Many growers use the water from catchment ponds as irrigation water. These ponds receive much of the run off from the greenhouse, rain water and often road run off. Water from these ponds is one of the primary sources of contaminating ions in fertilizer and nutrient solutions. Most commercial fertilizers and fertilizer chemicals contain very little in the way of allegedly detrimental ions such as sodium, chloride and sulphate. However, growers often report damage to crops which they believe is due to one or more of these damaging ions. Unfortunately, at this time there are no formal regulations in place in Canada to control the quality of effluent that runs into catchment ponds. The Federal government has no formal approvement process for activities on provincial lands, however, under section 36(3) of the Fisheries Act, it is an offense to deposit any harmful substances into water frequented by fish including water that may eventually enter other water frequented by fish. The Ontario government instituted a Ground Water Survey in which it set out guidelines as to the acceptable levels of various chemicals allowable in drinking water (Water Quality Survey, 1992). These levels are as follows: chloride, 0 - 250 mg·l-1; nitrate, 0 - 45 mg·l-1; sodium, 0 -200 mg·l-1; phosphate, no guidelines are given, concentrations below 100 mg·l-1 was considered acceptable (Canadian Water Quality Guidelines, 1984). To date no legislation is in place to reduce runoff from greenhouse crop production to zero, however, the Ontario Greenhouse Floriculture Research and Services Subcommittee has identified several research priorities related to greenhouse crop production (Ontario Greenhouse Floriculture Research and Services Subcommittee, 1999 Report to Ontario Horticultural Services Research and Services Committee, 2000). Priority 1A, Integrated nutrient and Water Management Strategies, identifies five specific goals related to irrigation, fertilization and run-off:
1. Identify the environmental impact of greenhouse run-off under different cropping systems.
2. Determine the root zone requirement of plants in closed circuit recirculating systems.
3. Determine the nutrient and water budgets for different production systems.
4. Determine nutrient loss or waste in bedding plant production systems.
5. Determine the problems, if any, related to organic root exudates.
The move towards reducing effluent from horticultural crops has been accelerated by government legislation with the most proactive program in the Netherlands where approximately $7.9 billion was spent in the environment in 1998. Approximately $674 million were in the agriculture sector, including greenhouses (Ivo Van der Loo, 1999). In the Netherlands, the discharge of water into the surface water has been regulated by the Act on the Pollution of Surface Waters. More recently, the National Environmental Policy Plan 3 (NEPP3) set out goals aiming to reduce pollution by toxic and hazardous substances to zero by the year 2000. This goal has been revised since it was found that this was not feasible. Reducing effluent into surface water will now be carried out in a stepwise fashion.
In the United States there is also legislation governing the reduction of greenhouse effluents. All agricultural businesses must have a discharge permit which governs the amount of irrigation runoff. The permits are usually 3-5 years in duration and require all runoff to be retained, most storm runoff must be retained, irrigation runoff must be disposed of, no pesticides may be discharged, nitrate and ammonia must be below 2mg·l-1, the pH of the discharge must be between 6 and 9 and discharge must have an acceptable level of suspended solids.
At this time some growers are still using conventional irrigation techniques which allow leachate to escape the greenhouse and end up in the environment, or at least back in the catchment ponds from which the water was drawn. Conventional techniques include overhead watering, hand watering and drip irrigation. These methods of fertilizer application apply the fertilizer solution to the top of the media where upon it percolates through the media profile and exits the root zone. Many growers are using recirculating methods of fertilizer application where the water is applied to the media from below, as in ebb and flow systems, or from above with drip or spray emitters. Overhead systems allow the solution to flow through the planting media before being collected and reused. This means that the return solution is different in composition from the initial solution that was run through the media. The solutions used in these types of systems are much more difficult to regulate since salts must be added back to the solution once it has been collected, if it is reused. Subirrigation, such as ebb and flow, allow a one way flow up into the potting media by capillary action. The solution returned and collected after irrigation is close to being the same as the initial solution applied to the plants. These solutions are much easier to maintain. In both cases the excess water/fertilizer is collected and stored in a tank for reuse. Unfortunately, those growers that are using top applied recirculating solution must dump the storage tanks periodically because ions in the solutions become unbalanced when plants take up ions at different rates. Growers can replenish certain ions in the returned solutions, however it is difficult to control single ions since an ion must be added with its counter ion in the salt form. Most growers replenish all the salts up to a desired electrical conductivity (EC). Feedback control of nutrient solution quality is typically achieved with pH and EC sensors. These are indiscriminate with respect to individual ions. Eventually, certain ions that are used to a lesser extent accumulate and may cause damage to plants. The most accurate analytical technology to monitor individual ions is HPLC which is very expensive so most growers cannot use them on a routine basis.
Since legislation may one day require a zero or very limited runoff tolerance from greenhouses, determining the extent to which an average greenhouse may contribute to the detrimental effluent in the environment will be useful. It will also tell us the relative amounts of these ions that can potentially contaminate irrigation sources.
Research case studies