potting soils tend to contain a good deal of peat and other forest products, meaning tree bark, etc. when I say 900+ is too high, it's because the peat or whatever is used, will trap fert in itself, so it can build up over time. 900 is more of a hydroponic feeding level because it just water, not potting or peat, so there a really high dilution. even if you use coco the fert can get trapped in the coco. you wouldn't expect coco to break down and feed the plants, but fertilizers can coat the fibers.