Kale has a good calciumhosphorus ratio, which is what you aim for, but unfortunately it is also high in oxalic acid, which binds calcium, so it can't be taken up. Resulting calcium oxalate will cause kidney stones.
Also, like any member of the cabbage family, kale is high in goitrogens, which can lead to the enlargement (and hyperfunction) of the thyroid.
Thus it is better to stick to dandelion, endive and chicory for greens (with dandelion being the most valuable!).

Gaby