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Improving Clay Soils, Methods & How to Improve Clay Soils

Ever since we became involved in growing vegetables when five years old with parents and grandparents, improving clay soils in Ireland and the UK has been a priority. And the challenge followed us to Spain. Here our raw red clay soil was used until fifty years by villagers to make home made bricks!

Clay Soil

Clay Soil – Cracked and Hard when Dry

The top soil being a combination of particles from the erosion of the side of the mountainside that we live on – 400 metres up , ten kilometres from the Mediterranean beaches – natural humus from wild plants before we cultivated it and the red earth particles carried from north Africa for millions of years by southern gales that fall as ‘red’ rain. Called that as the cars are that colour when they dry!

So we need to improve the clay soil in order to be able to grow our organic vegetables and fruit;

  • on a 800 square metre allotment,
  • a raised bed system we are currently constructing in the garden for the time we lose the allotment to the inevitable expansion of houses
  • in the containers used to demonstrate to apartment and town house dwellers that they can easily grow vegetables and fruit in just a couple of square metres of space as illustrated in our books ‘ Growing healthy vegetables in Spain’ and ‘ Growing healthy fruit in Spain’, without the need for excessive amounts of water and the use of manufactured fertilizers.

The table below illustrates what we mix into the raw clay soil to improve it:

The bulk ingredients are blended in by digging or rotovating down to a depth of twenty to thirty centimetres for vegetables and fifty for fruit trees or by tipping backwards and forwards between two 20 litre plastic builders/gardeners buckets when preparing soil/compost mixes for containers.

Improving Clay Soils

Additives to Improve Clay Soils
Purpose Additives
To lighten the soil, make it more workable, improve it’s water holding capacity and at time make it more free draining. Mix in two to three 20 litre builders buckets of homemade*, Eco-park **or bagged compost per square metre of soil.
To further improve soil fertility.** Mix in a bucket of composted or dried bagged manure per square metre.
To further improve it’s water holding capacity during hot dry weather. Mix in a little TerraCottem soil improving gel (
To further ensure it will be free draining and not become waterlogged. Mix in 5 to 10% course sand, grit or ground lava.****
To make minor increases in soil acidity and kill off lurking fungal spores. Dust surface with sulphur powder
To further kill off fungal spores plus young slugs Sprinkle surface with ground neem kernels ( for instance).
In areas for asparagus Mulch with partially composted seaweed from the beaches after storms.
For plants preferring acid soil such as potatoes and strawberries. Mix in compost made from pine needles or/and natural sulphur powder.


* Two nearby towns have Eco-parks that compost the green garden waste from municipal parks and professional gardeners and seaweed washed upon to beaches by storms. The sieved compost can be purchased with or without seaweed at only 3 euros a hundred kilos.

**We add comfrey leaves, rabbit and chicken manures to our five pallet built compost bins as accelerators and enrichments. We also have an experimental mini worm composter to produce compost to mix into containers and window boxes used for mini vegetable growing.

***We mulch rows of raspberries with comfrey leaves and grass cuttings.

The rows of comfrey we top dress with well composted chicken manure once as year. Comfrey leaves are wrapped round seed potatoes and mixed into soil where beans and peas will be grown. ( more on Comfrey )

****Naturally if we had a sandy soils we would leave out the addition of course sand, grit or ground lava.

Nutrients & Moisture

With such a mix plants have an ample supply of nutrients and moisture to support steady healthy plant growth. Only hungry fruit vegetable plants such as tomatoes peppers and squashes, and fruit trees (after a couple of years) are given supplementary feeds of a liquid comfrey feed.

Nearby Spaniards who have abandoned the annual improvement of soil with donkey, mule, horse, rabbit and sheep/goat manure during the past thirty years now use excessive amounts of water and inorganic fertilizers to grow vegetables for the local market.

Not only are they often rather watery and lacking taste but the forced growth is susceptible to insect and fungal attacks in the hot and humid Spanish spring and summer climates.

Although ecological/biological solutions as well as organic chemical solutions to such problems are now available so we avoid using even these if possible by growing naturally rather than by forcing. By the way we only grow vegetables for personal consumption ad not for shows – a phenomenon that is unknown here in Spain.

As said in our books feed the soil and not the plants for naturally tasty healthy plants.

This article is by Dick Handscombe who retired to Spain to live the good life where he writes and gives talks on gardening in the Spanish climate.


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