Crop receives its own organic substances back via recirculation of drain water
The roots of a plant secrete organic substances: this way, the crop tries to influence its environment. If the plant grows in a substrate (instead of in the soil) these substances will soon end up in the drain water. At a later stage, they return to the crop via the irrigation water through recirculation of drain water. The Greenhouse Horticulture and Flower Bulbs Business Unit of Wageningen University & Research, KWR and Stichting Control in Food & Flowers are investigating whether these organic substances subsequently influence the growth of the crop, and whether negative effects can be remediated.
Organic substances in irrigation water
By secreting organic substances, a plant can, for example, attract micro-organisms that convert nitrogen into an absorbable form, protect itself against harmful micro-organisms or warn other plants of an attack by a disease (such as thrips). Examples of those substances include amino acids and organic acids. They end up in the water around the roots and provide a favourable microbiology.
When growing in rockwool, for example, that water is drained away. The drain water is then disinfected so that it can be used as irrigation water, but the organic substances remain in the water. After all, they are dissolved in the water and are therefore not removed by a filter, for example: this can only be achieved through specialist techniques, such as reverse osmosis or an active carbon filter.
Exact effects unknown
Because of this, the organic substances are returned to the crop at a later time and at a different location. The exact effects of this are not known. However, it happens with some regularity that growers who recycle see unexplained growth inhibitions in their crop. This can be investigated more thoroughly through new measurement and analysis methods.
WUR, KWR and the Control in Food & Flowers Foundation are investigating the effects of organic substances in the cultivation of tomato and phalaenopsis. In addition, they are studying whether negative effects of the substances can be cancelled out, or whether positive effects can be enhanced, by means of a water treatment technique or by adding organic substances, for example. This should lead to a protocol with which growers can better control the levels of organic substances.