Answer:
Moisture
Step-by-step explanation:
The independent variable, in this case, would be the moisture levels of the soils.
The independent variable is the variable being tested. It is the only variable that is altered in the course of an experiment and which has a direct effect on the actual variable that is the main aim of the experiment. All other variables are kept constant while the independent variable is manipulated.
The tomato plants in the two groups were under the same conditions except that a group had more moisture in the soil. Hence, the independent variable is the moisture.