Right. Which is why we need a cross reference calculation chart. Type of shovel, cross referenced with type of soil, add in current ground moisture levels, potential obstacles like roots and stones! Remember to factor in depth as well since the soil composition may well change. Double check the weight of the character if it's dense soil and the character can stand on the shovel. Don't forget to include their strength modifier.
Phew. Okay, only a page or two of complex calculations, just be sure to cross reference with the soil type and density charts and we should be good to go! After all, don't want the DM just making up a number and going with it since most of the factors are arbitrary anyway.