Do you have the following information?

1) The actual depth sampled for each layer e.g 15cm

2) The bulk density for each layer sampled

3) Weight of soil sampled in each layer on a dry weight basis

4) Weight of roots recovered from the soil sampled in each layer on a dry weight basis

With the above information you should be able to calculate the weight of soil in a hectare for each depth sampled i.e length (m) x width (m) x depth (m) x Bulk density (kg/m3)

Then relate the amount of roots from your sample to the weight in a hectare for each depth