Hi Yahtzdog and welcome to The Tortoise Table! And congratulations on getting your little hatchling Hermanns - you will love them.
Regarding your questions: No, you don't have to sow seeds into organic potting soil, but what you need to do is to avoid fertilizers if possible. Even organic potting soil might have some extra (organic) fertilizers added to it. These won't kill your tortoise, and as the plants grow they will use up the fertilizers so there will be less in the soil. It's mainly nitrogen, phosphorous and potassium that are added, and eating too much of these (and the plants that grow in that soil will take up those elements into their structure) is not great for them. And it's important to have the right phosphorous/calcium balance. Tortoises need a lot of calcium (to support all that bone and shell), but calcum is poorly absorbed by the body whereas phosphorus is readily absorbed - so if a diet is heavy in phosphorus in relation to calcium, the excess phosphorus will prevent the uptake of calcium. This is why we recommend feeding weeds to tortoises -- they tend to thrive in 'poor' soil and so don't have an excess of nitrogen or phosphorous in their structure.
I wouldn't worry too much about what you have already planted, but I would avoid anything that is called 'potting compost'in future because that does usually have fertilizers added. In the UK, where we are based, you can get bags of ordinary topsoil to use (you just buy it from garden centers), and that has very little added fertilizer. Can you get that in the USA? You could probably also just use soil from your yard, if you have one -- that should be fine.
Hibiscus and Mulberry will be lovely and they should be fine to feed to your tortoise. However, the real danger with plants that you buy is that they might well have been sprayed with insecticides, to keep pests off them. Tortoises are very sensitive to insecticides, so unless you know for sure that they have not been sprayed, then you need to plant them and let them grow a bit and only feed the new leaves and flowers (which shouldn't take too long). The soil they have been grown in shouldn't do any harm.
Re the RepCal -- that should be fine, although I would say that it should be given at least every other day (which is what we do wth the equivalent here (Nutrobal). However, i'm not familiar with RepCal so I would just check the instructions. It usually calls for a small pinch (and if you wet the leaves first it sticks better to them), The instructions will give you the dosage and frequency, depending on the size and age of the tortoise.
Mazuri is a bit of a controversial subject. In the UK we don't like it much (a little high in protein and contains some ingredients that tortoises wouldn't naturally, like soybean meal and molasses), but it is very popular in the USA and there are some respected tortoise keepers who do feed it.
Regarding lists of safe plants, you can produce these using the filter on our website. I know this email has become very long, but I hope you won't mind if it gets a bit longer so that I can give you easy instructions on how to produce a list of safe plants. Here's what you do:
Our Plant Database on the website contains nearly 1,000 plants, only some of which are edible. If you would like to make a list of the ones that are safe for tortoises, or plants that should not be fed, you can do so by following these steps.
1. Go into the Website
https://www.thetortoisetable.org.uk/ where you will see 7 plant categories (Wildflowers, Garden & House, etc.) 2. Go into any of the Categories (for example, Wild Flowers).
3. Towards the top of the page you will see a dark green bar that says ‘Filter Results by Safety’, followed by a row of coloured boxes with ticks in them (Green, Amber-Green, Amber-Red, and Red). These are our ‘traffic light’ codings: Green for safe to feed; Amber-green for feed in moderation; Amber-red for feed sparingly; and Red for do not feed.
4. Remove the ticks from the Amber-Red and Red boxes (just click on a tick remove it), leaving only the Green and Amber-Green boxes ticked (or for the safest plants, leave only the Green box ticked).
5. Thereafter, as long as you are in the current session on our website, all of the Categories will only display plants that are safe for your tortoise to eat. If you exit the website and then return, you will have to do this again. And of course you can also filter on the Red coding to produce a list of plants that you should not feed, etc.
6. If you want to print off the new list that you have made, there is a print icon underneath the full menu that will enable you to do this. But please note that the list you are printing off is the shortened version of the plant entries, and you should always consult the full version of an entry to see all the details, more photos, and the icons that tell you which parts of the plant are edible.
I'm so sorry that this has been such a long email (you are probably half asleep by now), but i hope it helps and please don't hesitate to contact us again if you have any problems or questions.
Nina