Why do we need fertilizers?

Fertilizers increase crop yields
Plants need sun, water and nutrients to grow. The nutrients can be taken from air or soil. If there is an ample supply of nutrients in the soil, crops are more likely to grow well and produce high yields. If even one of the nutrients needed is in short supply, plant growth is limited and crop yields are reduced.
Fertilizers are needed to obtain high yields because they supply crops with the nutrients the soil lacks. By adding fertilizers, crop yields can often be doubled or even tripled. The UN Food and Agriculture Organization (FAO) Fertilizer Programme undertook extensive demonstrations and trials in 40 countries over a period of 25 years. The weighted average increase resulting from the best fertilizer treatment for wheat was about 60%.
(source IFA)
Fertilizers contribute to efficient use of land and water
Fertilizers ensure the most effective use of both land and water. Where rainfall is low or crops are irrigated, the yield per unit of water used may be more than doubled and the rooting depth of the crop increased through fertilizer application.