Garden Soils

Plants rely on the soil for water, nutrients and physical support, so the well-being of both plant and soil are closely linked. I’ve always believed that gardeners should understand something about the nature of the soil in order to manage it properly and also to encourage them to treat it with the respect that it merits. First, it must be realised that soil isn’t a single static entity but a changing mixture of different components. To look at these soil constituents in more detail, it is useful to divide them into three groups: non-living, once-living but now dead, and living.

Non-living matter

garden soil begins with the gradual erosion of rocks by rivers, the sea, ice and windThe non-living component of soil consists of minerals. A mineral is defined rather technically as one of a group of naturally occurring solid inorganic substances with a characteristic crystalline form and a uniform chemical composition’. In everyday language, the group embraces such substances as quartz, vermiculite, calcite and dolomite that many gardeners will have heard of. Minerals originate in rocks (a rock as we see it lying in our gardens is no more than a collection of minerals) but over thousands of years they have been eroded by the action of the weather, earth movements, seas and rivers to end up as small particles. Together, these millions of small particles form the basis of soil.

Mineral particles may be derived from the local rock underlying your garden or from parent rock situated some distance away. Mineral matter can be swept many kilometres from its origin by rivers or glaciers and it is for this reason that the chemical characteristics of your soil may be significantly different from those of the rock that outcrops on a nearby hillside. The degradation of the minerals from rock results in three main types of particle: sand, silt and clay.

There are two other non-living components of soil that are vital to plant life, water and air. These are held within the soil, usually in the spaces between the small solid particles, and the way that the solid particles are aggregated together to form crumbs controls the relative amounts of air and water and in large measure this dictates the soil’s characteristics.

Once-living But Now Dead Matter

When a small mass of mineral particles first accumulates in a particular site, it begins to be colonised by relatively simple forms of plant life such as mosses and liverworts. When these plants die, their remains become added to the mass of mineral matter and this blend then forms the very beginnings of soil proper. The dead plant remains are called humus or organic matter.

the closely packed, fine particles of a clay soil act as a barrier to the penetration of air and waterIn time, other plants grow, the relatively simple types soon giving way to more advanced, flowering plants, and they too in turn will add more humus to the soil. One of the commonest examples of this process, and one that many gardeners will have seen, lakes place on the undisturbed tiles of old roofs. Mineral particles are blown to accumulate in the crevices, cushions of mosses soon grow and within a few years, it they are left undisturbed, small flowering plants will be seen to colonise.

In gardens, much of the organic matter is removed before it has a chance to form humus; crop plants are harvested, weeds are removed promptly and dead herbaceous material is taken away at regular intervals in the cause of tidiness. This is why we need to add extra organic matter or humus to the soil in the form of composts and manures to compensate for what we have removed.

Living Matter

All of us are aware of the larger creatures that make their homes in the soil: mammals, like moles and voles; amphibians, like frogs and toads; and reptiles, like snakes and lizards. There are also, however, vast populations of much smaller living organisms: eelworms; earthworms; countless arthropods including insects; myriapods (centipedes and millipedes); mites and woodlice; and astronomical numbers of bacteria together with fungi, algae and representatives of other groups.

There are three main ways in which these living organisms can affect soil. First, fungi and bacteria, together, most importantly, with earthworms, digest the remains of plants and animals and break them down into chemically simpler nutrients that are then available for plants to take up through their roots. Some specialised bacteria also play a very valuable role in the recycling of the important element nitrogen by converting atmospheric nitrogen into inorganic salts which are also then made available to plants.

Second, there is the physical effect of the soil being disturbed and mixed as the animals themselves move around and make burrows which aerate the soil. The volumes of soil that are moved in this way are quite remarkable. As long ago as the nineteenth century, the subject fascinated Charles Darwin who calculated that over 100 tonnes of soil are moved per hectare every year solely through the action of earthworms. This activity improves drainage and makes it easier for plant roots to penetrate the soil. And finally, before we leave the subject of living things in soil, it should be remembered that many soil organisms do use plants as a source of food, so as well as the fount of nutrients and water for plants, the soil can also be a source of pests and diseases.

Many of the constituents of soil are easy to see simply by digging and handling it. What are harder to assess visually are the physical and chemical characteristics that affect plant growth, although I shall show you how, by understanding some simple science, and by observation and simple tests, much important information about your soil can be obtained.

The Physical Characteristics of Soil; Soil Texture

As I explained earlier, in the course of soil formation, mineral particles are broken down into sand, silt and clay, the difference between them lying in the size of the particles. Sand comprises particles 0.06-2.0mm in diameter, silt comprises smaller particles of 0.002-0.06mm while clay particles are minute, less than 0.002mm in diameter. The proportion of these three types of particle in a soil is used by soil scientists to classify soils, each type having a characteristic texture. You can feel die differences yourself if you rub moist samples of soil between your finger tips and thumb: the large sand particles will feel gritty, the silt feels smooth, and smaller particles of clay will be sticky. By shaking up a small sample of soil with water in a clear jar then leaving it to settle, you will see the proportions of sand, silt and clay in layers, the dense sand at the bottom, then the silt and then the clay.

Humus also contributes to soil texture; a soil with a high humus content will stain your fingers when touched and when shaken up with water the humus itself floats on top.

Soil Structure

Mineral particles clump together to form larger aggregates called crumbs. Within a crumb, and between neighbouring crumbs, are spaces. These are known as pores and it is within the pores that air and water are to be found. And whilst the relative amounts of sand, silt, clay and other materials make up a soils texture, the relative proportions of solid matter and pores are an expression of its structure. The ideal garden soil, one with a mixture of large and small pores, is therefore said to be well-structured.

Both the texture and structure of a soil will determine how well it holds water. Water will drain much more rapidly through a sandy soil where there are many more large pores than through a clay soil where the tiny particles are very close together. The addition of humus to a soil will improve its ability to hold moisture, humus acting like a sponge in soaking up and holding the water without causing water-logging.

Chemical Characteristics of Soil

Soil chemistry is a very complex subject but there are a few important aspects for gardeners to consider. The most significant are the relative amounts and relative availability to plants of soil nutrients, and the soil’s relative acidity or alkalinity, expressed as its pH.

the characteristic yellowing between the leaf veins which shows a deficiency of iron symptomatic of alkaline conditionsAlmost all the plant nutrients in a soil are derived from the mineral matter and are a feature of the type of rock from which that matter was derived. Some types of plant thrive better on some soils than others simply because of the different proportions of nutrients present. Plants growing wild rarely suffer from or display the characteristic symptoms of nutrient deficiencies. They tend to grow only in those places to which they are most suited and the nutrients taken up by them are returned to the soil when the plant dies and decomposes. The situation in gardens is rather different as plants are grown intensively together, often not on the most appropriate type of soil, and are then harvested and their remains removed. Although we attempt to compensate by applying additional organic matter and fertiliser, shortages of individual nutrients may still occur.

The expression pH stands for ‘potential of hydrogen’. It is a complex measurement related to the number of charged hydrogen atoms or ions present in solution and is expressed on a logarithmic scale. This means that a solution with a pH of 6 contains ten times as many hydrogen ions as one of pH7 and one hundred times as many as one of pH8. The practical significance of this is that a small difference on the pH scale can represent a big difference in acidity or alkalinity. The pH scale runs from 0 to 14 with pH 7 being neutral: values lower than 7 are acid and values higher are alkaline.

An alkaline soil means the presence of large amounts of calcium, often in the form of chalk or limestone. Under such alkaline conditions and the presence of large amounts of calcium, some nutrients are difficult for plants to absorb (they are said to be unavailable) as they combine with other chemicals in the soil. Many plants have no strong reaction either way to this situation but some have a particular ability to tolerate it (these are known as calcicoles) while some are quite unable to tolerate it (these are called calcifuges). The optimum soil pH in which most plants can take up nutrients satisfactorily is about 6.5 and this is the ideal to which vegetable gardeners especially aspire.

Soil Tests

The nutrient level and pH of a soil can be tested and analysed, either by taking a sample and sending it to a soil laboratory or by purchasing a test kit from a garden centre and doing it yourself.

Nonetheless, whilst I consider pH tests are worth doing, nutritional soil tests can at best raise more questions than answers and at worst be seriously misleading. I say this because the limiting factor for plant growth in most gardens is likely to be the amount and availability of soil nitrogen. Yet simple test kits don’t generally distinguish between total nitrogen in the soil and the proportion available for plants to use. While nitrogen levels can fluctuate considerably depending on weather and other conditions. Nitrogen is highly soluble and the level in soil can diminish very quickly following heavy rain; which is why gardeners often need to apply additional nitrogen fertiliser to their crops in significantly wet seasons. The nutrient status of an unknown soil is often rather effectively revealed by looking for plants that indicate specific nutrient deficiencies.

To check your soil pH, use an inexpensive chemical kit that relies on the colour change of chemical dyes rather than a pH meter. The meters sold for garden use don’t measure pH directly but an electrical parameter which is sometimes but not always directly proportional to it. Results from them can therefore be misleading. The kits are reliable but great care must be taken to obtain a representative sample of soil. You should collect between five and ten samples per 100 sq.m and these should be taken from soil before fertilisers and manures have been applied. Dig a small hole and take samples from the sides rather than the surface, remove any debris such as stones or fibrous organic matter, break up any lumps you find and mix the samples together with distilled or deionised water using clean equipment. You will then need to take sub-samples from your original sample in the small test-tubes provided. The kit will then require you to add a dye or use coloured indicator paper impregnated with dye. Follow the instructions offered with the kit and compare the colour produced by your test sample with the colours on the chart supplied to find the pH of your soil.

A Soil Profile

To examine a soil profile in your garden, dig a flat-sided hole in an undisturbed (not recently cultivated) area. Examine the vertical sides of the hole and you should see fairly distinct horizontal layers. By studying the depth and composition of these layers, soil surveyors classify soils and deduce the processes that led to the soil’s formation, for gardeners, it is sufficient to be able to identify the darker upper layer of top soil, the lower sub-soil and the underlying rock. Look also for the presence of a hard, crusty horizontal layer of deposited mineral salts about 20cm (8in) down within the soil. Such a layer is called a pan and it can impede drainage and root growth even on soils that otherwise are free-draining. The presence of a pan may explain why you have had problems with poor plant growth and water-logging; some deep digging will be needed to break it up.

Digging a straight-sided hole in your garden will reveal your soil profile: an upper humus-rich layer of top soil lies over a much poorer layer of sub-soil which in turn grades into the bed rockThe top soil is the upper-most fertile layer containing the fixing and once-living but now dead constituents. An ideal top soil should be at least one spade deep but the depth can vary greatly from one site to another. Sub-soil, which lies beneath, is often lighter in colour and has a coarser texture than top soil. Avoid mixing the two together as the sub-soil lacks organic matter and micro-organisms. Normally, organic matter should be added to the top soil region but if the top soil is shallow, or the site has been neglected for a few years, double digging will be needed to take it deeper. Double digging will also gradually enable you to increase the depth of a shallow top soil.

Soil Conditions

A clay soil is hard to cultivate, poorly drained and tends to be wet in winter then very slow to warm up in spring. In summer the surface can become hard and impenetrable. On the plus side, the minute particles present in clay do hold plant nutrients very effectively so fertility is high. A sandy soil, by contrast, is easy to work and quick to warm up in spring but plants can suffer as water drains through very rapidly, taking nutrients with it. Of course, there are plants that have become adapted to both but in order to grow the widest range of plants, the ideal soil is a medium loam, a readily workable blend of sand, silt and clay.

Most of us, at least initially, don’t have that ideal loam but soils at either extreme can be improved by adding humus in the form of organic matter. The sponge-like properties of humus will help a sandy soil retain moisture while the natural glues that it contains will help bind mineral particles into crumbs. Paradoxically, humus will also improve clay soils as it helps crumb formation. So opening up the soil and making it less sticky and prone to water-logging.

large clods left on the soil surface during autumn digging will be broken down gradually and efficiently by winter frostEventually, humus is broken down by micro-organisms so adding organic matter to the soil is a continuing activity. On a large scale it can be both tiring and expensive, so concentrate on one bed at a time. In a conventional vegetable plot, digging in organic matter should be an annual activity undertaken in the autumn when the bed is clear of plants and it can be rough dug. The advantage of autumn digging is that large clods of earth can be left over winter and will be broken down by winter rains and frosts. Then, in the spring, it will be easier for you to form a tilth, a fine surface of level soil suitable for seed germination. The digging will also help improve the soil structure as soil that has become compacted by heavy feet or equipment can be opened up. With a deep bed system, the incorporation of organic-matter may be done through the operation of double digging every four or five years.

In a herbaceous border or soft fruit garden, the whole bed can be dug over when plants are replaced, say every seven to ten years. Of course, organic matter will be added when individual plants are replaced or in the form of a loose mulch in spring that will gradually be absorbed into the soil during the summer.

After finding out the pH of your soil, you may wish to adjust it in order to grow a wider range of plants. Raising the pH of an acid soil is fairly straightforward and simply requires applications of garden lime (ground limestone). The best time to add lime is in the autumn and a rotary cultivator will help to incorporate it thoroughly. To determine the correct dose, follow the instructions on the packet (the amount to add will depend on the soil type and the pH) and remember to wear gloves when handling lime. Don’t be tempted to add too much as over-liming can cause nutrient deficiency problems. Also don’t apply lime within one month of applying animal manure to the soil, for the two can combine to liberate ammonia which is harmful. Check the pH again after about nine months and lime again if necessary but once the desired pH level has been reached, no further liming should be needed for four to five years.

Soils that are very alkaline (pH 7.5 and above) are harder to alter. Proprietary sulphur chips can be added over a small area to reduce the pH but the results are not as effective as when using lime to raise pH. Applying fertilisers that contain ammonium sulphate as a nitrogen source will also help to lower the pH but it isn’t good practice to add soluble nitrogen simply as a pH adjuster. The best approach is to grow plants that prefer alkaline conditions and to grow any favourite calcifuges in potting compost in containers. Any borderline plant that shows deficiency symptoms can be treated with a fertiliser containing sequestered iron.

Limitations of soil improvements

Most soil improvement will consist of adding organic matter or increasing the pH by adding lime. It isn’t practical to import into the garden large quantities of non-living constituents like top soil, sand, silt or clay in an attempt to change the entire garden. The quantity needed to effect a difference would make such an exercise extremely hard work and very expensive. Small areas, however, can sometimes be improved in this way, especially when there is a particular benefit: using fresh top soil when replanting a rose garden to help avoid rose replant problem, creating a raised bed to enable root vegetables to be grown in a garden with heavy clay, or digging in grit around shrubs that need a well-drained soil are instances of this. New housing developments often have the top soil removed during construction, and as plants will not grow satisfactorily in sub-soil anyone purchasing a new house should ensure that their builder is contracted to replace the top soil before they move in.

Chemicals are available that claim to help improve soil structure by aggregating the particles into crumbs. They are sometimes called soil conditioners and are of variable effectiveness. They include products derived from seaweed which are rich in alginates; whilst they may be helpful on a small scale, the cost of trying to improve large volumes of soil in this way would be prohibitive. Moreover, adding soil improvers will not help improve drainage if a heavy clay soil lies over a poorly-draining subsoil. Such severe water-logging can generally only be corrected with a drainage system which is best installed under professional guidance.

23. July 2013 by admin
Categories: Soil Cultivation | Tags: , , | Comments Off on Garden Soils

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