1. BACKGROUND

One of the alternatives for the use of methyl bromide (Braun and Supkoff, 1994) in protected cultivation is hydroponics or soilless culture. Now, tomatoes, strawberries, cucumbers, peppers, eggplants, and some flower crops, such as roses, gerbera and orchids are widely grown commercially on artificial substrates such as stone wool, coir, perlite and pumice. In the Netherlands the use of methyl bromide has been decreased to zero, because of the mentioned change to soilless growing systems and the legislation against application introduced simultaneously since the early eighties. Initially, hydroponic growing systems have been developed to get higher yields and a better quality, later, to avoid emissions of water, fertilisers and pesticides to the environment.

Characteristics:

Nowadays, hydroponic growing systems are common in horticultural practice in most of the European countries, although not in each country on a large scale. Advantages of hydroponic systems compared to soil grown crops are (Van Os, 1999):

• growth and yield are independent of the soil type of the cultivated area
• better control of growth by use of improved water quality and a better fertigation
• increased quality of products also by better growth control
• pathogen-free start by use of substrates other than soil and/or easier control of soil-borne pathogens

Of course, there are a number of disadvantages such as the demanded high water quality, high investments and high costs for fertilisers. In most cases open or run-to-waste systems rather than “closed” or recirculation systems are adopted. In such open systems superfluous nutrient solution freely leaches to ground and surface water. Now, on economic motives and environmental concerns soilless systems have to be as closed as possible, i.e. recirculation of the nutrient solution, reuse of the substrate and use of more sustainable materials. The advantages of closed systems (Fig 1.) compared to open systems are a reduction of the amount of waste material, less pollution of ground and surface water, a more efficient use of water and fertilisers and lower costs because of the savings. Closed soilless systems potentially have a few significant disadvantages. Apart from the demand for high quality supply water, there is a risk of rapid dispersal of soil-borne pathogens by the recirculating nutrient solution and accumulation of potential phytotoxic metabolites and organic substances in the recirculating nutrient solution.

Where such systems have been adapted commercially, growers attempt to overcome the problems of pathogen dissemination by disinfecting the water by heat or ozone treatment and UV-radiation (Runia et al., 1988; Van Os, 1999; Ehret et al., 2001). Such treatments need a high investment but functioned as an insurance: to avoid outbreaks of a root disease and loss of yield, you have to pay for the equipment. In the last few years, researchers became aware that the resident microflora may play a certain role in suppressing diseases and, consequently, the nutrient solution should not be sterilised. Preference should be given to passive disinfection methods which eliminate pathogens and keeps the resident microflora alive                   (Garibaldi et al., 2003; Postma et al., 2003; Van Os & Bruins, 2004; Postma et al., 2008).                 Slow sand filtration is such a treatment (Van Os et al. 2004), besides, it is a robust and cheap method (fig. 2).

Fig 1: Scheme of a closed soilless growing system

Fig 2: Scheme of a slow sand filtration system to eliminate soil-borne pathogens

2. RECOMMENDATIONS FOR GREENHOUSE CROP

For fruit vegetables multi-year rockwool slabs, perlite or coir enveloped in polyethene foil lying in a profiled coated steel or polypropene trough is recommended to get a sustainable system (Van Os et al., 2008). In north-west Europe, commercial firms mostly use single-year rockwool of several qualities (water content, density) and of several brands. Main reasons are the quick change of crops in winter and the clean restart. After the cultivation period rockwool might be collected and recycled by the manufacturer. In southern Europe there is more variety in the use of substrates. Especially perlite, delivered in bags, and locally found pumice are used very much. In these countries rockwool is used to a lesser extend because of its transportation costs.

For collecting the drain water growers try to find the cheapest way. Not only sustainable metal troughs are used but also the so-called drain profiles of polypropene or poly vinyl chloride (PVC). Drain profiles are not self-supporting and are laid down partly into the soil, because they do not have a flat bottom. Often you see a system where the trough is being made of foil in which a drain pipe is laid and upon which slabs or bags are placed. This system is also in use in the Mediterranean countries, it is the cheapest option, but not very sustainable. The foil has not a very long lifespan and must be renewed every year. Besides, the risk of leakages caused by sharp stones or mice is rather big. However, in the Mediterranean countries most of the systems are still open.

For cut flowers to be harvested more than once, e.g. roses and carnations, similar systems and substrates as used in vegetable production are recommended. In some cases it is economic to use rolling benches to increase the utilisation of space. Gerbera is grown on stages, because of labour efficiency. In that case only self-supporting troughs can be used. Crops growing span-wide with many plants per m2, such as chrysanthemums, lettuce and radishes, it is recommended to dig in a polyethene foil with pumice stone, flugsand or sand as a substrate. In the Netherlands these crops are not grown hydroponically, mainly because of economic reasons. In Belgium and Scandinavian countries there is an NFT-system for lettuce and herbs, at which the troughs can be spaced automatically. Instead of a span-wide system a bed system with aisles consisting of polyethene foil and concrete or aluminium side supports filled with a loose substrate can be used for freesia, alstroemeria and amaryllis. An additional demand of the latter group of crops is that of planting and yielding of the bulbs or tubers. At this moment there are only some small scale experiments.

Other aspects of soilless culture systems contain the control of the nutrient solution (Gieling et al.,1997). The total amount of ions resulting in a certain EC (Electric Conductivity) and pH has to be controlled depending the crop, climate and growing season.

3.LIMITATIONS

If you overlook the soilless growing systems used in Europe you see most developments in crops with only a few plants per m2 (fruit vegetables) or in crops at which you can increase the utilisation of the space (rose, strawberry). Economic factors are decisive (Ruijs, 1994). For a better environment you should not look to one aspect only (leaching of fertilisers) but to the whole situation on a business, such as energy, fertilisers, pesticides.

4.REFERENCES

• Braun and Supkoff 1994. "Options to Methyl Bromide for the Control of Soil-Borne Diseases and Pests in California with Reference to the Netherlands". Adolf Braun and David Supkoff, Pest Management Analysis and Planning Program, California Environmental Protection Agency, Department of Pesticide Regulation, Sacremento, CA. July 1994
• Ehret, D.L., Alsanius, B., Wohanka W., Menzies, J.G. & Utkede, R. 2001. Disinfestation of recirculating nutrient solutions in greenhouse horticulture. Agronomie 21: 323-339.
• Garibaldi, A., A. Minuto, V. Graso & M.L. Gullino 2003. Application of selected antagonistic strains against Phytophthora cryptogea on gerbera in closed soilless systems with disinfection by slow sand filtration. Crop Protection 22: 1053-1061.
• Gieling, Th.H., J.Bontsema and E.A. van Os, 1997. Monitoring and control of water and nutrient supply in closed growing systems. In: E. Goto et al. (Eds). Plant Production in Closed Ecosystems. P. 103- 121, Kluwer Acad. Publishers.
• Postma, J., B.W. Alsanius, J.M. Whipps, & W. Wohanka 2003. La microflora nei sistemi di coltivazione fuori suolo. Informatore Fitopathligico 3: 35-39.
• Postma, J., E.A. van Os and P.J.M. Bonants, 2008. Pathogen detection and management strategies in soilless plant growing systems. In: Raviv, M., J.H. Lieth (eds), Soilless Culture: Theory and practice; Elsevier BV. ISBN 978-0-444-52975-6, p 425-457.
• Ruijs, M.N.A. 1994. Economic evaluation of closed production systems in glasshouse horticulture. Acta Horticulturae 340: 87-94.
• Runia, W.Th., E.A, van Os & G.J. Bollen 1988. Disinfection of drain water from soilless cultures by heat treatment. Neth. Journal of Agricultural Science 36: 231-238.
• Van Os, E. A., 1999. Closed soilless growing systems: a sustainable solution for Dutch greenhouse horticulture. Water Science & Technology, Vol. 39, No 5, pp 105-112.
• Van Os, E.A., 1999. Recirculaciónde la solución nutriva: sistemas de desinfección. In: Cultivos sin Suelo II, Curso Superior de Especialización. Eds Milagros Fernández Fernández, Isabel Cuadro Gómez, FIAPA, Almería, Spain. pp. 383-398.
• Van Os, E.A., J. Postma, T.R. Pettitt & W. Wohanka 2004. Microbial optimisation in soilless cultivation: a replacement for methyl bromide. Acta Horticulturae 635: 47-58.
• Van Os, E.A., Th. H. Gieling, J.H. Lieth, 2008. Technical equipment in soilless production systems. In: Raviv, M., J.H. Lieth (eds), Soilless Culture: Theory and practice; Elsevier BV. ISBN 978-0-444- 52975-6, p 157-207.
• Van Os, E.A.,and M.A. Bruins, 2004. MIOPRODIS: Prevention of root diseases in closed soilless growing systems by microbial optimisation, a replacement for methyl bromide. Final report 01-03- 1999 to 28-02-2003. Agrotechnology & Food Innovations, Report no. 089.

Recently published books about soilless culture:

• Fernández Fernández, M., I. Cuadrado Gómez (eds), 1999. Cultivos sin suelo II, Curso Superior de especialización. Almería, ISBN 84-88246-13-7, 589p. Raviv, M., J.H. Lieth (eds), 2008 Soilless Culture: Theory and practice. Elsevier BV. ISBN 978-0-444- 52975-6, 587p.
• Savvas, D., H. Passam (eds), 2002. Hydroponic production of vegetables and ornamentals. Athens, Embryo Publications, Greece ; ISBN 960-8002-12-5, 463p.

Writing Date : 2008-07-18
Latest update Date : 2008-07-18