As you already know, green plants produce food, animals feed on them. On a more ecological level, we say producer is fed on by primary consumer, the primary consumer is also fed on by secondary consumers after which they are also fed on by tertiary consumers when the tertiary consumer dies, the decomposers, bacteria and fungi, will decompose them so that the accumulated nutrients are returned to the soil. When this happens, plants use the nutrient to survive and the process starts all over again and again. This process leads to the adding of nutrients.