The hydroponics agricultural system - Leca® ton as a substrate for the cultivation of plants

Dr. Heinz-Dieter Molitor was a scientist at the Institute for Urban Horticulture and Ornamental Plant Research as well as a lecturer at the Wiesbaden University of Applied Sciences.  During his active time, he was significantly involved in the development of hydroponics in Germany with studies in Geisenheim as well as technical articles in publications and seminar lectures.

What would you say is so fascinating about hydroponics?

First of all, it seems to make sense to me to define the term hydroponics a little more precisely in connection with indoor greening. In this case, the term stands for the "hydrocul­ture plant-holding system", which is known mainly in the German-speak­ing European area. The characteristic feature is the use of expanded clay as a plant substrate in an accumulation zone with a water level indicator.

As a plant nutritionist, I was fascinat­ed by the fact that hydroponics makes it comparatively easy to measure and optimize the supply of nutrients. Disturbances caused by interactions with the substrate can be largely ruled out. The nutrient and water supply can be easily determined by measuring the nutrient solution, as can the pH value and salt concentra­tion. The very complex taking of sub­strate or soil samples, the extraction in the laboratory and the often diffi­cult interpretation of the measured values are now a thing of the past. In some cases, important parameters can already be measured on-site and implemented promptly. The oxygen supply is always optimal, at least out­side the accumulation zone. Due to its production process, expanded clay is free of pathogens.

In addition, expanded clay as a plant substrate is structurally stable, not depleted and can be reused even af­ter years of use. This can be a decisive advantage regarding the increasingly demanded sustainability of a prod­uct.

What can you tell us about the history of modern hydroponics?

The development of the "planting system hydroponics" started with the discovery of expanded clay as a sub­strate for the cultivation of plants. Gerhard Baumann from Switzerland had this idea in 1959 and developed the Luwasa hydroponic system based on it. The system consisted of a wa­tertight container, a hydroponic plant grown in expanded clay, a special cul­ture pot, expanded clay as a substrate and an accumulation zone with nu­trient solution, controlled by a water level indicator. The plant was fed via nutrient batteries with the ion-ex­change-based slow-release fertilizer Lewatit HD5 developed by Bayer AG. This basic principle was taken up and improved by horticultural companies in Germany, Switzerland and Austria. Decisive progress was made through standardisation of the system components. The develop­ment was also promoted in the 1970s and 1980s, because almost all horticultural test facilities worked thoroughly on the "hydroponic system". The main focus was on ques­tions concerning the cultivation of hydroponics plants, nu­trition and water quality, the accumulation height and the quality of the expanded clay, or even plant protection. This contributed significantly to the introduction and dissem­ination in the private sector and in professional indoor greening. The quite simple and safe plant care combined with high-quality equipment and constant product inno­vations were very convincing. This development stopped in the 1990s when the "hydroponic system" became a mass product. Today, the "hydroponic system" is a perma­nent fixture in professional indoor greening, while its use in the private sector can only be found rather rarely.

What are the prerequisites for successful hydroponics?

The first prerequisite is the right choice of plants in terms of light, temperature and humidity. Avoiding plant species that are known to be sensitive to certain pests, as well as strict adherence to preventive plant protection measures contribute significantly to the durability of a planting.

Another prerequisite for successful hydroponics is the use of expanded clay that is subject to RAL quality assurance concerning substrates for plant cultivation. This ensures that the physical and chemical parameters defined there are adhered to.

It is imperative that the nutrition of the plants is adapted to the respective water quality. The choice of the fertilizer type, i.e. liquid fertilizer, ion exchange fertilizer or basic fertilizer, depends largely on this. Only high-quality products may be used as fertilizers, which in particular also contain all the necessary micronutrients in a stable form and in the right proportion to each other. The level of nutrient supply is based on the fact that in indoor greening, the aim is to achieve an optimal appearance of the plants with moderate growth. No compromises should be made with softened tap water, since this is unsuitable as irrigation water due to its usually high sodium concentration. A too high accumulation of nutrient solution must be avoided by all means because of the danger of oxygen deficiency of the roots in the overwatered area. Justified exceptions may include locations with high irradiation, where a quick consumption of the nutrient solution can be expected. Otherwise, the principle of "hydroponics" is to have as little accumulation as possible.

What is the current state of the art?

Users can choose from a wide range of high-quality vessels for every style of living. There is a high level of innovation in the development of new vessels in this area. By now, improvements in cultivation pots have been achieved by adding side slits in the upper part of the pots. This makes it easier for the plants to root out into the unaccumulated moist zone. According to our experience, users are very insecure when it comes to choosing the appropriate fertilizer and, in this context, the quality of the irrigation water. Regarding the quality of the quite large number of liquid fertilizers on the market, sometimes vast differences must be assumed. Most people prefer liquid fertilizers because of their easy handling, although in many cases they should not be used because of their insufficient Ca and Mg concentration of the irrigation water.

What innovations are possible in the future?

Future innovations will concern the display of the water level, an improved design of the culture pot and a convinc­ing concept for the reuse of used expanded clay.

The misleading indication "maximum" must be dropped from the water level display. The display should be limited to "watering" and "stop". Many users would certainly be delighted with an electronic display of the water level via an app directly on their smartphone. Other interesting pa­rameters, such as pH value, temperature and conductivity of the nutrient solution, might also be added.

Regarding the durability of a plantation, decisive im­provements in oxygen supply in the growing area could be achieved by setting low water levels. Previous develop­ments, however, have failed to make a breakthrough on the market.

The aim in the design of the cultivation pots must be to facilitate rooting out of the pot into the moist area that is not covered by water. The optimal solution would be a culture pot that dissolves over time in the relevant area.

In terms of sustainability, a convincing concept for recy­cling used expanded clay should be developed. As is well known, expanded clay is not used up even after many years of use and is much too good to throw away. After cleaning, it should be possible to use it again immediate­ly for hydroponics or other applications. This would be an undeniable advantage over all other substrates, including those based on organic materials.