Using the ruler to measure the height of the plant is a quantitative measurement. Quantitative measurement is a from of measurement that can be measured and numbers affixed.
The plant growth is the dependent variable in this experimental setup since it is the effect produced by applying fertilizers to the plant.
Using the ruler the botanist can tell about the growth of the plant in numbers and not by mere qualitative terms.
Other examples of quantitative measurements are: a mass of broth, height of an animal e.t.c.
