Erica's Original Notes Soil is made from: a) grist, which comes from broken-down and weathered rock; and b) humus, decayed animal and vegetable matter; it acts as a sponge to hold moisture.
Clay: by-product of the ceaseless warfare between weather and the rocks of earth's crust; clay particles are flat, overlap and interlock which accounts for their obstinate composition, only being relieved by the incorporation of sand or humus.
Lime: a soil conditioner liberating calcium for the plant's use.
Mineral traces: magic substances which are brought up from the sub-soil by 'weeds.'
Nitrogen: mineral that promotes leaf and stem growth; giving too much usually results in the lack of flowers.
pH+/-: pH 7 is neutral; below is acid and above is alkaline; pH6-6.5 is when the greatest number of nutrients are available to the plant.
Phosphate: used by the plant for the development of the root system; it is especially good for root vegetables.
Potash: useful for the general good health of plants. The name goes back to the time that it was used in the making of soap and was obtained by the leaching or running of water through the ashes of burnt wood, the residue being boiled in open kettles; hence the word 'ashes from pots,' or 'potash.'
Reclamation: of land in Holland was done by incorporating gypsum (calcium sulphate dihydrate) which displaced the salinity (sodium chloride) without aggravating the pH.
Seaweed: marvellous weed-free enricher of soil, being equal in nitrogen content to that of manure, having twice the amount of potash, but less phosphate and rich in iodine.
NOTES Updated 2013 (RP):
pH: This is a measure of the acidity or alkalinity of a solution. In chemical terms, it is a measure of the activity of the hydrogen ion in solution and is written as pH. Danish chemist Søren Peder Lauritz Sørensen first introduced the concept of pH at the Carlsberg Laboratory in 1909. According to the Carlsberg Foundation pH stands for 'power of hydrogen.' Other suggestions include this coming from other languages - German potenz (meaning 'power'), French puissance (also meaning 'power', based on the fact that the Carlsberg Laboratory was French-speaking) and others refer to 'potential.'
Gardeners are particularly interested in the pH of their soil because it determines whether certain plants thrive or suffer. It is possible to change soil pH with certain additives, but the results don’t last. The better option is to choose plants that like the pH of your soil. Rhododendron (including Azalea), Camellia, Erica, Gardenia, Magnolia and Vaccinium species are well-known acid loving plants. Lilacs (Syringa), Daphne, Forsythia and Clematis species are plants of choice for alkaline soils.
Hydrangeas are well known for their ability to have flowers of different colours in acid or alkaline soils. H.macrophylla, can be blue, red, pink, light purple, or dark purple. An acidic soil (pH below 6) will usually produce flower colour closer to blue, whereas an alkaline soil (pH above 6) will produce flowers more pink. This is caused by a color change of the flower pigments in the presence of aluminum. Acidity assists plants in taking up more aluminum from the soil resulting in blue pigments.
Different soil types are associated with different locations. Rain washes away elements such as calcium and magnesium from the soil and the soil becomes more acidic. Areas that experience high total rainfall such as our city and surrounds, tends to have acidic soil. Areas with a base of chalk and limestone have alkaline soils.
To find out a soil pH precisely it is possible to purchase inexpensive soil testing kits.