One of my first char added soil amendments was to a desk plant that was needing a repot. Every few weeks the pathos could be found collapsed from lack of water. So into the mix went some char and some rotted wood mulch and a bit of potting soil. I am not sure of the percentage of char (no science there!) and no fertilizer added before mixing it up. I did do a "miracle-gro" drench right after planting it. The plant became our posterchild for what NOT to do with biochar. It absolutely went into suspended animation for the next year. Not a single leaf.
But on the other hand it has never shown any stress over water shortage, in that whole year and I don't water it very often.
The same thing occurred with an avocado tree that I transplanted from the compost heap, suspended animation until late spring when I did a bit of creative supplementation with deep holes and worm castings and liquid manure tea. now it is lush and healthy.
All of the char added to my organic garden was either mixed with humanure compost or with a fermented vegetation mix as the fertilizer, which seemed to me the closest I could get to what the early terra preta makers might have had available. The garden is wildly successful. Fully half of the garden is new, soil that was under a tree that we had removed, pure red georgia clay, then rotted leaves and the char/mix was added. About two 5 gallon buckets per 14 ft x 3ft raised planting bed. All the beds are fed with homemade organic fertilizer(Steve Solomon's Formula) I make it up and mix in (4-6 inches deep)at seeding and sprinkled on top every couple weeks as heavy growth dictates. Will post a seed starter formula later.
A Better Way to Fertilize Your Garden - Homemade Organic Fertilizer
