why would measuring just root mass not be a totally accurate way of measuring the effect of fertilizer on root growth

Fertilizers affect the root growth in several ways such as:

Root mass of root is the measurement of growth in length and weight of the roots.

So, the effect of fertilizer is not limited to the growth in length and weight, there are several nutritional changes in roots which are also important for the measurement of effect of fertlisers on root growth.

The correct answer is - there are several other factors that affect the growth of the roots other than fertilizers.

Explanation:

Measuring the root mass only is not a correct or accurate way of measuring of the effect of the fertilizers on root growth as it is not the only factor that affect growth of the roots.

The factors that have significant effect on growth of roots of the plant are environmental conditions that are present in the root zone such as gas exchange, Water, Space, Carbohydrates, Non-limiting Temperature, Minerals, Microbial Associations and Low Soil Density.

Thus, the correct answer is -  there are several other factors that affect the growth of the roots other than fertilizers.

the fossil record provides evidence of a common ancestor to many species.