What are the pros and cons of eating earth (soil)?
The article (a PDF can be found here) suggests that this happens neither for feeding reasons (to get your belly filled, when anything else is available) nor that this is done for supplemental reasons. 

They suggest that this happens more for protective reasons. In the abstract they write:


  Our results indicate that human geophagy is best explained as
  providing protection from dietary chemicals, parasites, and pathogens,
  whereas animal geophagy may involve both micronutrient acquisition and
  protection.


In the paper they give a few examples for this. First the earth eaten is usually digged up from deeper layers of the ground and sometimes even cooked. Then they bring in the example of people, which eat soil, when they have gastrointestinal problems as a kind of medicine to calm down the problems. 

In my opinion there is not much nutritional value in eating soil. Organic compounds can usually not be metabolized (for example we don't have the possibility to break down cellulose). I am not sure, if we can get minerals from clay into our body, since a lot of them is present in soil as unsoluble minerals.