The control group is the ten pots that got none of the fertilizer.

Yara measures those ten plants. They have each grown about 20 centimeters taller too.

Yara's two groups of bean plants: how much taller each group grew in two weeks.

Bean plants grow about 20 centimeters in two weeks anyway, with no fertilizer.

The control group shows what happens anyway, when nothing is changed.

The fertilizer group grew 20 centimeters, and the control group grew 20 centimeters. So the fertilizer did nothing.

So, when Yusuf said the fertilizer worked and the other pots were just spares, he was not right.

The control group did the test's most important job: it showed that the plants grow just as tall anyway.