"what they are used for and signs of the plant is in need of that nutrient" - in faq under disease section. seminis and plantphys link
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different nutrients and additives we can put into the soil" - there are thousands of products that claim to do lots of things and many different soilmixes / soil types throughout the world, it would be a large task to compile them all and the information wouldn't be true for everyone. like adding crab meal to clay may not do anything for you, but adding them to a potting mix does(etc).
"I been doing a !ot of reading and see a lot of different nutrients and not sure when they are needed" - yeah there is a lot of information / disinformation propagated... good luck. i would advise you just pick one way then go with that..
First decide if you want to be organic or not, this decides which products you will use.
Next decide your method Soil building / amending, or providing nutrients on a schedule(usually with water).
Then read people who have experience with these methods like the sticky on aact or the sticky on soil building or my thread on hydro nutrients explained.
Finally browse glogs from 2015 and before(so that they are a complete season) of people using similar setups to what you want to have and see what they did / what lessons they learned.
here's a glog everyone should look at atleast once:
pic1 pepper graphic