In my RPG campaign setting the orcs are tougher than most other races. In hard times they can eat wood (small bushes and fresh pieces of trees). In really hard times they can eat healthy soil and survive enough to continue their march/siege, but they don't like it.
How do I begin to explain this process?
While I could hand-wave it away with the word "magic," I'd like to have at least a believable starting point
Starting from the ground up: how much caloric energy even exists in the things orcs eat? Energy metabolism for aerobes involves combining oxidizable with oxygen - in essence burning them. One can burn things in a bomb calorimeter to see what the caloric value is. I looked up caloric values for fats, ethanol, protein and conventional carbohydrates (e.g. wheat flour); then wood and leaves, then various soils.
from https://en.wikipedia.org/wiki/Food_energy
Fats and ethanol have the greatest amount of food energy per gram, 37 and 29 kJ/g (8.8 and 6.9 kcal/g), respectively. Proteins and most carbohydrates have about 17 kJ/g (4 kcal/g).
from source
from Caloric values of organic matter in woodland, swamp, and lake soils
Caloric values of organic matter in woodland, swamp, and aquatic soils (± standard deviation of the mean) $$ \begin{array}{c|cc} \text{Soil Type} & \text{Kcal/g ash-free dry wt} & \text{Ignition loss% dry wt} \\ \hline \text{11 woodland humus layers} & 5.04±0.04 & 57 .5±5.7\\ \text{20 swamp soils} & 4.87±0.04 & 77 .3±1.5\\ \text{9 lake and pond muds} & 5.24±0.05 & 59.3 ±3.6 \\ \end{array} $$
So: for a fire, carbs, proteins, wood, leaves and soils all have comparable energy value - 4 to 5 kcal/g. I was surprised to read it for soil. Why can't you make a fire from dirt, then? Maybe my efforts have used dirt with too much sand or clay; the authors of the soil article specify that this is topsoil rich in organic matter, not deeper mineral soils.
Then why can humans eat corn but not wood? The calories in wood, tree leaves and soil are mostly as cellulose: the evolutionary masterwork of the plants. It is made of sugar and very much a carbohydrate, as energy dense as starch, but very difficult to digest.
To digest it one must have commensal bacterial to do the job. These live in the specialized gut organ called the rumen in ruminants (like cows), also in the less specialized digestive tracts of pandas, and in the cecum (part of the colon) of rabbits.
The problem rabbits have is that the cecum comes after the small intestine, and the small intestine is where nutrients are absorbed from broken down food. What good is it for your bacteria to break down cellulose into sugar if you can't soak up some of the sugar because you are about to poop it out?
The rabbit solution: refection. Eat the poop. That poop is full of microbes breaking down the cellulose. Give them a while and that poop is full of nutritious sugars. Back in it goes to absorbed the second time around.
from http://www.columbia.edu/itc/cerc/danoff-burg/invasion_bio/inv_spp_summ/Oryctolagus_cuniculus.htm
European rabbits are ravenous eaters and indulge in a diverse diet of grasses, roots, tree bark, leaves, grains, fruit, seeds, and buds. Since this diet is low in nutritional value and high in difficult-to-digest materials, they are known to reingest their feces to obtain extra nutritional value from the food the second time around.
Your orcs are set up the same way. They can extract energy value from twigs, leaves and soils by letting their colonic commensal bacteria break it down, then re-eating the product.
This also explains why they don't like it.
It is possible, because it is real.
See documentary video "Haitians eat dirt cookies to survive" https://youtu.be/s3337cj4sJQ
Also, in Vietnam (use google translate to read): http://news.zing.vn/nuong-dat-de-an-o-vinh-phuc-post396355.html
Loam (consists of sand clay, silt and any organic and water mixed in) contains vegetation that is breaking down. Often forest floors have layers of leaves lost each year. The carbohydrates that make a leave structure will give on energy when oxidized. Same as other food. They can also eat live roots, shoots, grubs, worms, bacteria, slugs. Fungi are usually present as single cells even if they do not show a cap above ground.
It is likely that sand and clay cause constipation.
Compost piles can have many of the possible nutrients that are in loam. Smashing everything and manufacturing a large pile of rot seams appropriate behavior for orcs.
You could have a preferred fungus that the orc bands carry with them. The fungus will have the enzymes that convert chewed beta cellulose (wood) into simple sugars.
You could take a slightly different approach, if your Orcs are green you could make the argument that they are able to perform limited photosynthesis in their skin, eating the dirt gives them the nutrients they need to perform photosynthesis which they don't usually get from their usual diet of meat and more meat.
