Get Your Soil pH Right for Perfect Plants

Music 16/05/2011

We all know that certain plants do better in acidic soils while some do better in alkaline. But how do you get to know your soil's pH and find out how to change it so that you can get the very best from your favourite flowers and vegetables?


Acidic soils have a pH less than 7 (7 being neutral) and alkaline pH soils have numbers higher than 7. Most soils in New Zealand are acidic but you can change this by adding certain organic matter, in particular lime. It is important that you get used to altering the pH in your soil to get the very best out of your plants. For example, to grow great cabbages, you need soil with an alkaline pH to encourage good health and fast growing rates. Potatoes will get scab if the soil is not acidic enough and need acidic environments to flourish, just like camellias and rhododendrons.


It is very rare that gardeners in NZ will have to make their soils more acidic (unless it has previously had a lot of lime added, so more often than not we need to raise the pH (make it more alkaline) by adding lime (otherwise known as calcium carbonate). You can buy a litmus paper test to test the soil pH yourself, then find how much you need to raise or lower the pH by. To raise the pH by one unit, add about 100g of lime per square metre. It is harder to lower the pH of soil, but if this is necessary, add peat, flowers of sulphur or iron sulphate.


When you have lots of plants or vegetables in the same plot that all have different needs and requirements, it is best to apply lime to one third of the area every three years. Plant crops in succession in this area that like the alkaline soil


Plants that like acidic soil:

Tomatoes, blueberries, kumaras, gardenias, potatoes, yams, spinach, silverbeet, strawberries, watermelons, radishes, azalea, camellia.


Plants that like alkaline soil:

Celery, lettuce, onions, rhubarb, leeks, dahlia, beetroot, chrysanthemum, echinacea, sweetpeas, asparagus.