Knowing the difference between acid and alkaline soil will help you to much better understand which plants you can grow the best.
Most plants do well in soil which is neutral or slightly acid but there are some which require distinctly acid soil.
They will not grow in soil which is even mildly alkaline and they should, therefore, not be planted in regions where the soil and water is alkaline.
There are testing kits available for doing soil tests oneself or one can send soil samples to a professional who does tests of this kind. Generally, the gardener can assume that where the hydrangeas are blue the soil is acid, and where hydrangeas are pink the soil is likely to be alkaline.
The acidity or alkalinity of the soil is measured in what are known as pH values.
What Does pH Mean?
Numbers from 1 to 14 are used to express the degree of acidity or alkalinity. A pH of 7 shows that the soil is neutral – that is neither acid nor alkaline.
The change up or down of more than 1 is important as a change of a single digit indicates a tenfold increase in acidity or alkalinity.
Soil with a pH of 5 is ten times more acid than one with a reaction of 6.
Most plants grow well in soil with a reaction between pH 6 and 6.5, but plants which like acid soil may require a pH of 5 or even lower, while the lime-loving ones may need a pH of 8.
Acid soils tend to occur in regions of high rainfall and alkaline soil in areas where the rainfall is low, but this is of course, not universally true.
Can I Change The pH Of My Soil?
Where soil is too acid for good growth a sprinkling of lime should be applied to the soil at the time of planting. But lime should not be used unless the pH is lower than 6, and the plant requires a more alkaline soil.
Lime tends to make food elements in acid soil more readily available, but, in soil which is highly alkaline with a pH of 8 or higher the opposite happens and plants will die because the necessary food elements become unavailable.
In regions where the soil is alkaline it is advisable to choose plants which are tolerant of such soil.
The alkalinity of the soil can be reduced by the regular application of certain chemicals but it is nevertheless wiser to select plants that tolerate alkaline conditions rather than to have to treat the soil constantly.
Iron sulphate, aluminium sulphate and iron chelates, obtainable from garden stores, are quick-acting in changing the character of the soil.
How often it is necessary to apply this kind of correction depends on the nature of the water used in the garden. If it is highly alkaline the salts will build up in the soil fairly quickly.
Plants which like acid soil (e.g. azaleas) should never be given ordinary garden fertilizers as these have an alkaline reaction and can cause their demise.
Even compost to which lime has been added may have an adverse effect on them.
Such plants show chlorosis, i.e. yellowing of the leaves when the pH value of the soil is too high, and the ground should be treated around the base of the plant with one of the chemicals mentioned above. A mulch of pine needles or oak-leaf compost will also help to acidity the soil.