Hi Andie and welcome.
What OM means by a backfit kit is the parts to convert your existing light to a higher efficiency light. At the bottom of this thread
[URL="http
/www.fishforums.net/index.php?showtopic=96754"]http/www.fishforums.net/index.php?showtopic=96754[/URL] is a link to AH Supply. They have multiple kits depending on what you want. I didn't notice if you are in the US. AH Supply is. I'll come back to lighting after talking about nutrients.
You definitely should follow OM's advice about water test kits and cycling.
Since you are a master gardener, forgive me if you know any of the following. I'm trying to do a planted aquarium myself and have been studying as I made a problem because I started before I knew what I was doing. (I'm about to find out how well I have learned as I am ordering bits right now.)
My observation is that the thriving planted aquarium is a combination of lighting, macronutrients and micronutrients. The lighting is important because it limits what types of plants you can have, but it is not the most important thing. There are several examples in the Plants forum of low light aquariums that are absolutely stunning. (Check out the links in SuperColey1's signature. He usually posts in the Plants forum.)
My impression is that the macro and micronutrients are more important. There are four macro nutrients, CO2, nitrate, potassium and phosphate. CO2 typically gets more discussion than the others. I'm guessing this is because it is more difficult to add properly. CO2 is typically injected and held between 20 and 35 ppm. Going over 35 ppm is dangerous to the fish. It seems 30 ppm is a common target. The closer wattage gets to 2 wpg the more crucial CO2 becomes as algae becomes a major problem without it. CO2 is typically injected through either pressurized or DIY (soda bottle construction) methods. At 40 gallons, you probably have to go pressurized as the DIY is labor intensive and will take multiple bottles which need weekly service. It is very important to keep the CO2 level constant as algae takes advantage of the plants adjusting to varying CO2 levels.
To keep the cost of a pressurized system reasonable many seem to gather parts independently and assemble the whole unit. I am taking that approach and expect to spend $70 for the regulator (http
/cgi.ebay.com/NEW-CO2-Regulator-Solenoid-and-FREE-Bubble-Counter_W0QQitemZ230315642187QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item230315642187), $60 for the CO2 bottle (http
/cgi.ebay.com/NEW-5-lb-CO2-Aluminum-Cylinder-W-Valve-co-2-tank_W0QQitemZ140290586876QQcmdZViewItemQQptZBI_Fountains_Dispensers?hash=item140290586876), $15 for a homemade reactor (I probably could have built it cheaper but someone else was selling one so I bought it) and up to $120 for a CO2/PH monitor. The last part isn't necessary but I want low maintenance and it will automatically turn the CO2 on and off to keep the level constant. (I hope the links work. Sorry, I'm not very good at links yet.) This may sound expensive but the one complete system I found was $700+.
I'll also get a dropchecker so I can manually check the CO2 levels but I probably won't use it much once I get the monitor.
There are threads in the Plants forum about non-pressurized DIY CO2 (IMO, forget most of the CO2 systems in LFS as they are expensive versions of the DIY system.). As I said, it is a labor intensive approach that I want to avoid.
I found this thread
[URL="http/www.fishforums.net/index.php?showtopic=265886"]http/www.fishforums.net/index.php?showtopic=265886[/URL] did a good job quick job of explaining the other nutrients and how to add them, so I won't repeat what it says here. This pinned article
[URL="http/www.fishforums.net/index.php?showtopic=104737"]http/www.fishforums.net/index.php?showtopic=104737[/URL] does a good job of explaining the weekly routine for these other nutrients.
In addition to all of this is a substate, basically a layer of dirt/clay under the gravel or sand. It provides a base for the plants to root into as well as being a source of the nutrients. There is a lot of discussion about which substrate is best. Eco-Complete seems to be a popular choice but other substrates have advantages. In the interest of time and brevity I'll move on.
Okay back to lighting. The 2wpg rule is a rule of thumb which I made a big mistake with. It is based on the wattage of T8 or T12 bulbs and not on how much light gets into the aquarium. Different bulbs and reflectors put more (or less) light into the aquarium. In other words, smaller bulbs (T5, T6 and compact flourescents) put more light in per watt of output than T8 or T12. A better reflector (than no reflector or the cheap reflector in most strip lights) also directs more light into the aquarium. In addition, where the bulb is in relation to the reflector affects how much light is directed into aquarium. Thus the smaller bulbs are more efficient than standard T8 or T12 bulbs.
In my case, I used a 36 watt retrofit kit. Nominally I am under the 2 wpg rule as I have it on a 22 gal tank. But the new bulb and reflector are more efficient/direct more light into the tank than the old T8. For purposes of the rule, I have been told to add 50% to the nominal output before computing wpg. Doing that I am past 2 wpg and the algae problems (BGA, GSA, hair algae and probably a couple of other types) that arose confirm it. (This morning I am completing a three day blackout in an attempt to temporarily beat the BGA (nasty stuff). T5, especially HOT5, is even more efficient than the cfl I am told. Basically double the nominal output, so a 20 watt T5 with a good reflector over your 40 gallon tank and be ready for the algae wars.
This gets us back to the plants and nutrients. As I understand it, the plants can override the algae, if they can grow fast enough. The growth rate is highly dependent upon nutrients and to a lesser extent light. Algae is highly dependent on light. My experience bears this out. My son's 20 gal with a standard 15 watt T8 is now algae free with the addition of low light tolerant plants. I suspect I don't have enough nutrients in that tank as the hornwort is losing some leaves but algae isn't a problem except on the pennywort directly under the light. My 22 gal is a problem. Because I've blown past the 2 wpg rule and don't want to redo the lighting again, I am going to remove everything from the tank, add a substrate, then gravel, add a pressurized CO2 system and dose with the other nutrients.
I'm sorry this has gotten so long. You asked a simple question that doesn't have a simple answer, IMO. Hopefully this helps you. It took me more than a month of reading and re-reading threads and articles to get what I consider a basic understanding of a planted aquarium. I'm no expert in a planted aquarium, just some guy trying to have a nice planted aquarium for my wife (yeah I'll enjoy it also). If I have messed up, hopefully somebody from the plants forum (hey Aaron, Andy, are you out there?) will correct me.
buy another aquarium to house the fish in while the change was being done, then I was going to have to set it up because I can't have a nice unused aquarium sitting empty after the change. Seriously, thank you. Less work and more enjoyment of the aquarium is better right now. 
