CLAY PEBBLES Clay Pebbles, Clay Balls, Orange Balls, Coco Balls, or lightweight expanded clay aggregate as they are technically called. These little beauties have a highly porous inner core with a hard outer shell, created by heating clay in a rotating kiln. The high kiln temperatures expand the clay, and as the kiln rotates it produces the substrates rounded shape. Most often, you will find hydro shops stock clay that is quite coarse is size, with an average particle size of 12mm. Clay is often used on its own in flood and drain systems, and typically holds up to half its volume with water (20L of Clay Pebs will hold 10L of water), which compared to perlite and vermiculite is relatively low. This means mixing clay pebbles with soil will lower the watering holding capacity and improve drainage. However, due to the overall large particle size of the clay pebbles mixing them with potting soil does not create a very even mix. Instead of creating a consistent water and air content throughout the mix, as you would achieve with perlite, mixing clay pebbles creates a mix with relatively dry pebbles surrounded by much wetter potting soil. For these reasons mixing clay pebbles in is not recommended unless it is in large quantities of 50% or higher to create a free draining mix suitable for frequent irrigation.
Clay pebbles do however work well as a top dressing to prevent surface evaporation and to encourage root growth right up to the surface of the potting soil. Clay pebbles can be used on the bottom of pots to act as an insulating layer, or to prevent soil from spilling out the holes, but contrary to popular belief a layer of pebbles at the bottom of the pot does not help improve drainage. Ok, now you are armed with the info on how to amend the physical structure of your soil, let’s get dirty and talk amendments that will add nutrients and that all important beneficial biology!
WORM CASTINGS When it comes to adding a huge dollop of nutritional and beneficial biological goodness into your soil mix, worm casts are indispensable. Worm casts, or worm castings, are literally worm poo, the result if worms munching away on decaying organic matter. Worm casts will bring poor soil back to ‘life’ by adding an army of beneficial microbes, particularly bacteria but also fungi, and plants respond extremely well to a high quality worm cast product. They are not hugely rich in nutrients, but are usually higher in nitrogen that other elements and what is added tend to last around 4 weeks. The only problem is that not all worm casts are created equal, and some products called worm castings are more of a worm worked compost or ‘vermicompost’. The term vermicompost is given to a product where not all of the material has passed through the worm, so will contain only a proportion of casts and will be of lower quality compared to high quality worm castings. However, some worm cast products come from worms that are reared on junk food! We are not talking Burgers and Fried Chicken here, worms love carbon rich food sources and soil dwelling ‘free range’ worms out in the real world love to chow down on autumn leaves. Worm farmers know this, and captive worms primarily reared for the fishing trade where worm casts are a harvested by-product are often fed large quantities of
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recycled paper and card or paper pulp. This results in worm castings that are a greyish/brownish colour, instead of being a rich dark brown to black colour that you should expect from a high quality product. So in some circumstances, a good quality vermicompost can outperform a poor quality worm cast. When adding worm castings to your potting mix, add them at 5-25% by volume. Worm castings are safe to use in a mix for well rooted cuttings or seedlings. The high quality casts will need a lower application rate than vermicompost. You can re-apply more worm castings as a light top dressing every 2-4 weeks throughout vegetative growth and the early flowering weeks. The two most widely available worm cast products are Plagron’s Mega Worm and BioBizz Worm Humus.