I'm new at hydroponics so if my question is stupid sorry. I have a 40 site system and I want to a variety of different vegetables. How do I regulate the nutirants since every plant has different needs.
Good luck with your new system. It would be difficult to make separate meals for each plant in a small system when each share the same punch-bowl, and pH and EC issues also complicate life. You can make small adjustments like plant placement or restricting flows to parts of a system, but these are far from being a convenient solution to what we would like to do. For many hobbiests, having two separate systems, a high EC (stronger nute) and low EC (weaker) system gives some flexibility.
Please look at this thread and in general search the archives. If you have a specific system, and tell us the prioritized list in the order of what your really want to grow, people could give you some pointers.
I have had some success growing different crops in bato buckets using the same nutrient. I always look up the range of nutrient concentration and pH each plant can tolerate and try to keep everything I grow in that range. I have also successfully used foliar feeding to supplement tomatoes growing in a vegetative nutrient mix when they required a fruiting nutrient.
I have also been feeding the plants with beneficial bacteria cultured from worm compost making a compost tea. This seems to have the effect of increasing any plants ability to uptake nutrients and even expand its pH range. I lack the analytical facilities to back this up with scientific proof but have read of the benefits and the claims seem to prove themselves in my system.
You should look into stable plant food. Avoid liquid fertilizer, think granular, and keep NPK real at 1:1:1