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Groundwater and Your Greenhouse

Detection of significant quantities agricultural chemicals in drinking water in many parts of the United States has focused attention on the problem of groundwater and surface water pollution from agricultural sources. Regulations are being developed at the state and federal level to protect water supplies from contamination and these laws could have significant impact on the greenhouse industry. The extent of water pollution caused by greenhouses is not fully understood but the potential must be taken seriously.

Several years ago I attended the "Greenhouses and Runoff" conference sponsored by GrowerTalks' magazine. The conference brought together growers, scientists, government officials, and industry leaders to discuss the problem of greenhouse runoff. One of the most important messages was that growers need to develop and implement short-term "action plans" and begin work on long-term plans to eliminate leaching and runoff from their greenhouses. The aim of this article is to begin this process.

Potential Contaminants

Pesticides and fertilizers used in the normal course of growing plants are the most important potential threats to groundwater. Gasoline, fuel oil, coal pile leachate and other materials are also serious threats, but regulations and methods of reducing their threat are largely in place.

Pesticides having high leaching potentials, high surface loss potentials, or which are persistent in soil are of greatest concern. One reason aldicarb was removed from the market was its large leaching potential. Oxamyl, diazinon, and demeton-s-methyl are examples of some active ingredients in commonly used pesticides with large leaching potentials. Dicofol, benomyl, endosulfan, and fluvalinate are examples of chemicals which move readily in sediment on the soil surface. Method of application, pesticide formulation, soil type, and microbial activity in the soil are some other factors which affect how much chemical may reach the groundwater. Fertilizers are a significant pollution threat because of their high solubility and the frequent application of large volumes of irrigation water. Nitrate-N (NO3-N) is the major element of concern, but other elements are potential pollutants as well. Current standards for drinking water in most states, including Massachusetts, allow no more than 10 ppm NO3-N. Compare this to the 200 - 300 ppm N applied in most fertilizer programs and the fact that container leachates commonly have levels of NO3-N well above 10 ppm and the problem is readily apparent.

Protect Your Water Supply

One of the areas most sensitive to contamination is the immediate source of water which enters your operation. This may be the private wellhead or the water line(s) which carry public water. The goal here is to prevent the introduction of contaminants into the water lines or soil close to the wellhead. Wells provide a direct entry point for pollutants to the groundwater. Pesticide and fertilizer mixing and storage should take place away from the wellhead to reduce the chance of contamination. This is particularly important for shallow wells and those in sandy soils.

Back flow preventers should be installed when chemicals are injected into the irrigation water regardless of source. Many localities require these devices by law. Water lines or hoses used to fill tanks during mixing should never be emersed in the solution because back-siphoning may occur. An "air break" between the water source and the chemical solution is as effective as a back flow preventer under these circumstances. Making these modifications will help protect your workers, neighbors, family and yourself from drinking water contamination.

Develop Action Plans

Long-term action plans should have as their goal the complete elimination of leaching and runoff of pesticides and fertilizer.

Recirculating subirrigation systems (ebb and flow, flooded floors, troughs) can do the job, but the large capital investment is prohibitive for many growers. However, such systems should be part of a 10 to 15 year plan and should be seriously considered now by anyone considering new construction. It may be possible to design and construct low cost "homemade" subirrigation systems and, at the very least, small scale trials of subirrigation should be made. Fears of disease infestations and the occurrence excess soluble salts should not prevent the adoption of these systems; both problems are easily and routinely handled by growers who are using recirculating systems.

Short-term action plans can be quickly developed and implemented. With pesticides try to reduce the use of materials which are persistent, have high leaching potentials, or move readily on the surface. A complete list of the chemical properties of pesticides can be obtained from the author or your regional floriculture agent. Adoption of IPM techniques will also help. Proper timing of application and subsequent evaluation of the resulting level of pest control are important steps in reducing pesticide use.

Much can be done today in the areas of irrigation and fertilizer practice. Drip irrigation systems eliminate runoff of water missing the pot during overhead irrigation and the volume of water applied to the pot can be controlled. In theory it should be possible to greatly reduce or eliminate leaching from pots by simply turning the system off as container capacity is reached. Recently Dr. Heiner Leith and coworkers at the University of California/Davis described a new way of controlling drip systems (GrowerTalks', September 1990). It consists of a tensiometer placed in the growing medium to sense moisture tension (level) and a small computer programmed to turn the system on or off when preset moisture tensions are reached. Using this system in a commercial greenhouse, Dr. Leith was able to reduce runoff from potted chrysanthemums and poinsettias to nearly zero! Perhaps a commercial adaptation of this system will be available soon.

Water trays and saucers, depending on their shape and spacing on the bench, can greatly reduce runoff and leaching by containing the water draining from pots and holding the water which misses the pot during overhead watering. They are inexpensive and reusable. Water which collects in them should be given adequate time to evaporate or be absorbed by the plant before further irrigation. Avoid tight plant spacing and poor ventilation to prevent disease problems when using this technique.

Controlled-release fertilizers can help reduce fertilizer leaching if properly used. A single large application shortly after planting has little benefit in controlling nutrient leaching as remarkably large quantities of NO3-N and other nutrients are released at this time which the plants do not need. A better approach is to split the single application into smaller amounts and apply them two or three times during the growing season. This technique greatly reduces NO3-N in the leachate compared to the large single application and conventional liquid fertilization. Split applications probably provide more constant nutrition for the plant as well. Using controlled- release fertilizers in this manner is currently being tested at Umass.

Here are some final ideas on protecting groundwater from contamination. Fertilizer leaching and runoff can be reduced by "fine-tuning" fertilization and irrigation practice. Review your past soil tests to see if you are historically a "heavy feeder". If the soil tests reveal this tendency try reducing your fertilizer rates (ppm) by 10 to 20% or reduce the frequency of fertilization.

One common recommendation is to try to maintain the amount of leachate draining from the pot at about 10% of the volume applied during irrigation. In most greenhouses leaching percentage is as high as 40%. Reaching 10% with hose watering is "easier said than done" and will require considerable discipline for many waterers. However, maintaining a small leaching percentage and reducing the amount of water which misses the pot during hose watering would go a long way to reducing water consumption and chemical leaching. If you are successful in reducing the leaching percentage using any irrigation method, fertilizer rate can also be lowered because more fertilizer will be retained in the pot.

Douglas Cox
Plant and Soil Sciences
University of Massachusetts

August 1993