Out of the $105$ elements discovered so far,how many have been found in plants?

Factors which determine to a large extent the vegetation of any area are:
$I.$ $pH$ of soil
$II.$ Mineral composition of the soil
$III.$ Water holding capacity of soil
$IV.$ Weather condition
Choose the correct option.

Plant ash is an indication of:

Magnesium is mainly present in the form of

Humus is important for plant growth because

Woodward $(1669)$ observed that plants grew better in muddy water than in rain water because: