Why are manure and fertilizers used in fields?

State one importance of photoperiod in agriculture.

Sidak,when visiting a supermarket,was surprised to see yellow,green,and red-coloured capsicum available in the stalls. It was a drastic change in the quality and quantity of capsicum.
$(a)$ What is this change due to?
$(b)$ How are such crops produced?

What do we get from cereals,pulses,fruits and vegetables?

Which one is an oil-yielding plant among the following?

To get a better yield,Jaikishan increased the use of pesticides and fertilizers. Though in the initial years he got higher yield,after that,it declined. He was much worried. His friend Mahesh persuaded him to abandon this practice and start using organic manure.
$(i)$ Why are fertilizers used in a field?
$(ii)$ State the possible reason for the decline in yield.
$(iii)$ List two values shown by Mahesh.