Lettuce on Two Rooftop Corners

A Rooftop Lettuce Investigation

A city community garden grows lettuce on the roof of a public library. Gardeners wanted to compare two places on the same roof. They placed equal numbers of young lettuce plants in two matching trays. The trays held no soil. Each tray received the same small amount of dissolved mineral nutrients, the same amount of water, and the same amount of light. The gardeners kept the temperature as similar as they could. They dried a few matching young plants to find the starting dry mass, and they measured the dry mass of the tray plants after six weeks.

The Windward Corner was open to the usual rooftop air. The Screened Corner was inside a cabinet supplied with air that had less carbon dioxide than the usual rooftop air. Water, minerals, light, temperature, and the number and kind of plants were kept the same as in the Windward Corner. The gardeners repeated the comparison with another pair of matching trays.

To measure growth, the gardeners rinsed the plants, dried them in a safe laboratory dryer, and weighed the dry plant material. Removing water made the mass easier to compare. Here are the records from one pair of trays. The mineral solution added to each tray contained 2 grams of dissolved minerals during the study.