The Real Costs of Using Cloth and Stones in Wicking Boxes and Bed

A common mistake made when making wicking beds and boxes includes the use of stones and a sheet of cloth covering the stones. Rocks waste space where the soil biology should be, the cloth stops the wicking process and the amount of water held in the open spaces between stones is less than in soil.

A common mistake when making wicking beds and boxes is adding stones and covering them with cloth. This reduces efficiency for multiple reasons. The stones take up valuable space meant for soil and soil biology, and the space can hold more water when filled with soil than sand or stone. Meanwhile, adding cloth creates airlocks that stop the wicking process.

Healthy Soil Biology is Essential for Healthy Plants

The Soil Biology required for healthy plants need surfaces to attach to and moisture to thrive. While stones provide a very limited surface area, clay, when spread out, offers far more—equivalent to the size of a tennis court. Because of this, using stones in a wicking bed is a waste of space that could instead be occupied by nutrient-rich soil.

Soil Biology constitutes the activity of the viruses, bacteria, archaea, fungi, algae, slime moulds, protozoa, and nematodes which feed the plants and create many compounds the plants require for healthy growth. You can learn more about creating a suitable soil by following our guide to create and improve your own soil.

Using Cloth Will Create Airlocks and Stop Wicking

Some people add cloth to the bottom in an attempt to prevent soil from washing into the reservoir pipe, this is not an issue in properly made wicking beds where the holes on the reservoir pipes point down.
The fine roots can penetrate the cloth and larger roots grow around it to reach the water. As the water level drops, this space is filled with air as it moves in to replace the water, this creates an airlock between the cloth and the water.
Since air does not wick moisture, the bed above the cloth ends up functioning like a regular garden bed rather than a wicking system. The space below the cloth then becomes useless, making the setup inefficient and unnecessarily expensive.

Soil Holds More Water than Sand or Stone

A good soil mix retains more water than the same volume of stones, coarse sand, pebbles, or fine sand. To test this, we measured the water retention of one litre samples of three different beds and stored them in ice-cream containers. These samples of soil, stones, and sand were dried in the hot sun, weighed, soaked, and then measured again to determine how much water they could hold.
Since a gram of water is equal to to one millimetre, we could use the difference to tell how much water there is in each litre of soil.

In the tables below, ‘Inert material’ refers to rocks, stones, sand, loam, and clay.

Results for a Soil Mix

We continued the trial by repeating the process on four more containers. Each container was filled with stones, coarse sand, pebbles, and fine sand respectively. As before, we ensured they were all bone-dry by drying them in the hot sun for days. We weighed them, filled them with water, drained it out and weighed them again.

Comparison the of Five Materials

Conclusion

Our soil mix holds more water than stones, which holds more than pebbles or sand. Much of that water is held in the bodies of the soil biology. You can get started at home with our guide to create and improve your own soil.