What substrate should I give harvester ants to dig in?
It depends on the species! "Harvester Ants" encompass a wide range of habitats so It's hard to recommend a single substrate.
The Florida Harvester Ant (Pogonomyrmex badius) for example is well adapted to Florida's sandy soils, and do well with pure sand as a substrate. Many of the Western Harvester Ants, P. occidentalis for example are typically adapted to more sandy loam/clay soils.
So what substrate should you provide your harvester ants? It can depend on the species, but you can pretty much never go wrong with a mix of clay, and sand.