Iron (ii) -, iron (iii) -, copper (ii) -, zinc -, manganese (ii) -, cobalt (ii) - and nickel (ii) - chelates of n, n '- to - (2 - hydroxybenzyl) - urea derivatives, those containing pflanzenkunstduenger and n, n' - to - (2 - hydroxybenzyl) - urea derivatives as well as a method for the preparation of the latter and of the former