too seriously for these substances have only a localised and temporary effect on a limited range of organisms and, in any event, contaminated soil will soon be brought to the surface from below. With few exceptions, nonetheless (club root and onion white-rot in particular), most soil pest and disease organisms will die away if crops prone to the particular problem are not grown on the affected site for about four or five years, the principle behind crop rotation.
Does leaf mould contain harmful pests and diseases?
Not really but it is best not used for young seedlings because creatures such woodlice may attack them and some mould fungi that are harmless to mature plants may also be present.
You will see all manner of fungi and micro-organisms growing among, and crawling about in, the leaf mould (as you will in garden compost), but these organisms are all helping to break down the material into a form that plants can utilise. Provided you don't use 'raw' leaf mould or compost in the greenhouse or for seed-boxes (where the combination of weak young plants and extra warmth means that even a normally harmless soil organism could cause damage), there is no risk. Leaf mould is a very valuable soil conditioning and mulching material. Indeed, as leaves rot down more slowly in a compost heap than most other types of organic matter, it is worth making your own leaf mould separately from compost if you have a quantity of trees in your garden. A simple wire netting leaf mould cage is all that is needed, with periodic watering of the layers of leaves and dusting with a compost accelerator.
What exactly is peat and why I am always being told not to use it?
Peat is the partly decomposed remains of plants and it should only be used in gardening when there is absolutely no alternative because much of it comes from important and irreplaceable ecological sites. Any peat that you buy and that we sell comes from approved sites.
Peat is the partly decomposed remains of plants; only partly decomposed because the plants have died in poorly aerated, waterlogged conditions, where the bacteria and fungi that are responsible for decay processes do not thrive. Peat has proved valuable in gardening because of its water-holding capacity (several times its own weight), its acidity and ability to hold fertilisers. But today it should only be used when there is no alternative and is derived from an approved site. Moss peat is derived very largely from mosses, especially species of Sphagnum. It is very acidic, with a pH of around 3.5 - 4. Sedge peat contains the remains of sedges, with some wet-land grasses and heathers. Sedge peats tend to be blacker than sphagnum peats, are more decomposed but have a lower moisture retentiveness, and are generally somewhat less acid. The most convenient way of buying peat, if more than a very small amount is needed, is in the form of bales; they are lighter to transport than bags for the peat is compressed and has had much of its water squeezed out.
Which is the best type of compost for seedlings and potting on?
Either a soil-less or soil-based mixture that is sold specifically for young plants. These have the low levels of nutrient that young plants need.
Not so many years ago, almost all potting composts, for sowing seeds and raising seedlings, for growing on more mature plants or for striking cuttings, included at least some sterilised loam, with greater or lesser amounts of added sand, grit, peat and/or fertiliser. Largely because of the shortage, and consequent cost, of adequate quantities of sterilised loam, however, soil-less composts have become very popular. Most are now based on alternatives to peat. Both soil-based (John Innes) and soil-less composts have their advantages and disadvantages, some of which are fairly self-evident, others more technical. Soil-based composts are more expensive to prepare properly and thus are more expensive to buy. They are also heavier and much less convenient to handle. Nonetheless, they contain a wider range of nutrients and plants grown in them are much less dependent on supplementary liquid feeding. Ultimately, for seedling composts especially, cost and ease of handling mean that most gardeners now use soil-less composts and find them satisfactory for most purposes. The main factor to watch closely is watering, for a soil-less compost especially can easily become waterlogged but, conversely, may very difficult to re-wet once it has dried out.
Why can't I grow rhododendrons, azaleas, camellias, magnolias and heathers on some soils?
Because these plants prefer acid conditions and will not thrive on alkaline or chalky soils.
The appeal of many of the most beautiful flowering shrubs is unfortunately limited by their preference for acid soils. Plants such as rhododendrons and azaleas and, to a slightly lesser extent, camellias and even magnolias are less able to take up many of their required nutrients from alkaline soils with the result that they lack vigour. In particular, they suffer from an inability to take up the element iron and consequently develop yellowing of the foliage which is thus impaired in its efficiency to manufacture essential food-stuffs.
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