Plantex CSM+B is a well known and highly used dry fertilizer for planted aquariums, it's basically all of the essential micro nutrients that your plants require, plus added boron. Here's a list of the nutrients it contains...
Plantex CSM + B analysis
Total magnesium - water soluble chelated magnesium 1.5%
Copper - chelated 0.1%
Iron - chelated 7.0%
Manganese - chelated 2.0%
Molybdenum 0.06%
Zinc - chelated 0.40%
Boron 0.04%
EDTA - minimum content 55%
These are nutrients that plants need to grow properly, and healthy.
So I fill empty capsules with dry CSM+B powder and mainly use them when setting up a new tank... The reason is because I've found that using a high CEC (cation exchange capacity - I'll explain) ratio substrate like Flourite (I prefer flourite), and flooding the tank with nutrients that I get better plant growth to begin with.
Cation Exchange Capacity or CEC; Is when a substrate is able to soak up nutrients in the water and release them back into the water slowly... So what are high CEC substrates... Examples would be Flourite, Eco-Complete, Special Kitty All Natural Kitty Litter... All are able to soak up nutrients and then release them slowly back into the water column.
When dosing ferts most people follow what's called a dosing regime, I follow the classic EI (estimative index) regime, other people may follow the PPS-PRO; The difference between them is with EI you're actually sort of overdosing the tank with nutrients and the plants feed off what they need leaving excess nutrients in the water, PPS-PRO you're only dosing a small amount of nutrients every day for the plants to feed on.
Example of my EI dosing schedule
Mon-Weds-Fri - Macro nutrients
Tues-Thurs-Sat - Micro nutrients (csm+b)
Sun - 50% water change every week
I use a fert calculator -
http://yanc.rotalabutterfly.com/
This calculator tells me the amount of each fertilizers I should be dosing the tank with, and how often I need to dose these amounts. I use a digital gram/ounce calculator I bought off ebay, to measure out the exact amount of each nutrient I need to be dosing, and I put the nutrients into this...
http://www.amazon.com/gp/product/B0036C0RT8/ref=oh_details_o00_s00_i00?ie=UTF8&psc=1
I put the Macro nutrients together in ever other little container, and then the Micro nutrients (csm+b) into the every other empty container. There's 30 little containers so I normally use a sharpie marker to draw lines between the containers and then mark the days of the week with 1 or 2 letters, this way I know which one is which, and what day of the week I need to dose it.
Never mix macro & micro nutrients together because if you mix phosphate with iron one of them will precipitate the other making it useless, and with EI you're dosing on alternate days between macro and micro nutrients.