Answer:
Nitrogen
Explanation:
Nitrogen is one of the elements required by living beings. It makes 78.09% of the atmosphere. However, molecular nitrogen cannot be used by living beings. Plants cannot absorb and utilize molecular nitrogen (N2). Plants can use ammonia which is a biologically usable form of nitrogen. The proportion of ammonia in many soils is less. The symbiotic nitrogen-fixing bacteria are present in the root nodules of the legumes such as soybean and convert the N2 into ammonia. Therefore, planting the legumes makes the soil rich in ammonia.
