Final answer:
Leguminous crops can fix atmospheric nitrogen through a symbiosis with rhizobia bacteria, enriching the soil naturally, which makes additional nitrogen enrichment unnecessary. This contributes to sustainable agriculture by reducing the need for chemical fertilizers.
Step-by-step explanation:
Leguminous crops such as alfalfa, clover, peas, beans, and soy do not require nitrogen enrichment because they possess a unique ability to fix atmospheric nitrogen through a symbiotic relationship with rhizobia bacteria. These bacteria colonize the root nodules of the legumes and convert diatomic nitrogen (N₂) from the air into ammonia (NH₃), which the plants can then use to synthesize proteins and other nitrogenous compounds.
When legumes are ploughed back into the soil, they contribute significant amounts of nitrogen, enhancing soil fertility for subsequent crops. A notable example is how a crop of lucerne ploughed back into a field may add up to 350 kilograms of nitrogen per hectare. This process, known as symbiotic nitrogen fixation, offers an environmentally friendly and sustainable alternative to chemical fertilization, thus greatly benefiting sustainable agriculture.