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versão On-line ISSN 0718-3429
Idesia vol.30 no.1 Arica abr. 2012
Volumen 30, N0 1. Páginas 3-6 IDESIA (Chile) Enero-Abril, 2012
Technologies of soilless culture and their sustainability
1Julio Muro Erreguerena
1Professor Department of Agricultural Production. Universidad Publica de Navarra, Pamplona-Navarra-Spain.firstname.lastname@example.org
Soilless culture is the term used to denominate a group of techniques and equipment which have in common their independence from the use of natural soil to obtain vegetable products.
The development of soilless culture is linked to the agro-technical and economic development of countries, Holland being the best example, and also to the opportunity to obtain the advantages of its use (better yields compared to cultures in soil and better quality of products), as in the case of ornamental plants in Latin America.
There are a number of systems, from pure hydroponic culture (roots given only nutritive solution, with no solid substrate) to systems with high aeration capacity substrates which receive the nutrient solution, which are currently the most used methods.
Pure hydroponic culture, in rafts or containers, was first developed in modern times by W. F. Gerike, although there are historical precedents such as the Hanging Gardens of Babylon (VI century BC) or the cultivation of the chinampa islands in the lakes of what is currently Mexico City, developed by the 4 first Aztecs. With technological evolution there are now principally two systems of pure hydroponics, the nutrient film technique and the New Growing System. Both are systems with nutrient recirculation, that is, the solution applied to the roots in culture is collected at the end of the canals (in which the crop roots develop) and re-used in the system. These systems are called recirculation systems or closed systems; they require considerable technology, inversion and especially chemical knowledge about both the nutrient systems and the control of their evolution over time which obliges immediate decisions of chemical correction (pH, conductivity and composition), since the absorption of nutrients by the crops change composition of the solution. To these difficulties must be added the sanitary control (especially fungi) of the circulating solution.
The second kind of current hydroponics is the systems of lost solution or open systems, in which the nutrient solution is only applied once to the crop, adjusting the amount to its needs. When the solution is removed it is discarded and not used again in the system.
This is the system most used in Spain and Europe, since it is not as technically complicated or costly as the other system. The nutritive solutions are prepared according to the quality of the water used and the needs of the crop. Normally two solutions are used for a crop, one for the vegetative phase and one for flowering-fruiting. These solutions only need to be calculated once by a hydroponics technician, and the producer only has to prepare the same "recipe" for each of his or her crops. What makes this technology even easier is that the application of the solution does not require special systems; drip irrigation systems (drippers and valves), which are well known and relatively cheap, work well. The nutrient solution is applied on elongated (more than 1 m) sacks called panels or sausages with a capacity of 20 to 401 which contain very porous substrates. Possible substrates include coarse sand, ground volcanic rocks (called tezontle in Mexico and picón in the Canary Islands), laterlite, perlite, mine tailings (from carbon mines in Spain), rock wool, cocoanutfiber, wood fiber, etc. The high porosity required of these substrates is indicated by the fact that peat moss does not provide sufficient air and is not used in hydroponics. The substrates most used in Europe are rock wool, perlite and cocoanut fiber, in that order.
The sustainability of hydroponic systems is based especially on two environmental aspects, management of residual water and management of the material used as substrate in the cultivation sacks.
The waste water of open systems which has been collected in deposits at the end of its useful life (about one month) must be later used in ferti-irrigation of other crops in soil. These solutions contain nutrient elements which may be used by other open-air crops by incorporating it into the water applied with gravity, spray or drip irrigation systems. The waste solutions should never be re-used as solutions for hydroponic culture (culture hygiene) nor dumped in uncultivated soil (contamination of sub-surface water) or rivers (risk of eutrophication).
The use of different substrates is important in terms of sustainability. The type of substrate used in the majority of cases depends on the commercialization system, since normally the substrates offered on the market are the ones used. This is true in most of Europe, in which rock wool is used because it is the most available, as well as being easy to manage and with great technical support from the companies which sell it; however, its use is not sustainable, since after use (2 years) when it is dry it becomes dangerous to manipulate due to the microfibers which it releases and which are breathed by those people who handle it. Although it has been claimed that its use is not dangerous, its users do not agree, and thus in France, Holland and Germany the companies which make this material are obliged to take it back after it is used. The second most used material in Spain is perlite, which is an inorganic material without as many drawbacks as rock wool (it does not release microfibers), but still produces a residue which must be disposed of, usually by taking it to dumps. It is also used occasionally on crops with clay soil to improve drainage, since it is an inorganic calciferous material which is innocuous for the soil. Cocoanut fiber, the third material most used as substrate in Europe, is the most innocuous of the three; after use it can be and should be applied to soil as an organic additive. The aspect of sustainability is very important, since it provides the opportunity to use aerating organic materials of local or national origin as substrates for hydroponics. In our most recent studies we have developed an easy to obtain substrate based on pine wood fiber (not sawdust), which is produced by the techniques of defibration used in the paper industry. This new substrate works perfectly well in tests compared to perlite and rock wool; there were no significant differences in tomato production. The use of this kind of native organic material provides a great opportunity for this kind of hydroponic production. In Mexico, for example, studies are adapting the use of the residue of the agave after the sweet material has been extracted for tequila production; there are analogous studies in Spain with crushed almond shells. There are multiple possibilities for obtaining organic materials from lignocellulose material of local resources; wood products based on the byproducts of sawmills are the principal starting point for this objective. There is also the possibility of using local inorganic materials, such as the tezontle in Mexico and picón in the Canary Islands mentioned above. These have very limited possibilities of expansion due to the cost of transport, since they are heavy materials; however, they are very apt for local use in which transport costs are low.
Local lignocellulose materials, which may be transported long distances, and inorganic material with very limited possibilities of transport, are two groups of sustainable materials which can compete with the standard materials offered by the companies which dominate the market of substrates for hydroponics.