A controlled environment

Resource, May 2000 by Both, Arend-Jan, Albright, Louis D

Test facility protects lettuce from the elements year round

As world population continues to expand, quality farmland shrinks and pressures on food production systems increase.

A challenge for the future will be to produce highquality foods using minimum inputs such as seed, fertilizer, labor and capital. The focus will be on maximum returns to growers, processors and retailers while keeping consumer costs low.

Fewer farmers will have to sustain more consumers so production system efficiency must continue to improve. Controlled environment production facilities such as greenhouses and plant factories can help by minimizing the negative effects of weather and optimizing shoot and root environments. Costs to build and operate such facilities must be offset by increased quality and productivity to make them economically viable.

In the early 1990s, Cornell University's Controlled Environment Agriculture (CEA) Program began to explore economic and production feasibility of hydroponically grown vegetables in the Northeastern United States. At that time, the New York State Energy Research and Development Authority (NYSERDA) also projected a bleak longterm future for fossil fuels. If predictions of future petroleum reserves ring true, the Northeast will experience reduced supply and increased costs for fresh out-of season produce. Greenhouse production of high quality, fresh, outof season crops surfaced as a possible solution because:

This type of production system could boost the struggling New York agricultural sector.

High population density in the Northeast creates numerous and varied marketing opportunities.

Highly controlled indoor plant production systems can help reduce food-borne diseases.

Closed-system greenhouse production produces little environmental discharge.

Local fresh produce production has shown an increasingly strong marketing advantage.

With these incentives in mind, NYSERDA assumed the lead in sponsoring the CEA research at Cornell that initially studied butterhead lettuce, spinach and pak choi. Other sponsors included New York State Electric and Gas Corp. (NYSEG) and Niagara Mohawk (NiMo). Lettuce became the main focus to be studied most extensively using nutrient film technique (NFT) and floating hydroponic (FH) systems. Floating hydroponics is also known as deep trough hydroponics. Altho; gh both systems have advantages, the FH system maintains a larger volume buffer - of water and nutrients around plant roots. It also permits easy plant transport from one end of the greenhouse to another.

Many, often inter-dependent, variables influence plant production and these parameters must be considered simultaneously to understand the entire system's responses. Cornell's project called for a multi-discipline approach, mainly involving engineering and horticulture but including disciplines such as economics, entomology, food science and plant pathology.

Experiments determined temperature, light and carbon dioxide and plant nutrient concentration effects and interactions. Cultural practices such as cultivar selection, seedling growth medium type, transplant and respacing timing, plant spacing and optimum plant age at harvest were examined. Costs for heating, lighting, ventilation and carbon dioxide enrichment systems were evaluated to find the most economical solutions.

Researchers then expanded their study to quantify the impact of the light environment on the crop. Controlling continuously fluctuating solar radiation is difficult. In many ways, light is as important for plant growth as temperature. Greenhouse engineers have spent years developing systems that shield plants from daily outdoor temperature changes. Light control for keeping the daily light integral consistent soon became a major goal of this research. The daily light integral provided to lettuce for vegetative growth correlated with biomass production. This daily light integral - photosynthetically active radiation (PAR) - was used in environmental control.

For lettuce, accurately controlling the daily light integral prevents the physiological disorder tipburn from making the crop unmarketable. Sufficient transpiration is also needed to move enough nutrients, especially calcium, to the most rapidly growing leaves, which also helps prevent tipburn.

Overhead fans can increase plant transpiration by reducing the boundary layer at the leaf surface, which reduces resistance to water vapor transport from leaf to air. In the growing system developed for this project, fans continually blew greenhouse air onto the crop. The lettuce grew more than one-third faster and suffering no tipburn.

The daily light integral was controlled using a movable shade curtain when the light integral threatened to overshoot the target value. A supplemental lighting system was used when solar radiation was insufficient to provide the target light integral.

A rule-based control algorithm governed the shading and supplemental lighting systems. This algorithm ensured that the daily light integral target was met throughout the year despite the solar radiation level. The algorithm provided as much lighting as possible during off peak electricity rate hours. Lettuce requires no fixed dark period so it can be grown under continuous lighting.