Soil formed after leaching and rich in $Al$ and $Fe$ is

What is the optimal soil $pH$ for plant growth?

What is the optimal $pH$ range for the growth of most plants?

If a dried leaf is taken in a crucible and heated to $600^{\circ}C$,a gray-coloured powder is left behind. It is referred to as:

Addition of a chelating agent to hydroponics is necessary for:

Which soil type has the highest water-holding capacity?