The 3 main ingredients for healthy plants are; sunlight, water and nutrition (fertiliser). Fertiliser needs to be applied differently according to the soil type and the plant species as every plant and every soil is different.
Fertiliser contains elements. Those same elements that we learned about at school in the periodic table. Each element has a different effect on the plant as it is metabolised by the plant. Keeping those elements in balance is difficult if you grow a multitude of crops. They each have different needs and some are greedy for elements that others need too.
Usually organic fertilisers are applied to the soil and washed into the root zone by either rain or irrigation. However some nutrients are more readily absorbed through the foliage. Liquid foliar fertilisers such as humate and worm wee are quickly metabolised through the leaves.