Hi Achantal,
Calathea, Maranta and Ctenanthe are one of the hardest plants to make happy in an indoor environment. They grow on the forest floor and in the lower understory of the rainforest where there is very little light, and also actually very little moisture that makes it all the way down to them from actual falling rain. A lot of people hear 'rainforest' and think 'wet', which in some plants is totally true, but in others (the denizens of the floor and lower understory) is not true so much. Their natural environment gets only 2-5% of the rain that falls, but the humidity is extremely high and the forest floor is always very damp.
Over time, the Calathea alliance has developed unique ways to live in that environment. The 'praying' habit of closing the leaves is an adaptation to collect and channel any rain that does come directly down the middle of the plant to the rootball.
The often bright and patterned colorations are meant to discourage animal predation (because many animals have learned over eons that color can =poison)
The often purple or red leaf undersides have specialized cells and anthocyanin pigments that allow the plant to collect what is called 'green light' that bounces up from the forest floor.
And the leaves have specialized cells that allow them to absorb available moisture from ambient humidity as a form of self-watering. So humidity is very important to these plants.
You seem to be on the road to collecting tropical plants of several genera, so if I might give you a tip...
Tropical plants, no matter which genera they fall in, like a growing substrate that is light, chunky, and airy, that will easily uptake water but just as easily shed excess water so that the substrate does not stay 'wet' but merely evenly moist.
Many aroid botanists, and older tropical plant collectors of many different generas of plants, have thrown out what we refer to as 'the old wive's tale' of only watering a potted plant when the top inch or so of soil 'feels dry'. I know that this will be absolute BLASPHEMY to some of the advice givers here, but this has been proven over time by many serious growers....its better to alter your mix than try to put a plant on a watering schedule which is so very subjective.
If you have the right mix, its more difficult to over or under water.
The mix advocated now is something like this, and is being used by 1000's of people who grow plants in indoor environments with great success. Of course its more trouble to do this, you can't just go down and buy a bag of readymade stuff and stick your plant in it. But the benefits are great.
I use a mix of equal parts of the following:
Fir bark in 1/2 inch pieces. This is something you may have to order unless there is a place near you that sells a lot of orchids. In a pinch, if you don;t want to order from online, you can use bagged 'BetterGrow' orchid bark that has the coarse perlite and charcoal bits mixed in from a box store...the chips are a little larger and not uniform but better than nothing
Natural wood crushed charcoal chunks (not for the BBQ but made specifically for plants) in 1/2 inch pieces (again, available where orchids are sold or at box stores but you may have to further crush it)
1/2 inch COARSE perlite not the little itty bitty balls of perlite. The maximally expanded stuff. Available from orchid supplies, not generally available in box stores
coconut husk FIBER chopped into 1/2 inch fibers. You can use pre-chopped coconut husk, which is available at Petsmart, but be careful, this form of coconut holds a lot more water for a lot longer so reduce the amount in your mix and ALWAYS premoisten this, it expands exponentially from a compressed brick
You can use an optional addition of compressed expanded clay pellets (Aliflor, LECA) or crushed lava rock for epiphytes, and for terrestrials, a portion of very good quality potting mix preferably without fertilizer added which is getting harder to find nowadays. Some people substitute peat or a coconut product called Eco Earth for this. Petsmart carries Eco Earth in the reptile department.
This mix holds water long enough around the roots for the plant to absorb it, and lets the additional drain very quickly. Its chunky airy texture allows air pockets, cracks and crevices that allow for more extensive healthy root development because the roots can breathe.
And secondly, specifically to the Calathea alliance, boosting humidity is a necessity. Heavy misting with a spray bottle a few times a day and keeping your plant as far away from an air conditioning/heating vent outflow as possible...or using a mist chamber once or twice a day (enclosing the plant in a confined space and running a humidifier or hand spraying extremely heavy until dripping wet and allowing a slow evaporation of the water over a period of hours. Also grouping plants closely together helps them hold more humidity. But be careful to watch for pests in this scenario.
