You randomly select 3 plots, dig out all the roots (and tubers) in a set volume of soil, then calculate the volume of the fresh biomass of roots. But it is more usual to measure dry weight of all roots of plants, which is easier :)

Thanks Dr. Andrew for this nice suggestions.

I understood in the following way:

Let the set volume of soil, roots and tubers be e.g. 0.3 m x 0.3 m x 0.3 m = 0.027 m3. After taking out of this volume, the roots and tubers are taken out and the remaining volume is measured, say, x m3. Then the volume of roots and tubers will be (0.027 - x) m3. The volume of tubers can be measured easily by putting in a graduated cylinder with known initial volume of water; the increased volume will give the volume of tubers, say y m3.

Then the volume of potato roots will be (0.027 -x-y) m3.

From this samples dry weight of roots will be taken.

The volume of tubers can be measured easily by putting in a graduated cylinder with known initial volume of water; the increased volume will give the volume of tubers, say y m3. Then the volume of potato roots will be (0.027 -x-y) m3. From this samples dry weight of roots will be taken