What procedure should have been changed to avoid errors when comparing plant transpiration rates at different humidity levels?

To accurately compare plant transpiration rates at different humidity levels, it is essential to ensure that all variables are controlled as much as possible. Using the same soil type for both groups at each humidity level eliminates any potential effects that different soil types could have on water retention, nutrient availability, and overall plant health. If different soil types were used, variations in soil composition could influence transpiration rates due to factors such as drainage, moisture retention, and root health. By controlling for soil type, any differences observed in transpiration rates can be more confidently attributed to the effect of humidity levels alone, leading to more reliable and valid results in the experiment.