Matter in Two Rooftop Growing Bays

Two Growing Bays Above the Library

A city library has a rooftop garden with two covered growing bays. A science club planted matching bean seedlings in each bay. The seedlings began with an estimated dry plant mass of 12 grams per bay. Both bays had the same kind and number of plants, the same light, temperature, soil, mineral solution, and amount of water. The club measured dry plant mass after eight weeks. Drying the plants removed water before weighing them, so the weights could be compared more fairly.

The air in Bay N came from the usual outdoor air. Bay L received air with much less carbon dioxide. The club kept the roots moist in both bays and changed no other planned condition. The club measured the dry mass of the plants and the dry mass of the soil at the beginning and end. These were the records: Bay N had usual outdoor air, a starting dry plant mass of 12 g, an ending dry plant mass of 70 g, a dry soil mass change of -3 g, and 18 L of water. Bay L had much less carbon dioxide, a starting dry plant mass of 12 g, an ending dry plant mass of 30 g, a dry soil mass change of -2 g, and 18 L of water.

The club also read a research note. Scientists can trace atoms in carbon dioxide from air and atoms in water after plants take them in. The traced atoms can later be found in newly made plant tissues. Mineral nutrients from soil or a solution are important for healthy plant growth, but plants use them in much smaller amounts than the large amounts of carbon dioxide and water involved in building plant material. The club used this note with its own records rather than treating one measurement as the whole explanation.