I have done planted tanks no for about 21 years. I have done low light easy plants, moderate light with a bit more demanding plants and one tanks using pressurized C02. So lets start with talking about supplementing CO2 and why or why not this may be needed.
Almost everything the plants get in a tank is determined by us. Even when it comes to fostering gas exchange between the water and the air, a natural phenomenon, we can influence it. However, everything that the plants need is determined by them. So, what this means is that our plants will do best when we supply what they need and that we remove what they don not so it doesn't accumulate (i,e, water changes).
There are other variables involved, such as what critters we keep what plants we choose and what equipment we have for keeping the tank healthy. But when it comes to CO2, things are pretty simple to understand. There is naturally CO2, as well as other gasses, dissolved in the water in our tanks. When there is not enough of either of these, issues will ensue.
Drilling down into CO2, the issues is how much do the plants require and then, how much is naturally present in the water. As long as the plants do not use the CO2 faster than it can be replaced from surface agitation. There are some simple guidelines one can use based on a couple of consideration re plant load.
The forst is the type of plants we want to have. In this respect we assess how demanding any plant species is. Faster growing plants use more nutrients than ones which grow slowly. Some plants need more light and ferts than others. So we can select our plants by considering this aspect. The more demanding a given plant, the more nutrients and light it needs and that also means the faster it will use these things and this includes CO2.
Next, there is the total plant mass that must be considered. The more plants, of any type, we put into a tank, the more total nutrients. light and CO2 they will need. So, a tank heavily planted with easy care, low light demanding plants can still benefit or even need extra CO2 or a CO2 alternative. The same applies to what sort of ferts and how often they need to be added.
The best part about all of this is planted tanks have been very popular in the hobby and increasingly so in the past 20+ years. So we can research the plants before we buy. We can know what their specific needs are which should help us to select the plants for which we will know how to care before we get them. If we choose to buy the most demanding plants, then they will require the most care. But if we choose the easy to care for species and only plant moderately, we can have great results with little effort. The plants can often get everything they need without much addition input by us.
There is a final component here. When we plan out a planted we make out plant selection. If we do this merely based solely on how a plant looks rather than its need, we will can end up with a mismatch. It is easy to get leggy plants, or plants that shade each other which may mean some plants do not get enough light.
Keeping live plants is not much different than keeping the fish. We need to choses ones which can thrive in our water parameters. We need to feed them the proper nutrition etc. So the trick to this all is do your homework first. Then talk to peole who have actually done it and succeeded. Bear in mind that there is more misinformation on the net than there is good advuice. So you need to go wfor good answers. Wile it is not he most comprehensive source, it is one of the best, imo, is the Tropica site. It is available in a couple of languages, here is the link to the English language version
http://tropica.com/en/
I would remind folks that Tropica sells plants and products for their care. This is true of many sites. My advice is to pay attention to the factual parts and do not assume you must use only Tropica products to succeed. Our juob is to find the products that work well for us and then to use them. To that end I can report the following.
We all love to save money. Most of us in the hobby also know that we can buy a bale of peat from our local plant nursery for a few dollars. Or we can buy a tiny box of it marketed for aquarium use for many times the price. The same applies to a lot of the items we use for out plants. I have been fertilizing the substrate since I began keeping planted tanks using Jobes Spikes for lush ferns and palms. They work great and are so cheap it is almost a sin. They can be cut into any size needed, are easy to push into the sunbstrate and I replace them every 3 months or so. But I normally let the plants show me when they need more. My guess is I have spent about $20 over 21 years on substrate ferts. There is only one caveat if you use them. If you uproot them because you rescape often, then you will very quickly earn your MBA (Mastered By Algae). The spikes need to be left in peace down in the substrate.
When I began doing planted tanks there was much debate about water table v.s. substrate ferts. At some point I recall reading a Tom Barr post on his site about the odea that one should be using both types of ferts in most tanks. As Byron pointed out plants take up nuitrients both via leaves and roots and that different nutrients may be taken up by each route.
One last comment re fetilizing. For over 20 years I have used only one brand of ferts and trace elements- Tropica. While I gave ip my CO2 added tank many years ago. I still keep 8 planted tanks. In all but one of them I use Flurish Excel (for CO2) once a week after water changes. I do not overdose it and I do not consider it an algacide. I undestand what the ingredient is and why some folks will not use it. However, after 20+ years of doing so I have never seen a detrimental effect on anything. But I do not hold it against those who whose not to ever put it into their tanks. Each of us must reach our own decision on this.