Adding lime to soil: Side effects

There are hardly any  side-effects of adding lime to garden soil, but when you add lime in order to reduce soil acidity, you also trigger off a series of other events. The first is purely chemical and connected with the availability of some of the plant nutrients existing nat­urally in the soil.

When the land is strongly acid (a pH at or below 5.5), nitrogen, phosphorus, potas­sium, sulphur and calcium are rendered less available to most plants.

These elements are not necessarily in short supply, but the acidity has the effect of lock­ing them up in a chemical form that the plants cannot use.

A similar chemical reaction happens on strongly alkaline soils, only here it is the iron and manganese which are immobilised. On acid land, lime liberates some of the elements that were locked up.

The second important effect from liming is purely physical. Gardeners on clay have long known that a dressing of lime or chalk after autumn diging improves the soil structure, aiding drainage and making the soil easier to cultivate.

In uncultivated, heavy clay soil, the indi­vidual soil particles are minute and stick together in a solid amorphous mass with hardly any air spaces. Thus a clay soil lies cold and wet in winter and sets like concrete in the summer.

Lime encourages the tiny clay particles to cluster together into crumb-sized lumps, thereby paradoxically breaking up the solid clods. You can see how the process works yourself by stirring a teaspoon of clay soil into a glass of water.

The liquid will stay muddy after you stop stirring because the tiny particles are too light to sink to the bottom. Now add a quar­ter teaspoon of garden lime to the glass. Stir it in, and you can see the soil forming crumbs and rapidly sinking to the bottom, leaving the water clear.