Soil Rejuvenation

I have a large planter box that has no soil, but only bags of mix/mulch. What specific nutrients do I need to add to the mix/mulch to keep it at its best? My vegetables didn't have a good crop this summer. 

Because we rent, I am gardening in containers such as tubs, buckets and even a bath! This is my second year, and the vegetables (beans, zucchinis, lettuce, tomatoes, beetroot etc) are as good as last year’s crop. How do I keep the soil viable for next year’s crops?

I pulled some flax out of my garden. What sort of nutrients can I put into the soil to get decent growth as nothing there is taking off?

Inspired by the Netflix documentary Kiss the Ground, we would like to regenerate our soil. What mulch did you recommend for our flower and rose beds? We have a clay soil that has been treated with gypsum to improve aeration, but the soil is lacking in rich nutrients. Also, I recently read that bark depletes the soil of nutrients as it breaks down, is this true?

My question is how the different mixes (e.g. the tomato mix) help the plant grow? Also, would they work for other plants and do they need to be changed after some time?