Plant Doctor Archive

Improving clay soils

I have horrible clay soil in my garden that always seems to be soggy. How can I improve the drainage of this soil?

Clay soils can be improved by adding organic matter, such as manures and compost, and calcium in the form of gypsum or calcium sulphate.

There is a simple test you can carry out to ascertain whether you need to add calcium to your soil or not.

Put about 2 cm of soil in a large jar, fill it with water, screw on the lid and shake thoroughly. If the water clears within half an hour you will not need to apply any calcium. If it takes one to two hours for the water to clear, apply about 50 g of calcium per metre squared to your soil to improve drainage and structure. If the water has still not cleared after several hours, add about 200 g per metre squared to your soil, digging it in at least 30 cm deep. Repeat this process every four weeks until new tests show soil improvement.

Spent mushroom compost is an excellent mulch for clay soils because it is high in calcium. As mushroom compost can be very alkaline, however, avoid applying to alkaline soils or where phosphorus-sensitive plants, such as proteas, banksias and grevilleas, are planted.

Advice by Dr Dan Blanchon from Unitec's Diploma in Sustainable Horticulture and Bachelor of Resource Management.

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