Clay in potting soil to control growth

11 april 2019

Sometimes growers want to slow down the growth of their plants, for example in the bedding plants sector. However, the use of growth regulators in horticulture is more and more under pressure. Therefore, in order to grow a more compact plant alternatives for chemical growth regulators are looked at more and more. A potting soil containing clay certainly offers possibilities when it comes to controlling the growth. How does this work exactly?

Clay in a potting soil causes the crop to remain more compact. This is because clay can lower the availability of water and phosphate (P). Slowing down the growth can be enforced by using a potting soil with a low EAW level (Easily Available Water) and combining it with dry growing. Clay materials in potting soils have an effect on the availability of water. The EAW is the amount of water that a plant can easily absorb from the potting soil. The dosage of a number of clay products influences the availability of water to the crop. The finer the clay products, the more the decrease of the EAW. Coarse clay materials in general have a decreasing effect on the EAW when applied in high dosages.* As the EAW is lower and the plants thus need to appeal to more scarcely available water, the growth of the plants is slowed down and they become more compact and sturdier. In order to use the EAW as a tool for managing the crop development, the crop needs to be grown dry; the root ball is kept at a lower moisture level.

* There is a difference in clay materials. Fresh clay still contains water. Dry clay granules therefore give more effect, because it effectively contains more clay mineral.

By adding clay to a potting soil mixture the availability of phosphate for the plant decreases. In combination with no extra phosphate fertilization, the growth of the crop may decrease. Therefore, a low phosphate fertilization in potting soils may be used as a tool for the desired growth inhibition. This works for all kinds of clay.


So clay can be an aid in controlling growth. However, this depends in particular on the mineralogical compositions and fraction of the clay used and how the culture is grown and the experience of the grower. There are many different qualities of clay on the market, all with their own specific characteristics. Contact the suppliers of clay and growing media for advice on achieving the desired effect.

RHP slowing down growth with clay

More about clay?

Download the information sheet concerning clay. This contains further explanations of the effects of clay in a potting soil mixture. RHP certified companies can also find this information sheet in the RHP Toolbox at My RHP.

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