ROCK ON!
22nd October 2018
The Appraising Landscapes session last week (18th October) consisted of laboratory work investigating the qualities of natural rocks from Britain and Europe then some local soil samples which were tested for texture and pH values.
A basic knowledge of geology is important for two reasons. First, it determines the essential character of the broader landscape (hills, valleys, slopes, and consequently the formation of rivers and lakes). Second, rock type has a major influence on the soil composition (sandy, clayey, well-drained, nutrient-rich etc). A wide range of igneous, sedimentary and metamorphic rocks were studied for hardness, texture, reaction to acid, colour.
Soil texture refers to the degree of coarseness or fineness of the mineral particles in a soil, which can have a direct effect on soil drainage, water and nutrient (chemical elements) availability for plants. Among the samples studied were heavy clay soils from the Vale of Gloucester and much more sandy “podzols” from high on the Malvern Hills.
Soil pH is the value of soil acidity and alkalinity, specified in relation to the amount of free (dissociated) hydrogen ions in the soil. The pH scale runs from 0 to 14 with 7 = neutral, and the lower numbers denoting increasing acid reaction. The students determined the pH using the BDH colorimetric test, involving mixing a small sample of soil with barium sulphate and an indicator (litmus) solution. The resultant colour is matched against a calibrated colour chart.
The Malvern podzol turned out to have a very low pH (4.5) which reflects the lack of base chemicals in the upper soil horizons and the higher incidence of rainfall on the uplands leading to greater downward leaching of these nutrients. Only coarse grasses, bracken and heather will grow there. The soil from the Cotswolds is developed upon the underlying limestone and consequently had a higher pH score of 7.5. This “rendzina” soil is typically thin, well-drained and nutrient poor.
Bob Moore