(i) Addition of chemical fertilizers in plot B will result in sudden increase in yield due to more availability of N, P, K nutrients. The gradual decline may be due to killing of useful microbes in soil as a result of persistence of chemicals in the soil. This decreases soil fertility.
(ii) In plot A, manures supply nutrients slowly to the soil manures contain large quantity of organic matter which is slowly decomposed by microbes to release nutrients in small quantities. Thus, manures enrich soil fertility gradually.
(iii) Different patterns in two graphs are due to the reason that use of manures is beneficial for long duration in crop fields to keep soil fertility high whereas chemical fertilizers lead to problems when used continuously for long time. Loss of microbes lead to decrease in decomposition of organic matter and as a result soil fertility is reduced.
