why iron in soil is not decompose and dead plants and animales decompose?

Answer 1

Decomposition is the disintegration of something into its constituent parts. Iron (Fe) is an element so it has no parts to break into. Rusting is not decomposition since the iron goes from the elemental form to the compound oxide of iron - iron oxide, Fe2O3.
Aristotle said the definition of an element – was one of those bodies into which other bodies can be decomposed and which itself is not capable of being divided into other.
Now we say an element is one of a class of substances that cannot be separated into simpler substances by chemical means. The periodic table is a chart showing all the known elements of matter.
http://www.chemicalelements.com/

Answer 2

Iron decomposes - it forms rust (iron oxides) when exposed to oxygen and moisture.