¹ For scientific work the soil reaction is usually expressed by a figure known as the “Hydrogen Ion Concentration” or “pH value.” The standard of neutrality is approximately 7: if the soil is acid the figure is lower, if basic it is higher. The usual reactions encountered on calcareous soils range from pH 7 to pH 8. These figures are shown on the B.D.H. colour chart, and correspond to the bluish-green colours of the Indicator liquid. More accurate results are obtainable by using indicators which react over a narrower range of values and for this purpose the B.D.H. Capillator Outfit is usually employed. Test papers impregnated with indicators are also available for use instead of liquids. In the laboratory it is possible to measure pH values directly and more accurately by electrometric instruments.