In order to grow and survive, a plant needs water, sun, nutrients and air. The process through which plants "feed" is photosynthesis. Through this process, light energy is converted into chemical energy used to fuel the plant's physiological needs.
In her experiment, Jo has taken into account almost all the factors that are necessary for a plant's growth. As sunlight is a type of energy, it has no mass to affect the experiment. However, plants are mostly made from carbon. This is not obtained from the soil (which is carefully weighted), but from the air during photosynthesis.
Jo has forgot to take into account this vital component to plant's growth. Thus, her experiment is flawed and will produce erroneous results.
Furthermore, she will be unable to use the plant's growth as proof because the plant itself is not ne only living organism. All soil has its own microorganisms that closely interact with the plant itself. Some of these bacteria and fungi will also use different components of air and thus their mass will change in a way that the experiment does not take into account.
If the amount of air entering and leaving the system could be measured, then the experiment would be accurate. During photosynthesis carbon dioxide is transformed into oxygen, so the plant will require a continuous supply of fresh carbon dioxide. My measuring volume of air entering the system as well as exiting the system, the mass of air components (carbon and perhaps others) absorbed by the plant and potentially soil microorganisms could be approximated.
