I was one of only two kids in my Singapore state school to join the gardening club. Yes, I was deeply uncool from a very early age. After lessons each day, we had to move around great sacks of sand and dig it into the sticky clay soil under the tropical sun. We were taught by our stern teacher, Mr Lau, that this backbreaking work was necessary to help improve the drainage of dense, heavy soil, which was prone to turning into an impenetrable mass of mud in the rain and baking rock hard in the sun.
This idea dates back at least to standard horticultural advice from Victorian Britain and is still widely espoused today. Even at school, I always wondered why this never seemed to make a huge difference when the monsoon turned the whole area into one big puddle. But what does the science say?
Advertisement
It is true that clay soils, made of billions of tiny mineral particles, can be terrible at allowing water to pass through. The particles are packed tightly together, with little space for water and air. This is why clay soil is used to line ponds, and made into pottery. But it can have a deleterious impact on garden plants.
Sandy soils, on the other hand, have large gaps between their comparatively enormous particles, allowing water to freely pass through. So, it makes perfect sense that simply adding sand to heavy clay would give you a better soil. Indeed, sand and gravel are widely used in potting mixes for species that require fast drainage, like cacti.
However, when this is scaled up to the level of entire gardens, things begin to change, and this is all down to the of clay particles compared with sand. Anyone who has mixed potting soil will know that levels of sand have to get up to around 50 per cent before things become . Before this point, all that happens is that the tiny clay particles fill in the air gaps between sand grains that makes them free-draining.
At the scale of even a small garden, reaching a ratio of around 50 per cent sand would mean literally tens of tonnes of sand before any benefit would be observed.
Fortunately, we now know there is a much easier way to improve drainage of clay soils, by simply mimicking what happens in nature. The action of microbial life in soil naturally creates a crumb structure, grouping fine clay particles into much larger pellets, which allow water to pass through.
These microbes feed on decaying organic matter, so all you need to boost their levels is to add large amounts of dead plant material, like fallen leaves, lawn clippings and compost. The best bit is that you don鈥檛 even need to dig it in: the natural action of worms will do this for you, while they create a network of tunnels to further improve drainage at the same time. How I wish someone had told Mr Lau this in 1989.
James Wong is a botanist and science writer, with a particular interest in food crops, conservation and the environment. Trained at the Royal Botanic Gardens, Kew, in London, he shares his tiny flat with more than 500 houseplants. You can follow him on X and Instagram @botanygeek
For other projects visit newscientist.com/maker.
