The structure, fertility and pH of your soil all have a tremendous influence on which plants you are able to grow and how healthy those plants will be. Anyone who has moved house and merrily planted their old favourites in their new garden will understand that not all soils are created equal.

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It all starts with soil structure. Mineral soils are mainly made up of ground-up rock particles such as silt, sand and clay, while organic soils are mainly made up of living and dead fungi, plants and animals. Most soils are a mixture of both minerals and organics and each have properties that determine the success of our harvests and the beauty of our flower gardens.

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In both mineral and organic soils there is something called cation exchange which determines how successfully an individual soil particle (ie one grain of sand or one minute grain of clay) is able to hold on to the nutrients that are floating around in the water that circulates in the pores between each soil particle. Clay has a very high cation exchange capacity, as does humus. This means that soils that have a good clay or humus content (or both) will produce healthier plants with a faster growth rate than plants grown under exactly the same conditions with exactly the same feeding routine but in coarser or sandier soils. This isn’t because the soil particles themselves are more nutritious, just that the soil particle can hold onto and then release to the plant roots the nutrients that are available. This is partly due to the size and therefor the surface area of the individual soil particles and partly to the fact that the individual particles of clay and humus are negatively charged so they attract the positively charged cation nutrients that are floating about in the water. These particles of clay and humus hold the nutrients in a way that they can be taken by the plants but cannot be washed away unless they are exchanged or replaced with more cations. Hence the exchange in cation exchange.

A high clay or organic content also means that it takes large numbers of calcium cations (the element needed to raise pH) to replace the large numbers of hydrogen and aluminium cations (the elements that are responsible for reducing pH) which are held by the soil particles. This is called the buffering capacity and simply means that it takes a lot more lime to raise the pH of clay soils than sandy soils.  By the same token, sandy soils are vulnerable to fluctuations in pH because of their low buffering capacity, and so can be easily over-limed. For this reason it is necessary to get to know the make-up of your soil before trying to adjust the pH.

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Another great influence exerted by the size of soil particles is drainage. Sandy soils obviously drain far more freely than clay or organically-rich soils, both because the particle size determines the size of the spaces between the particles and because the  sand grains have no capacity to swell and absorb the available water.

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So the first thing to do is to determine the structure and texture of your soil.  Begin your investigation by taking a soil sample about the size of a walnut and moistening it slightly. Rub the sample through your fingers to break it up and to remove any stones or large particles. Try rolling the sample into a worm shape; sandy soils will not be able to retain the shape, even if very wet.  Silts and loams (mixtures of sands and clays in different proportions) can be rolled into a worm, but will hold the shape with difficulty. Very clay soils will retain the worm shape with ease and will not readily fall apart.

To further define the type of soil rub the sample between your fingers; any feeling of grittiness indicates the presence of sand, silkiness indicates silt and stickiness confirms that there is a significant amount of clay. A large proportion of organic matter in your soil is easily determined by a silky and spongy texture.

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Organic matter is a tonic for all types of soils. It reduces the stickiness of clays, improving their drainage capacity, their ability to release water to plant roots and reduces their tendency to cake and bake in hot or dry weather. Organic matter also serves to increase soil temperatures in clays, both because of the aerobic activity of microorganisms and the decay process, and also because waterlogging is reduced.  This last property is incalculably beneficial in the spring.

In sandy soils the addition of organic matter vastly improves the water holding ability and cation exchange capacity (and therefor the nutrient availability) and increases the buffering capacity, thereby shielding plants from fluctuations in pH.  Sandy soils benefit enormously from the addition of plenty of organic matter.

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Soil structure is inextricably linked to soil pH.  The term pH stands for the percentage of hydrogen, which determines how acid or alkaline your soil is. The pH scale goes from 0 to 14, with 7 being neutral; anything under 7 is acid and anything over 7 is alkaline.

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The acidifying hydrogen ions can be replaced by calcium ions, but to understand the difficulty of altering soil pH it is helpful to know that a soil with of a pH of 6 contains 10 times more hydrogen ions than a soil with a pH of 7 (not 1 more as may be assumed) , a soil with a pH of 5 contains 100 times more than a soil with pH7 and a soil with a pH of 4 contains 1000 times more hydrogen ions, and so on. Needless to say the converse is true with the reduction of hydrogen ions as the scale goes above 7 into the alkaline zone.

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pH affects plants by influencing the availability of nutrients in the soil. At a pH of 5.5 to 6.5 most essential plant nutrients are available in a chemical form that is accessible to most plants.  As you slide up or down the scale other nutrients are made available at toxic levels. Obviously, then, it is usually best to try and keep your soil within the neutral zone.

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Some plant groups, brassicas among them,  prefer a slightly alkaline soil, while others, such as plants belonging to the blueberry family, need the acid conditions of their homelands. Plant preferences may be governed by nutrient availability or by the prevalence of certain diseases at a particular pH level. The living community within the soil -microbes, fungi and bacteria - work their best within the neutral  zone and will die in extreme conditions.

Testing the pH value of your soil is a quick and easy task, and while you can send a sample off for analysis, it is easier to pop down to your local nursery and pick up a test kit. Simply make up a sample from several areas of your garden, mix the soils together and then follow the developing instructions on the kit. Compare the resultant colour of your sample with the chart supplied with the kit. These tests, properly done, are reliable, easy and inexpensive. Alternatively take a look at the weeds that grow naturally on your plot, as they can offer a fairly reliable indication of soil conditions and pH. Sorrel, mullein, stinging nettle and wild pansy all thrive in acidic soils; salad burnet, scarlet pimpernel, nodding thistle, stinkweed and campion prefer alkaline conditions.

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Soil pH in mildly acidic soils is easy though sometimes time consuming to alter. The application of calcium in various forms of chalk or limestone takes time to take effect. The fastest acting form of limestone, builders lime or hydrated lime, can be irritating to skin and eyes and needs to be applied with care. Ground limestone, calcified seaweed and ground chalk are slower acting while the slowest acting Dolomitic limestone, also known as magnesium limestone,  will raise the pH and provide a good source of magnesium into the bargain.

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Lowering pH is more difficult. At one time raised beds made of peat blocks filled with peaty compost were used to nurture acid-loving plants, but thankfully we have moved on with our awareness of the fragile state of the world’s peat reserves. Now, in any attempt to lower the pH, it must be recognised that any alteration will be temporary and so any precious ericaceous plants may well benefit from being grown in containers where pH can more easily be controlled.  For acid-loving plants peat-based composts can be easily be replaced by composted pine bark, coirs and composted green waste. If Kew gardens and the National Trust can do it, it should be a doddle for the rest of us.

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Sulphur is the least expensive and least harmful of the agents commonly used to acidify soils. Aluminium sulphate (often used to turn pink hydrangeas blue)  and ferrous sulphate have been shown in some trials to damage the soil ecology and have a detrimental effect on invertebrates while in larger quantities they interfere with the availability of the essential plant nutrient,  phosphorous.

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Always take the time to test your soil before adding any chemical or mineral. The routine application of any substance is pointless and may even be harmful. Overall the very best way of adjusting soil pH, while improving soil structure and nutrient value at the same time, is to add as much micro-organism-laden organic matter as possible. Mix in spent mushroom compost and locally sourced composted oak or pine leaves or bark to lower the pH; add wood ash and carefully calculated amounts of ground limestone mixed with garden compost to raise the pH.  All the mineral based soil improvers such as lime, phosphorous and potassium are mined, generally in vast open cast mines, and their extraction does dreadful harm to the environment, so these need to be used with restraint.

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Plant nutrients are loosely categorised as macro-nutrients and micro-nutrients (or trace elements), depending on the quantities required for healthy plant growth. Until relatively recently it was thought that there were 3 macro-nutrients  - nitrogen (N), phosphorus (P) and potassium (K) - but it is now known that calcium (Ca) magnesium Mg) and sulphur (S) are required in similar large quantities.

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There are a further 7 micronutrients that are no less important but are simply required in smaller quantities: boron (B), chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), and zinc (Zn).  This is not an exhaustive list as other elements are being added all the time. Some are required in minute quantities to activate other elements, while some are valuable in their own right. Many plant diseases are caused by deficiencies in one or more of these nutrients.   Chlorosis, or yellowing of the leaves, is a classic sign of iron deficiency but it may also be caused by a magnesium deficiency or be an indication of an acid soil where the iron is present but chemically unavailable to your plants. Here is where a good book on plant diseases comes in handy. Careful detective work may answer the question, but if not test kits are available on-line that will diagnose deficiencies in some of the macro-nutrients. Unfortunately home test kits are not reliable enough to test for the presence of minute quantities of the micro-nutrients. For this you need the costly assistance of an agricultural testing service.

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 If, however, you were to compost everything that comes out of your garden in the way of kitchen waste, shredded green waste and woody prunings, leaf mould and grass clippings and return it to the garden, you will have little need for anything else.  Add to this the judicious use of plants such as deep-rooted comfrey which can extract  nutrients from the subsoil, the use of nitrogen-harvesting green manures alongside routine additions of composted farmyard manure and you will just about wipe out any chance of suffering from any nutrient deficiency. 

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If you were able to find a tiny measuring spoon that would snugly hold a single grain of coarse sand, you would be able, in that same spoon, to hold 1000 million particles of clay!