A farmer investigated the effect of different concentrations of a new selective herbicide on creeping buttercup plants growing in a pasture.
To find the most effective concentration that is also cost-effective, she marked out six 1 m2 plots in the pasture.
She counted the number of creeping buttercup plants in each plot.
She then prepared six different concentrations of the selective herbicide and applied a treatment to each of the six plots.
After three weeks, she recorded the number of surviving creeping buttercup plants in each plot.
The results are shown in the table below:
| Concentration of herbicide (%) | Number of creeping buttercup plants in 1 m2 | |
|---|---|---|
| Before applying herbicide | Three weeks after applying herbicide | |
| 0 | 12 | 12 |
| 10 | 11 | 11 |
| 20 | 14 | 9 |
| 30 | 12 | 4 |
| 40 | 13 | 1 |
| 50 | 11 | 0 |
Give one variable the farmer should control in this investigation.