This is the measure used to indicate the active acidity of the soil. The term is based on the balance between the hydrogen ions and the hydroxyl ions and it is important to remember that a low pH figure indicates acid soil which for most crops requires correcting by the addition of lime. pH 7 represents neutrality. Below pH 6.5, the soil is acid and at pH 6, the degree of acidity reaches the point, where only acid-loving plants, such as heathers and rhododendrons will thrive. If the figure is up to pH 8, the alkalinity of the soil is such that certain essential foods are locked up and plants show signs of starvation. The scale used is logarithmic, so that pH 5, is ten times as acid as pH 6, and pH 4, is one hundred times as acid as pH 6.
Simple and reasonably priced outfits are available to determine the pH value and it is advisable to take samples from various parts of the garden since the degree of acidity may vary. To raise the pH value (which means neutralizing the soil acidity) lime is added. In general the lighter the soil, the smaller the quantity of lime required; a heavy clay soil needs about half as much again as a light sandy soil. As a rough guide, hydrated lime, applied at the rate of 225-335g (8-12oz) per sq m (sq yd), according to the soil texture, every few years should be adequate, except, of course, where chalky soils are concerned which require no additional lime, or very acid soils, which would require more. The pH measurement is a useful check to be applied when the crops show signs of lime shortage.
It should also be remembered that excessive lime dressings over a long period produce deficiencies of various traceelements and encourage scab in potatoes on soils liable. to produce the disease. Lime can be applied in hydrated form or as ground limestone or chalk. About 0.5kg (1 lb) of hydrated lime is equal to 1kg (2 lb) of the other two forms. If spent mushroom compost obtained from farms using chalk in the casing soil is used in the garden as a mulch, the pH should be watched.