|Themes > Science > Earth Sciences > Geology > Soils > Soil Composition > Cultivated Soil Composition|
Your soil and it's condition is the starting point when it comes to creating a garden as it determines the type of plants that you can grow. This sounds like an ominous statement and may send you rushing for an analysis kit, but it's not as bad as it sounds. Most plant books give the soil conditions preferred by various plants, but these are ideal and few gardens can supply them all. The truth is that trial and error usually determines the range of plants that we grow. After all you can only work with the soil you've got. There are only a few plants which have strict requirements for growth and the rest will perform with varying degrees of success in differing conditions. Most plants are very generous and will grow almost anywhere, with weather conditions and size being the determining factors when choosing. This is why the expected height and spread of a plant usually varies from the quoted figures, and the shade or colour of flowers may differ. You should choose the plant by it's shape and colour first, then consider the growing conditions, unless it is horrendously expensive and the risk of failure is great. Below is a brief explanation of the formation of soil and how it influences the plants we like to grow.
Igneous rocks, eg bassalt, were the first to form when the molten lava cooled, then the erosion began. The process is sometimes reversed producing sedimentary and metamorphic rocks by compression, chemical action or extreme temperature. Sandstone is formed by chemical action and compression and further pressure changes it to quartzite. Further compression of limestone produces marble. Shale and slate are compressed silt. Fossils are found in these sedimentary rocks when they were laid down in a short time trapping the original creature, eg. during flooding.
Soil is rock in it's final state of decay. After millions of years of weathering it has broken down to fine mineral particles, the composition of the original rock and the processes which have acted on it, have determined it's present form. Water, wind, movement, chemical action and varying temperatures act on the rock dividing it into smaller and smaller particles. In the British Isles the soil is unlikely to be sedentary, ie. formed directly above the parent rock with a gradient of particles becoming fissured rock as you dig down. The soil here in Northern Ireland is transported soil which was moved great distances during the Ice Ages and has a definite boundary between soil and the bedrock on which it was deposited.
Most cultivated soil is predominately mineral particles 2mm or less with "fines" of silt and clay. The proportion of the particle sizes determines if it is described as a clay or sandy soil. Particles between 0.06mm and 2mm are sand, silt particles are between 0.002mm and 0.6mm and clay particles are less than 0.002mm derived by chemical weathering and mostly have a negative surface charge. This surface charge is important in the exchange of nutrients, so although clay is full of nutrients they tend to be bound to the particles and are not available to plant roots. The addition of lime and organic matter causes the opening up of the particles and nutrients become available. The calcium ions in lime have a positive charge and the negative clay particles "flucculate" around them forming minute crumbs so opening spaces. Organic matter (humus) forms a coating on the particles holding them in loose crumbs and giving the darker colour characteristic of topsoil (sometimes called vegetable soil). Both these actions allow a better flow of air and water, essential to supporting life in the soil. With a higher proportion of sand, water flows quickly washing out nutrients so sandy soil tends to be poorer. The water content is a factor in soil temperature so clay soils tend to warm up more slowly.
A typical cultivated soil will have 50 to 60% mineral particles, 1 to 5% organic matter, and 40% pore spaces between the particles which will have varying amounts of air and water in them. Rubbing a small sample between finger and thumb will give a rough guide to the texture of soil. Sands have little cohesion and don't bind; sandy loam has a gritty feel; loam can be formed into a ball and does not have a dominant feel of grittiness, silkiness or stickiness; loamy sand can be rolled into a "worm"; silty loam has a silky feel; clay loam binds together strongly, does not readily deform and takes a polish if rubbed; clay readily binds and takes a polish if rubbed. The amount of organic matter and calcium in the soil can change the feel, chalk can give a silky or gritty feel and organic matter makes it more sticky. Other descriptions of soil are "heavy" or "light", these refer to clay soil and sandy soil respectively
The distribution of the particles determines the structure of the soil and how porous it is, it is important to maintain a good structure. Compaction reduces the porosity, the addition of grit and organic matter increases it. Wetting and drying, freezing and thawing, root growth, soil organisms and cultivation are all actions which change the structure. Sand or silt particles do not aggregate well and tend to fall apart when wet, clay particles adhere better so their presence increases the stability of soil structure. The addition of humus, chalk and iron oxides bind the particles together, forming small clumps or crumbs but clay particles are the main stabilizers in a good loam.
The top 50mm of soil should have a good 'tilth' with stable crumbs of 0.5mm to 5mm. Wetting and drying can cause a breakdown of the tilth to a fine dust which dries to a cap or pan, subsequent watering fails to penetrate and seeds cannot break through. Sandy and silty soils are more prone to this due to their poorer stability. Over cultivation or cultivation at the wrong time can break down the structure of poorer soils. A good time is in the autumn, leave the cultivated soil in rough clods to be acted on by the winter weather. If cultivating in the spring check for wetness by squeezing into a ball, if ready it should break apart easily. Deep pans can be created by smearing of the subsoil with cultivator blades or when developers spread out site debris and cover it with a sprinkle of topsoil and call it a garden. The tracks of their heavy machinery create pans and subsequent plantings have difficulty establishing on them. You should avoid walking on cultivated soil when it is very wet, as the 'puddling' action breaks down the structure.
Digging, ploughing or rotavating (tillage) all improve the structure of the soil by incorporating residues of previous planting and relieving compaction. During cultivation only the top 20 to 30cm should be involved, the subsoil should never be brought to the surface. If the layer of topsoil is thin, the subsoil can have organic matter forked into it using the double-digging method. There is a method of minimum or zero tillage where organic matter is spread on the surface to be incorporated by worms and during planting, this preserves the natural structure, but an initial digging to remove weeds and stones would probably be necessary. The stones are useful for making paths. Harrowing or raking produce the surface tilth.
Acidity or alkalinity of the soil is
determined by its mineral content. It is described scientifically as the
pH value, this is a measure of hydrogen (H) ions available, an acid soil
has a low pH, neutral is pH 7 and alkaline is greater than pH 7. Most
plants grow best at a pH of 6.5 as most plant nutrients are available for
uptake by the roots at this pH level, the pH range of most soils is
between 5.5 and 7.5.
It is fairly easy to make your soil more alkaline by the addition of lime. To raise the pH by one point requires about 4.5 kg of lime to 9 square metres, but making it more acidic and maintaining it that way is more difficult. You could try substituting sulphur for lime at the above rate. If your soil is derived from chalk then there will be a constant supply of calcium which will keep it alkaline. The addition of plenty of organic matter will improve the situation slightly. To try to change it chemically, eg. adding sulphur or iron, could introduce a whole new set of interactions which could be toxic to the plants. It is usually best to grow acid-loving plants in specially prepared raised beds or containers rather than trying to change the native soil. Acid-loving plants, eg. rododendrons, usually demonstrate their dislike of alkaline soil by yellowing of their leaves . This happens because the calcium present displaces free iron which is essential for making chlorophyll, the green matter in leaves involved in photosynthesis - iron deficient chlorosis. It can be counteracted with sequestered iron which is applied to the foliage, thus bypassing the roots.
To find out the pH and structure of your soil take about fifteen to twenty spoonful-sized samples from all over the area at about 150mm depths. Mix them all together and send away for analysis or small kits can be purchased at garden centres. Gardening from Which?, part of the Consumers Association, provide a service for their readers. For a rough idea check out the neighbourhood to see if rhododendrons are thriving, in acid soil the hydrangea blooms should be blue (not always accurate if an acidifier has been used to produce this effect). A normally acidic or neutral soil can be turned alkaline by cement rubble or leaching from a concrete surface or wall.