Horfield and District Allotments Association

The importance of adding Lime

The Importance of Lime  


The importance of adding lime to our plots cannot be over emphasised. Apart from its ability to break up heavy clay soil – a serious consideration on our ground, its influence on the pH (acidity/alkalinity) of our soil is crucial. It must be said, heavy clay soils are more likely to be acid rather than alkaline. I’ve talked about the addition of lime with many of my neighbours over the last year – it seems very few people now apply this essential ingredient. We keep adding cow manure, horse manure, green manure, etc., but fail to take into account the affect these have on the pH of our ground. We look at our crops – some doing well, some doing okay and some failing, although we keep adding manure – what can we be doing wrong? Do have a look at the tables below:-


Check out – www.hydroponics.co.nz/images/large/pH_chart.gif



























N- Nitrogen, P - phosphorus, K - Potassium    - the main nutrients

S - sulphur, Ca - Calcium, Mg - Magnesium, Fe - Iron, Mn - Manganese, B - Boron, Cu & Zn - Copper and Zinc, Mo - Molybdenum.


I have noticed when reading various gardening sites and chat lines that Mg - magnesium and Mn - Manganese are often mixed up. It is important to know the difference when providing the nutrients and trace elements for our crops



I hope the chart above makes sense – the thickness of the shaded lines indicates the optimum uptake of each the essential plant nutrients. For our soil it should be around 6.3 – 6.5. Where the pH is below 5 the plants’ uptake of nutrients tails off significantly and the ground can become toxic. Although the nutrients may be present the plants are unable to use them. The word ‘toxic’ seemed to me to be a somewhat dramatic statement. When I asked why ‘toxic’? I was told that in very acidic conditions, minerals such as aluminium can increase in the soil to toxic levels.


A pH meter is inexpensive and easy to use. It will give you an indication of the pH of your plot, if not a totally accurate read-out but, can tell you almost instantly where lime should be added. For a totally accurate reading you will need to invest in a proper kit - for me this is laborous, expensive and time consuming. I just need an approx. estimation of the need for lime.


But, don’t add it to the ground before planting some crops, for example - potatoes. Do check out the soil conditions needed for individual vegetables, and don’t apply at the same time as manure. The lime and the nitrogen in the manure will react and any nitrogen which might have been available will disappear as a gas. It is not a good idea to apply a lot of lime in one dressing. If the pH of your ground is very low, ( in some patches on my plot it has been as low as 3! - good grief, how could that be?),  it is best to apply a light dressing over several years to bring the pH up to a reasonable level. It is important to note that a high alkaline soil will have a similar effect as  very acidic soil, although for heavy clay soils it would take some time and a lot of lime to build it up to toxic levels.