Lime – What is Lime Used For in Gardening?

For those of you new to gardening, the issue of soil preparation may be a mystery. Believe me it is more than just turning over a sod or two with a spade. I was talking to a friend today about PH and Lime application, and it was apparent from our conversation that this whole area is a mystery to some.

One of the most misunderstood issues in gardening relates to that magic phrase “PH” – so what’s it all about? Well, all soils are generally speaking acidic in character, to a higher or lower extent, dependent in each case on a number of variable factors at work in the soil forming process for that individual “soil.” Most gardeners recognise the importance of the following influences:

  1. The surface features or topography of the area
  2. The prevailing climate particularly rainfall leaching.
  3. The influence of time
  4. The nature of the soil itself
  5. The type of living organisms active in the soil
  6. The level of fertilizer use

All of these factors are at work and the synthesis of their interaction over time results in a particular level of acidity for that soil sample. So why is acidity so important? Well, it depends on the plants you are growing of course and some plants do better in acidic conditions, but as a general rule high soil acidity is a “no – no” for most plants.

So how do we work out soil acidity? It is obvious that we need some simple measure of soil acidity so that corrective action can be taken to reduce acidity and improve growth conditions. Well, this is where PH comes in. Without going into too much technical jargon, PH is simply a way of calibrating the level ( or potential “P”) of hydrogen ions ( H+ ) in a solution of water. It gives us a measure of acidity.

By mixing a solution of soil and distilled water in a 1:1 solution, the PH of the water solution in equilibrium with the soil can be accurately recorded. The lower the PH the more acidic the soil sample. The higher the PH the more alkaline the soil sample. PH is usually measured on a scale of 1 to 14, so that at a PH of 7, the soil sample can be said to be neutral. The lower the PH level the more likely it is that metallic components are present. So below a PH of about 6.0 the incidence of metals such as copper, zinc and manganese increases to toxic levels. Above 6.0 the incidence of nutrients such as calcium, magnesium, phosphorous and potassium increases to beneficial levels for plant growth.

Each plant species is suited to different growing conditions of course, and slightly acidic soil can help to keep down blight in potatoes for example. On the other hand, slightly alkali soil has a higher level of salt present which may not suit your particular crop. Generally however, gardeners tend to aim for neutral PH of around 7.0 in most cases.

So, faced with an acidic soil, what can the gardener do to improve soil quality? Well, this is where lime comes in. You can raise soil PH by applying Lime to the soil. Lime is made by crushing limestone or chalk, whose main active ingredients are calcium and magnesium carbonates and oxides. ( Don’t worry – you can buy it at your local Garden Centre ). On application, a chemical reaction occurs which changes some of the hydrogen ion concentration into water and carbon dioxide – in effect diluting the hydrogen ion level and raising the PH measure to a more alkali friendly number. Liming in this way can provide a source of calcium, improve water penetration and increase bacterial activity. Be careful to understand however that Lime is a chemical and overliming can be harmful too.

Lime can be applied throughout the year, but most gardeners will apply it during Winter or early Spring. Remember that Lime is insoluble in water so thoroughly mix the lime with the top soil. Once moisture is applied the lime will start to chemically react, so thorough mixing in dry conditions is very important. Don’t forget that different plants thrive in different soil conditions, so make sure you know which PH level your plant needs before deciding how much or how little Lime to apply.