1 Asociacion Mexicana de Productores de Hortalizas Bajo Invernadero (AMPHI)

Table 2. Solar Radiation Comparison - PAR (Mols/M²)

With today’s superhighways and climate-controlled trucks, it’s no longer necessary to grow greenhouse vegetables near population centers. Prior to 1970, the greenhouse vegetable industry was located near high population centers, mainly in the states of Ohio, Michigan and Massachusetts. After 1970, with the rapid rise of energy costs to heat greenhouses, along with the construction of superhighways to transport fresh products from southern regions, the above-mentioned states became importers of tomatoes.

The energy required to transport fresh vegetables from the southwestern region of the United States and Mexico is much less than that required to heat a greenhouse. For example, in conventional greenhouses in Ohio, nearly 40,000 kcal of energy is required to grow 1 kg of tomatoes vs. only 4000 kcal in the open field.

Today, with superhighways and high energy costs, light is considered the most important factor for greenhouse vegetable production. In North America, the highest light levels are in the southwestern desert regions extending into northern and central Mexico. This is especially important if a grower is to grow greenhouse vegetables during the winter when prices are at their highest (Table 2). Generally, a 1% decrease in light reduces yield 1%.

A greenhouse in a high winter light region at optimum temperatures will produce more than 250 tons of marketable tomatoes per acre per year. Producing such yields in northern latitudes is only possible if the crops are grown through the summer period when market prices are at their lowest.

Along with the light factor are temperature considerations, especially in the southwest desert. For example, if tomatoes are selected as the crop to be grown year-round, low elevations must be avoided, due to the difficulty in maintaining desirable temperatures in the greenhouse during late spring and early fall, even with fan and pad cooling. In the late 1960s, hydroponic installations were installed in low elevation regions in Texas and Arizona. In most regions of Texas, evaporative cooling is ineffective due to high ambient humidity. Escalating energy costs in the 1970s added to the costs of cooling in the summer, as well as heating during the winter months. This, coupled with insect and disease problems and high amortization costs, especially when growers were purchasing turnkey greenhouse systems rather than building their own growing system, caused most hydroponic installations to fail financially. This was true not only in Texas and Arizona, but throughout the United States.

Given the high cost of fan and pad equipment, future hydroponic growers will be selecting sites at specific elevations that have summer temperatures that do not require evaporative cooling, therefore sparing the costs of such cooling equipment. At the same time, an elevation should be selected that is not too high in order to avoid high heating costs in winter. In southern Arizona, such an elevation for tomato production would range from 1250 to 1675 m and for cucumber production, 600 to 1250 m.

Proposed as an alternative to fan and pad cooling are high pressure fog systems. Recent experiences have proven this method of cooling desirable if the feed water is absolutely free of any undissolved or dissolved solids. It is important for the greenhouse structure to have ridge vents to accommodate ample air exchange for prescribed temperature and humidity control. Any time a grower deviates from the prescribed growing temperatures for a given crop, yields will be lowered. The more a grower has to cool or heat a greenhouse in order to maintain recommended temperatures, the greater the cost to operate the facility, therefore lessening financial return. If evaporative cooling systems are used, locating the greenhouse in a region of low outdoor humidity is important.

Especially important is selection of a site free of insects that might be vectors for severe virus diseases. Early hydroponic ventures did not consider this. In the United States and Mexico, sites were selected where white flies existed. These can be a vector of gemini viruses, which are extremely lethal to most solanaceous and cucurbit crops. Screens on air intakes do not always work, as the white fly almost always gains entry into the growing area. Growing in regions where there are mild winters normally increases the incidence of insects and diseases due to the continued life cycle of the pest. Selecting a site that isn't already a major producer of vegetable crops is also advisable.

Energy and Water

There are many choices of energy sources, such as natural gas, propane, fuel oil and electricity. Many early hydroponic growers did not consider cost differences between the types of energy. Many used propane, which proved to be very expensive. The only economical choices are natural gas and fuel oil. Coal was once used but air pollution standards and regulations make the use of this fuel prohibitively expensive.

There is new interest today in lighting greenhouses with high intensity-discharge lamps. Both the capital and operating cost of such systems are extremely high and will not, in the foreseeable future, permit competition with winter greenhouse vegetables grown in high light regions. An exception may be in Quebec, Canada, where the electrical rates are extremely low.

Water quality has become a major concern of greenhouse growers, especially where large amounts of water are applied to a restricted volume of growing medium. Plant growth is affected by the interaction of the dissolved chemical elements in the water supply, the chemical properties of the growing medium to which the water is applied, and the fertility program employed.

In selecting a greenhouse site, a grower must be aware of several chemical properties that might cause problems for greenhouse growers: pH, alkalinity, soluble salts, calcium, magnesium, boron, fluoride, chloride, sulfates, sodium, carbonate, and iron. The cleaner the water, the greater the opportunity to achieve maximum yields. The water designated for use in a greenhouse must be analyzed for agricultural suitability during greenhouse site selection.

Structures and Environmental Control

The European glass structures that today are commonly being built for vegetable production in the southwestern part of the United States and Mexico are very different from the polyethylene/fiberglass houses used in hydroponic production between 1965 and 1990. The European structures are much higher. It is becoming increasingly common for the height of polyethylene greenhouses to be like the glass structures.

To achieve a more uniform growing environment, without rapid temperature fluctuations, more total volume of space is being allotted within a greenhouse; today the gutters of greenhouse structures are commonly more than 5 m above ground level.

The types of polyethylene sheet films are much the same except those introduced over a decade ago that retard the loss of infrared heat. These films are reported to reduce 20% of the heat loss from a greenhouse and have become common in today's industry, especially in Europe. Other glazing materials, such as fiberglass, polyvinyl chloride, Mylar and Tedlar, have proven either less appropriate, inconvenient, or in most cases, much more expensive than polyethylene, even though the latter may have to be replaced more frequently. Newer materials, such as polycarbonates and acrylics have become much more common, but their popularity has been offset by high costs.

Greenhouses are expensive, however, and controlling the environment within a greenhouse requires considerable energy. Starting 20 years ago, there was major research emphasis on the use of solar energy and reject heat from large industrial units. Although solar energy as a greenhouse heat source is technically feasible, it has not proven economical because of collection and storage costs. The economics of using waste heat from generating plants favors incorporating the heat-use system into the overall plans for new plants, rather than modifying existing ones.

In the last 10 years, there has been interest in the development of co-generation plants; small electrical plants receive government assistance if designed to use the waste heat from the electrical generators. Several such facilities have been established that use the waste heat either to heat greenhouse vegetables or water for fish production. While such opportunities are inviting, excess government regulation and red tape have discouraged many investors from taking advantage of such opportunities.

Whatever the source of energy, it should be conserved once it is in the greenhouse. In regions of cold winter weather, thermal curtains of porous polyester or an aluminum foil fabric are installed to reduce night heat loss by as much as 57%. In the deserts of the southwest, especially at low elevations, winter temperatures are not severe enough to warrant curtains. While curtains will provide energy savings, they are not sufficiently effective to warrant their high cost.

Computers can operate hundreds of devices within a greenhouse (vents, heaters, fans, hot water mixing valves, irrigation valves, curtains, lights, etc.) by utilizing dozens of input parameters, such as outside and inside temperatures, humidity, outside wind direction and velocity, carbon dioxide levels and even the time of day or night. Unlike early hydroponic controls, computers are used today to collect and log data provided by greenhouse production managers. A computer can keep track of all relevant information, such as temperature, humidity, CO₂, and light levels. It dates and time tags the information and stores it for current or later use. Such a data acquisition system enables the grower to gain a comprehensive understanding of all factors affecting the quality and timeliness of the product.

Hydroponic/Soilless Culture

While there are many types of growing systems, the two most popular growing media today are rockwool and perlite. Due to the high cost of rockwool, root volume is being reduced. Growers in Arizona are growing six tomato plants from a rockwool slab no bigger than 7.5 x 130 x 15 cm. Each plant has a root volume no greater than 2438 cm³. (A gallon contains 3608 cm³.) The irrigation system may be activated more than 30 times per day. At the University of Arizona, excellent tomato crops have been grown in a container no larger than 956 cm³. In this case, the irrigation system was left on continuously to optimize root aeration, pH and nutrition. Maximum yields were 12.8 kg of tomatoes per plant over a 6-month period.

In the future, growers will provide little root volume in order to not only reduce media cost but to maximize control over mineral nutrition, pH, aeration and root diseases. Unbelievably high salt levels are maintained in the root systems where the E.C. of the feed solution will approach 3.5 and the drain water at an E.C. of 4.5 to 5.0. This helps to control plant growth as well as flavor of the tomato fruit. All systems in the future will be closed, with no drainage, preventing any loss of mineral elements and the contamination of groundwater. For health reasons, hydroponic systems may be used to reduce nitrogen levels in leafy vegetables at harvest. This is especially true in Europe for such crops grown under low winter light intensities.

Pest Control

Early hydroponic operations were devastated by pest problems. White flies, leaf miners, pin worms, nematodes, Cladosporium leaf mold and viruses, as well as root diseases such as Pythium root rot and bacterial wilt were common. Today, unlike 20 years ago, the drain solution is often sterilized. The options are heat treatment, ozone and ultraviolet radiation. The University of Arizona has a program to control certain root diseases with surfactants or by using nonchemical approaches. While the results are not yet practiced in hydroponic systems, the results look promising.

Today integrated pest management (IPM) is of particular interest to Americans in CEA because of the paucity of pesticides with legal clearance for use in greenhouses. The frightening ability of some pests to develop resistance to chemical pesticides has revived worldwide interest in the use of natural enemies of insect pests, particularly when used in association with horticultural practices, genetics and other control mechanisms. Tomorrow's growers may be growing crops without applying any chemicals to control diseases and insects. Crop production requires both the identification of possible crop disease and insect problems, and the ability to properly integrate disease and insect prevention and control practices into a total management plan.

Overview

Hydroponic culture is an inherently attractive, often oversimplified technology, which is far easier to promote than to sustain. Unfortunately, failures far outnumber the successes, due to management inexperience or lack of scientific and engineering support. Thus, interest in hydroponics has followed a roller coaster ride since its conception. However, in recent years, extensive research and development programs in Europe have vastly improved hydroponic production systems. These new technologies are today being successfully transferred to the United States, proving hydroponics a technical reality in the high light regions of the desert southwest.

Proceedings - Table of Contents