rhizo, that's true. I've grown a couple of things with chemical fertilizers and supplements. You don't need, the microorganisms, but why kill them with chemicals when all they're doing is helping? IMO a more balanced diet is provided to the plants when microbial life is encouraged to flourish, however I do think that advancements in aeroponics and such are making chemical fertilizer use way more sustainable, so it is definitely a viable option. I'm experimenting with organic aeroponics and hydroponics in coco coir this summer. Here is a link that might be useful: The Anticlimactic

If you're growing indoors, your zone isn't nearly as relevant as when you're growing outside. The cat thing is tricky. I've had some luck putting things in a place that the cat won't view as stable. It also helps if the diameter of the plant isn't very different than the diameter of the pot, so there isn't a big expanse of tempting dirt, beckoning to be used as a litter box. You could also consider some sort of less-tempting soil covering, like decorative stones or moss, which might also help you with moisture retention in the desert.

Fungus gnats!! They infect every single plant indoors. They fly to each plant, lay eggs, eggs hatch and turn into maggots that chew through the roots. Weaking plants and then they get sick and die. You need a few things to get rid of them. First, trap as many as you can with yellow sticky traps layed horizontally on the edges of ALL pots. Then, get some Bt insecticide, the type that will kill the fungus maggots in the soil. Once maggots are mature, you get more tiny flies. The two procedures are the ONLY way to effectivly kill both cycles of these tiny bugs. They also love outdoor gardens and can spell doom for anyone who has a big greenhouse. I use a benefical nematodes as well, and they are watered into my garden soil in summer, to help control the fungus gnats. A few years back, my brussles sprouts were all loaded with these tiny flies. Nothing worse than eating some of the sprouts and seeing tiny dead black flies in the cooking liquid YUK! The ONLY way to prevent these nasty bugs from getting into indoor plants is to always use STERILE SOIL or sterile seed starting mixes. These mixes contain no bugs or pathogens, and if the gnats are in the plants indoors (even in ONLY one pot) they spread quite fast and you will have ALL indoor plants infected within a few weeks. Wish I could help further, but I guess no one seems to read my warnings about the kinds of potting soil to use indoors.

Seed starting soil is needed. Its also sterile and has no insects or diseases that can kill young seedlings. Suspect that they are being over watered, and lack sufficient light. If you do see yellowing leaves thats a sure sign of too wet soil. Potting soil might be fine for well established plants for outside, but you may also have very tiny fungus gnats in it that could infest every single plant indoors or out. The use of yellow sticky traps placed horizontally near the pots, as well as some careful observation is necessary. Do not use fertilizers for these young seedlings. Seed starting medium is made of things like shreaded coconut husks (coir) and a little perlite. I start seeds for many herbs and flowers indoors. Right now, I have over 150 plants growing from seeds, and have neary no losses once they are ready to be translanted outside. The fastest germination I had was with Brussels Sprout seeds. They took only two days to sprout and were over an inch tall in 3 days! Right now, all my petunias, oregano, impatiens, peppers, tomatoes, watermelon, and cantaloupe seeds are doing great under plant lights. I have the lights on a timer for about 18-20 hours per day. Plants do need a small amout of darkness time, but because plant lights are only low power substitutes for sun, they do need to be very close to the seedlings, like about 2-3 inches. My plant shelves are metal with 2'x4' shelves and use two 4 foot shop light fixtures with two lamps each fixture per shelf. They are attached with chains and are raised when the plant growth starts to touch them. In a tray of 32 pots of 2"x2" each will have single plant once they are at 2 inches tall. Natural Beginnings is a seed starting medium from Gardens Alive. Even though their prices are a bit high, I always prefer to use some of that medium with added Jiffy mix, a similar product, and any other sterile seed starting medium. These are all actually 'soilless'. Dill can reach a few feet tall and doesn't do as well indoors. The pots need drainage holes in the bottoms. All my seed staring has been very successful for well over 40 years now, and I will be picking peppers and tomatoes in early July, while everyone else still has only a few blossoms showing. I also use heat mats under my trays, and ove each with clear plastic dome lids. The lids get removed once the leaves touch them, then I also unplug the heat mats. If the mats are left on, they tend to dry the soil out very fast, so I shut them off and just grow by the room temperature after that. Before being planted outside, each tray is moved to my small glass greenhouse outside for about a week to 10 days, where the direct sun will 'harden them off'. This reduces translant stresses and gives the plants a lot of strength beforehand.