Kevin decides to find out which fertilizer is best for tomatoes. He pots up three plants, all with the same soil, giving one no fertilizer, giving the second type A fertilizer, and giving the third type B fertilizer. In this experiment, what is the control, or Kevin's standard for comparison?

The control is the one without the fertilizer.

Control in an experiment usually is the one without the variable to be tested, which is fertilizer in this case.

Therefore he can use the one without the fertilizer to compare to the others with different types of fertilizers, to see how fertilizers affect their growth.