This picture shows some strange objects I have recently seen in three places near my home 鈥� twice on verges of country roads and once near a stable鈥檚 muck heap. They are small spheres, between 2 and 4 centimetres in diameter, and very hard. They are nearly black inside, with terracotta-coloured skin. Each site had several hundred of the objects in a rough heap about a metre across. Have they been manufactured for a purpose? If so, why are they being dumped? Or are they natural? If so, what created them?
鈥� These are expanded clay pellets or aggregate. They are produced by firing natural clay at high temperatures in a rotating kiln to produce spherical pellets with a terracotta-like surface. A black porous core is formed by burning organic materials within.
鈥淭he pellets are produced by firing natural clay at high temperatures and are very resistant to rot鈥�
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Aggregate is manufactured for many purposes, including for use in building materials such as blocks, slabs and concrete. It is useful for this because the clay pellets are lightweight and are excellent insulators of sound and heat, and they are resistant to rot and chemical degradation.
It is also often used in building situations where loads need to be minimised, such as in the substrate above a tunnel where the pellets also provide stability and efficient drainage. Additionally, the pellets can be used for water treatment due to their high surface area, which aids the absorption of pollutants.
The ones shown have most likely been used as a growing medium and discarded. The clay allows for the absorption and retention of water as well as allowing the drainage of excess water from around the roots of whatever is planted in them. This ensures that air circulates throughout the medium and the porous pellets, maximising oxygen uptake.
They are often used with a constant or regular water supply such as a drip feed or an ebb and flow system. The pellets can maintain a neutral pH as well as providing a large cation-exchange capacity for the retention and provision of plant nutrients added to the water.
Because they have been left in relatively small heaps by the road, it鈥檚 not unreasonable to suggest that they have been used as a growing medium for the cultivation of cannabis, dumped within a rural environment to avoid suspicion.
Patrick Melia, Brighton, UK
This article appeared in print under the headline 鈥淥dd jobs鈥�