(a)How do plants obtain food?
(b) Why do plants need nitrogen? How do plants obtain nitrogen?

(a) Plants obtain food by the process of photosynthesis. 

Photosynthesis is a process by which green plants prepare their own food by utilizing sunlight, chlorophyll, water, and carbon dioxide gas to produce food in the form of starch.

(b) Plants need nitrogen because it is the major component of amino acid. Nitrogen is also a building block of protein. 

Plants obtain nitrogen from the soil by inorganic salts like nitrites or nitrates in the form of organic compounds produced by bacteria using the atmospheric nitrogen. Plants also obtain nitrogen from the NPK (Nitrogen, phosphorus, and potassium) fertilizers.