Why does a farmer put fertilizer on his crops?

That's a bit of a rhetorical question, don't you think? Because that's what leguminous crops are used for: to put nitrogen back into the soil. Nitrogen is one of the essential macro-minerals that plants need to grow and thrive. Rotating cereals with pulse crops helps increase the nitrogen in the soil and decreases the costs the producer has to pay to put nitrogen fertilizer in the soil at seeding.