How to improve clay soil

Volcanic or sandy loam soils are best for growing plants. Unfortunately, almost 80% of New Zealand soils are classified as clay or clay loams. Clay is made up of very fine particles which pack together tightly making it very difficult for plant roots to penetrate and to take up water and nutrients. In addition, clay soils do not drain, and water builds up in the planting hole.

To ensure success gardening in clay soils it is necessary to —

1. Raise the planting area by adding lots of compost and mixing with the clay top soil. Adding gypsum into this mix will assist. This applies to vegetable gardens, fruit trees and shrubs.

2. Continue to add a layer of compost each year.

Dig a wide shallow hole, and mix large quantities of compost with the clay topsoil. Add sufficient compost to create a mound or raised area 15 – 20 cm above surrounding level. Whilst it is commendable to make your own compost from the garden green waste, it will be insufficient to improve and maintain a clay soil. Hence it is necessary to buy in large amounts — a trailer load or in some cases a truckload.

Contrary to popular belief, Gypsum or lime will not break up the impervious clay layer, but these products are effective in assisting the compost in opening up the clay in the top soil. Gypsum and lime also add calcium to the soil which ensures phosphate is available from the fertiliser applied.