It will be seaweed/kelp like all root boosters but Iv not used the pro one.I stopped using all GH now TA. as they're now owned by Monsanto.A lot of shops took them off the shelf.They've been about years as General Hydroponics.Iv just checked.The 2 drops is for foliar feeding,spraying and the 0.2ml per litre is every watering.Its not a nutriant its whats called a bio stimulant.Kelp has a number of growth hormones in it which help keepthe seedling strong.I'm suprised they say to only use it for 2wks though.I guess they have another product with it in to take over after 2wks.Seaweed/kelp is in so many things and so helpful.All root booisters and all veg boosters and even some flower boosters have it in. House and Garden roots excellarator isnThe Best on the market and can be used for 5wks.So its 0.2ml per litre,1ml-5litres every watering.It doesnt mention about there being any different feeding for different substrate,soil/cococ/rockwool so every watering.Its also organic which makes its harder to harm your plants if you do make a mistake but kelp is one of the organics that will burn your plant if you give it too much.