Learning how to adapt soil for your crops is a great skill to have – here, the RHS explains how
Easy guide
Soil can be acid, alkaline or neutral. The lower the pH value the more acid, and the higher the pH the more alkaline, with pH7 being neutral. Most plants favour slightly acidic or pH6-6.5, although ericaceous or lime-hating plants such as blueberries prefer a pH of about 5.5.
The main influence on pH is the amount of calcium present. Alkaline soils have less calcium and alkaline ones more.
High pH soils ‘lock up’ almost all trace elements, except molybdenum, and especially iron. Ericaceous plants cannot acquire iron as easily as other plants and typically develop chlorosis or yellow leaves where the veins remain green. This reduces their growth and cropping. In acid soils almost all trace elements are readily available, but so is aluminium. Aluminium ions are very damaging to plants and plant growth, other than for ericaceous plants, and is often poor on acid soils due to aluminium toxicity.
Plant nutrients including the very important nitrogen, potassium and phosphorus are most available to plants in slightly acid soil, and becoming increasingly unavailable as the pH tends to the low or high ends of the scale. Therefore plant growth and crop yield are optimal at slightly acid pH levels, and fertiliser gives best value for money when used on soils with the optimum slightly acid pH.
Discovering soil pH is easy; test kits sold in garden centres are inexpensive and give a usefully accurate pH although a laboratory test is better. Even looking at what is growing in neighbouring gardens can give a clue. Areas rich in camellias, heathers and rhododendrons, and where hydrangeas are blue flowered, are likely to be an acid soil area. The action of rain and certain fertilisers lowers pH bit by bit over time so repeated liming every five years or so is often required. On the other hand compost and manure tends to raise pH. Checking the pH levels every four years is enough given the slow rate of change in soil conditions.
Calcium rich materials called ‘lime’ raises the pH. Garden lime is usually finely ground limestone, but could be chalk, hydrated lime from builders’ suppliers or wood ash. Liming materials should be finely ground into dust, as coarsely ground lime takes many years to react having much less surface area then materials with finer particles. Sandy soils need much less lime than clay soils and guidance charts are supplied to calculate the amoun
