The brief wave of positive electrical charge that sweeps down the axon is

I believe Action potential is the brief wave of positive charge that sweeps down the axon. Axon is part of the neuron that conducts impulses from the dendrites towards the cell body along the neuron. The action potential is brief since the sodium channels can only stay open for a very brief amount of time. As it travels along the neuron there is a change in polarity across the membrane of the axon .

The independent variable would be the fertilizer as it is what the dependent variable (the growth of grass) depends on.