Apart from sunlight and water, plants need different chemical elements for their growth. These chemicals are called plant nutrients. They may either be available naturally in the soil or could be supplemented using fertilizers.
Some of them like oxygen, carbon and hydrogen are available in plenty in the atmosphere. The other mineral based nutrients are classified as micronutrients and macronutrients. As the name implies, macro nutrients are needed in large amounts and micro nutrients are needed in smaller amounts by the plants for their growth. Macro nutrients are further classified as primary nutrients and secondary nutrients.
The elements nitrogen, phosphorus and potassium come under primary category. These are very important for manufacture of basic building blocks like the plant cells. Hence the primary nutriets are needed in large amounts by the plants and are generally supplemented using potash fertilizers. Calcium, magnesium and sulfur are referred to as secondary and are normally available in the soil.
The elements iron, zinc, manganese, copper, cobalt, boron and molybdenum are needed by the plants in very small amounts. These elements are collectively referred under micro category.