Sand offers very little in the way of nutrients to plants... it will hold the roots & any solids like fish poop, that may provide some nutrients... in the case of aquatic plants in aquariums... most nutrients need to be either dissolved or suspended in the water
When there is sand - or gravel - on the bottom of the tank, you need to add fertiliser.
For plants which feed through the roots, fertiliser must be added to the substrate using fertiliser tablets. This is for plants like amazon sword plants and cryptocorynes.
For plants which feed through the leaves, liquid fertiliser should be added to the water. This is for plants which grow as stems.
you mean will nutrients from sand dissolve into the water???
if so, most sands are made of pretty hard rocks & don't dissolve much into the water... crushed coral or limestone can leach some into the water but most actual sands are are made of materials too hard to dissolve anything into the water
This is added to the aquarium once or twice a week. The bottle has instructions for how much to use. If there are many plants, use the amount in the instructions. If there are few plants, use less than instructions.
Seachem make many products called Flourish, but buy only the product in the link.