Dr. Merle H. Jensen
Assistant Dean, College of Agriculture, The University of Arizona
Dr. Merle H. Jensen is Assistant Dean for Sponsored Research and Associate Director of the Arizona Agricultural Experiment Station at the University of Arizona and Professor of Plant Science. He is a graduate of California State Polytechnic, Cornell and Rutgers Universities. He has been involved with numerous programs for research and development of new methods of food production, including serving as team leader to develop the agricultural systems for “The Land” at EPCOT, Walt Disney World.
Dr. Jensen is one of America’s leading agricultural scientists on the subject of controlled environment agriculture and future opportunitiesfor food production. He has published nearly 100 research and technical articles, and his research and development projects have been featured in many popular publications and television programs.
Many factors have spurred the development of more extensive and more effective ways to use greenhouse structures for the production of food and floral crops. Greenhouses may be glasshouses, they may be constructed with plastic or simply screened enclosures. Such structures offer an effective way to conserve water usage, to control pests, to produce crops without soil, to utilize geographic areas unsuitable for crop production and to secure very high yields from limited space.
Some of the locations where greenhouse culture has been spectacularly successful are the deserts in the Arabian Peninsula, southwestern United States and northwestern Mexico. Greenhouses have permitted the extensive production of flowers in Columbia where the structures are used primarily to protect plants from excessive rainfall. In Malaysia screen houses are used to protect plants from insects and in China plastic greenhouses are used extensively to attain very high yields of vegetables from limited space.
The key factor that has facilitated the rapid expansion of greenhouse culture has been the development of hydroponics, a technology for growing plants in nutrient solutions with or without an artificial medium. Hydroponic technology completely eliminates the need for soil as a source of plant nutrients. Various inert materials may be used to anchor plants such sand, sphagnum moss, rock wool, vermiculite and other substances, but there are numerous hydroponics operations where plants are grown exclusively in a liquid medium. All of the essential plant nutrients are dissolved in water that has been adjusted to a favorable acidity level and this solution is constantly circulated around the roots of plants growing with or without an anchoring medium. As the nutrients are removed from the circulating solution, they are replaced by injecting the depleted elements into the solution to keep concentrations at optimum levels. In some hydroponic installations the nutrient solutions are discarded after a certain period of use. With either system the plants are constantly provided with optimum nutrient levels and favorable acidity levels as required for each stage of plant development. Moreover, water is used very efficiently and soil borne pests (weeds, insects and diseases) are eliminated. Some insect pests such as aphids and white flies are effectively control with biological techniques that reduce the need to employ pesticides.
Environmental factors such as air temperatures and humidity can be controlled in most greenhouse structures and in some cases carbon dioxide levels may be increased to optimum levels. One environmental factor that limits the use of hydroponic greenhouses is available light. If natural light is relied upon, the greenhouses have to be located where light intensity and day length are adequate. Optimum day length, suitable light intensity and moderate temperatures are found in semi-tropical locations at relatively high altitudes. Artificial light may be used to augment day light in northern areas but power costs may make this alternative economically unattractive.
Controlled environment greenhouses using hydroponic systems are high technology and capital intensive. This type of agriculture is not cost effective except in certain localities, but in appropriate settings results have been highly productive and economically successful. In 1977 there were 100 acres of hydroponic/controlled environment greenhouse in operation in Colorado, Nevada and Arizona. Since then many more similar operations have been brought into production in southwestern United State, Mexico and China.
It is estimated that 700 acres of hydroponic greenhouses have been constructed in Mexico for the production of cherry tomatoes for shipment to the United States. Other installations in Mexico are growing lettuce in hydroponic greenhouses using Styrofoam rafts that float in shallow vats containing a nutrient solution. Lettuce yields of 250 tons per acre per year have been obtained. In Arizona yields of tomatoes have reached 350 tons per acre per year in hydroponic greenhouses. In China a million acres of hydroponic greenhouse are producing vast quantities of fresh vegetables using clean water that eliminates the danger of E. coli commonly found in field grown vegetables irrigated with sewage. Hydroponic techniques are being employed in a highly insulated and artificially illuminated structure in Antarctica to provide fresh vegetables scientists stationed there. Studies are being aggressively conducted to devise ways to use hydroponic systems to produce plant products for future occupants of space ships.
Because of the demonstrated success of hydroponic/controlled environment greenhouse in different part of the world, research activity has increased. The exact nutrient requirements, environmental conditions and pest management needs are being determined for a variety of crops. Plant breeders and biotechnologists are investigating the development of plants that are more attractive, tastier and better adapted to this type of agriculture. It is believed that the expansion of food plant production in hydroponic greenhouses will rapidly increase in the future.
In an effort to provide wide-ranging views and perspectives regarding the practice of and issues surrounding agriculture, the Philadelphia Society for Promoting Agriculture (PSPA) seeks speakers representing a variety of perspectives. The statements and opinions they present are strictly their own and do not necessarily represent the views of PSPA.