Enhancing Environmental Quality in Agricultural Systems

Greenhouses have been extremely successful in providing abundant, cheap and high-quality produce, by using resources (water, minerals, pesticides) with a very high economic efficiency. Marginal agricultural land is being rapidly converted into protected cultivation in many (semi-arid) regions of the world, hoping to prosper both from primary and secondary activities. Water use efficiency of greenhouse production is about five times as high as field production of vegetables. However, in spite of using resources more efficiently, greenhouse areas have an enormous visual and environmental impact: diversion of limited good water resources; contamination due to pollutants released with over-abundant irrigation; production of plastic and mineral waste and biological by-products; contamination due to plant protection chemicals and emission of “greenhouse” gases (CO2) by heating with fossil fuels in Northern countries. In addition, greenhouse production has an “image” problem: there is a general perception among European consumers that such an “industrial” production of food is non-natural and unhealthy, although in the Americas, for instance, the “cleanliness” of the production process is considered an advantage. Since, the “polluter pays” very seldom, environment-friendly production is more expensive. Therefore a large market in “eco-labels” has developed in response to consumers’ misgivings and in the hope of recovering (part of) the costs through higher prices. However, there is little clarity about agricultural practices associated to each label and there are doubts about enforcement. This paper analyses advantages and draw-backs of greenhouse production, and attempts to review the items where improvement is necessary in order to ensure that greenhouse production is sustainable, yet profitable also in the future. INTRODUCTION Consumers in developed countries are so used to get appealing, clean looking and cheap vegetables the whole year round, that very few of them are aware that such a luxury was unthinkable just 20 years ago. This abundance is most commonly generated under shelters (protected cultivation) that, for a large fraction of the year, ensure growing conditions much better suited for crop production than open field. This is not new: the advantages of ambient modification in this respect were certainly known in ancient Rome, as Plinius (77A.D.) maintained that the emperor Tiberius could eat year-round cucumbers from plants that were withdrawn under semi-transparent shelters whenever conditions were unfavourable. Similarly, orangeries (buildings devoted to create growing conditions of citrus where citrus would not grow) have been built in the gardens of rich people for many centuries. What is new of the last 20 years is that year-round abundance is so cheap as to be taken for granted. This has been caused by the large-scale application of horticulture under plastic that has transformed formerly marginal agricultural land into a highly efficient and profitable crop production. The most striking example is the “explosion” of protected cultivation in the Mediterranean region: according to the FAO, from nihil in 1950 to 120 000 ha of greenhouses and tunnels in 1985, on to 200 000 in 1997 (Baudoin, 1999). ECONOMIC WATER USE EFFICIENCY One of the world’s largest concentrations of greenhouses is in the province of Almeria (South-Eastern Spain), whose economic ranking among the Spanish provinces Proc. IS on Greenhouse Salinity Eds: A. Pardossi et al. Acta Hort 609, ISHS 2003 278 has grown from third from bottom to third from top, in 20 years, largely thanks to this and the associated secondary industry. Recent estimates of the fast-growing greenhouse area in the province put it at 25 000 ha, with a total production of 2.6 million ton/year of vegetables, whose value is € 1600 million (a mean of 64 000 €/ha). Value of quality-labelled tomatoes from insular Italy easily exceeds 120 000 €/ha. Some flower-crops in Dutch glasshouses gross more than 400 000 €/ha. Little wonder that other regions and countries are developing their greenhouse industry fast. The whole agricultural sector in the Mediterranean basin is facing a decreasing amount of water resources, often coupled to an increase in their price. The ongoing economic development causes a shifting in priorities, whereby, under scarcity, water is allocated to sectors of high social priority, such as household, or economic importance, such as tourism. This tendency towards allocating water where it is most economically used, is strengthened by recent European regulations enunciating the “cost recovery” principle. That is that the price of water must reflect the real costs of provision, including environmental costs; in short the “polluter pays” principle, such as stated in the “Water framework directive” (European Parliament and Council, 2000). The combined pressure of these two factors (scarcity and price) on water resources for agriculture (which is still the largest single user by far) simply reinforces the above-mentioned tendency for “marginal” agriculture to disappear from many regions, to be replaced by cash crops, of high water use efficiency. That greenhouse production has higher water use efficiency is the consequence of at least three factors: reduced potential evaporation (less sun radiation; less wind and higher humidity); increased production (better control of pathologies; better control of climate parameters); application of advanced irrigation techniques (drip irrigation; re-use of drain water) As Table 1 makes clear, the combined effect of these factors can increase water use efficiency by a factor up to five. Since greenhouse products, being more attractive and of more uniform quality, often sell for a higher price than field products the income produced per unit of water used has increased by even more (Table 2). Presently about 2/3 of the greenhouse area in Holland is substrate cultivation. The reason for growing on substrate is purely economical: better returns. A very recent study comparing returns of substrate and soil-grown sweet pepper in Almeria (Table 3) points to the same trend. Substrate cultivation of course makes it rather easy to collect drain-water and reuse it (closed systems), which allows for some saving on the costs of fertilisers and water. Indeed, Dutch growers started to grow on closed systems for this purely economic reason (Ruijs & Van Os, 1991). Later, however, environmental considerations (high population density, shallow water table) have caused the government to issue regulations whereby recollection and re-use of drain water is getting increasingly compulsory. Obviously, the main point of national regulations is not water saving (not in Holland), but to prevent percolation of nutrients. BACK-SIDE OF COIN: EMISSION, CONTAMINATION AND SOLID WASTE Indeed, in spite of the high economic efficiency of greenhouse production (and also the very high resource use efficiency), it is beyond doubt that such an intensive growing method has a large impact on the environment. For instance, it has been recently estimated that yearly plant-protection chemicals application in Dutch greenhouses is 31 kg/ha (active component), Table 4. Application rates in Mediterranean countries are at least as high. Very little is known about the deposition and dispersion rate of sprayed plant protection chemicals. Van Os et al. (1994) have calculated that 30 to 50% leave the greenhouse through the air. This fraction applies to ventilation rates of 0.5 h and would be higher in more ventilated greenhouses, such as in the Mediterranean region. Total fertilisers application on greenhouse tomato in Sicily is some 6 ton/ha per year, which is comparable, for instance, to the Dutch estimate of 1200 and 250 kg/ha of the elements N and P, respectively (Table 4). Nutrient uptake, however, does not exceed 500 kg/ha, so that more than 50% of the application percolates in the subsoil, a fraction that can be reduced to virtually nothing in closed soil-less systems (Table 5).