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                                                                                                Soils


It is obvious to the casual observer that the flora of Britain varies dramatically with location. Their are a number of of significant factors that determine which plants are successful in any particular area. These include: temperature extremes, rainfall, winds, sunshine hours, aspect, altitude and soil composition. Whilst the meteorological parameters change, the soil composition does not in the short term (unless human activity forces it).



The gardening enthusiasts will be interested in soil pH (this is the logarithm of the hydrogen ion concentration), which determines the acidity or alkalinity. A pH of 7 is neutral, less than 7 is acidic, more than 7 is alkaline. Soils in the UK are rarely outside the range 5.5-8.5.  I have tried simple chemical and electronic measurements of soil pH, with very unsuccessful results. Most of the home garden kits are poor, and do not even point out that the water used in the mix can influence the results-you need to use de-ionised water as local tap water might have an influence on the answers.



The main constituents of soil are silt, clay and sand.  The definition of these is based on particle size, and there is an agreed international size scale. Sand is normally up to 2mm in diameter and consists mainly of quartz(silicon dioxide). Silt and clays go right down to a few microns (one millionth of a meter) in diameter, and consist of many different minerals and trace elements. In general, the smaller the particle size, the more they tend to stick together, called flocculation (but is affected by salinity).  You will know that clay in the garden is a problem as it sticks together, and has a lot of very small particles in it.



In addition there is organic matter, insects, bacteria and many fungal spores etc.  Organic usually means decomposing vegetable and animal matter. There are also nutrients (nitrates, phosphates and potassium) and trace elements. Soils lacking any one of these constituents are likely to be classified as deficient in some way, for which remedial treatment is necessary for successful plant growing. There will also be air (oxygen, nitrogen, carbon dioxide and other gases including methane), and of course water. Soil consistency is determined by the various combinations of all these, ideally producing a friable and moist growing medium with aeration. Garden experts can sometimes identify soil deficiencies by a close examination of plant growth problems such as leaf colouration and growth deformities.



Organic gardeners seek to supply all nutrients by adding only organic materials to the soil(usually by composting and manuring). Alternatively they can be added by using various chemically derived fertilisers (which have the advantage of being sterile and thus free from weed seeds and insects). The composition of some of these can be found elsewhere on this site.  Manufactured fertilisers and composts are continually changing to reflect improvements, public opinion, market demand and profitability. Probably the earliest were the controlled formulations prepared by the John Innes Research Institute, and are still very popular today.  They are based on sterilised soil with additives. You can find data on these at John Innes or the seedsite.