If a farmer plants the same crop every year, the soil becomes less fertile. What causes the loss of fertility?

The nutrients lost depend on what is being planted over and over again. If its corn, nitrogen will be lost over the years. Thats why if a farmer wants corn in the same field over and over again he will have some sort of nitrogen enriched seed or fertilizer that he puts in the planter. Have you ever seen a soybean field that has little corn stalks in it? This is because farmers plant soybeans in a field after corn has been there because soybeans put nitrogen back into the soil

The plants need nutrients to grow. once planted they germinate and use up all of the nutrients found within the seed. as they grow out of the ground and continue to live, they must pull extra nutrients from the ground. now each crop uses different nutrients, so if you plant the same crop again and again the soil becomes less fertile because it has less of a variety of nutrients.