I use an Eco friendly soil, that doesn't contain fertilizer or any other chemicals.. The reason I like it is because I can get the base humid but the top bit stays relatively dry. Then I turn the soil in certain places where I want the humid part.
Keep the housing relatively warm about 30*c with a dish with fresh water just enough to cover the torts bottom.
make sure you lighting is adequate, most of us use MVB's
Calcium should be given daily in moderation with the correct diet
I found cypress mulch to work well in terms of holding humidity and keeping those pesky little black flies at bay. Babies should have higher humidity. In my dry Northeast apartment, that can be a struggle, so I made made up for the dry air with daily soaks (or every other day sometimes....)
I have had good sucess raising Eastern, Western & Dalmatian Hermann's as well as other Testudo species on a substrate consisting of about 60-70% organic top or potting soil, 20-25% coconut coir and the remainer ground sphagnum moss and leaves. Thoroughly mixed and about 2-2.5" deep it retains moisture well and allows the tortoises to dig themselves into it while still allowing for solid footing on top. In addition there are several flat stones strategically placed as well as a few hides within the enclosures. I spray them twice daily and once a week thoroughly saturate and churn the substrate to keep it moist, but not soaking wet. Generally speaking, most Northern Mediterranean Testudo species, of which Hermann's are, do best in an ambient air humidity of 50-75%.
Yes, correct humidity level is a big factor is the health, growth and development of a 4 month old Hermann's as well as younger and older Hermann's tortoises. The three key elements to good hydration are: Amibient air humidity, substrate moisture and actual body hydration, i.e.-drinking water.