Soils having low soluble salts coupled with lower soil pH and biologically more active are preferred by livestock besides diets of livestock deficient in certain nutrients..
There are certain active organisms in the soil and also the chemical properties like low ph and low soluble chemicals and compounds(as rightly suggested by ANOOP sir) helps the livestock in the completion of their nutrient diet. In some cases it is also seen as a remedy to certain problems like indigestion, constipation etc...
I think it is not only for Livestock but some pregnant women in a certain country prefer to select particular type of soil when they are deficient in Nutritional requirement especially in Iron. Thus, I suggest you to check the feeding type of the livestock ……… may be this will give you an indication if unbalanced nutritional amount of the livestock have to choice for the type of soil.
A wide range of animals in the Kalahari (e.g. Kudu, Hartebeast, Wildebeest, Ostrich) congregate at natural salt licks occurring in depressions (pans) were clay and salts have accumulated. Salt (Na), Mg and P are usual suspects.
There is also incidental soil intake in addition to preferential ingestion. This can be significant but often depends on condition of the land grazed. Attached is an older report discussing this.
In Ethiopia, soil ingestion and soils ingested by animals is unique. We have got extremely black soil that is traditionally under use to heal some livestock diseases apart from its importance for fattening. That soil is completely different from clay eating we knew. I think there is some chemical constituents that makes the soil especial. Any idea please? To say its salt content, the area is where the soil found is humid. One amazing point is that there exists a blacky water resting over the soil. The soil was dug below water and given to animals.