A long time ago, farmers discovered that if they grow the same crops on a field, then after a few years, the crops grow more slowly. Why does
this happen?​

Because they don't have fertilizer.

Explanation:

Well, if this was a long time ago, then farmers did not have fertilizer. So, there are bacteria on the roots of plants that convert nitrogen into a usable form. It's very important for growing, and when farmers harvest the crop, not much of the nitrogen is returned to the land, so it is not as fertile. Fertilizer is largely consisted of nitrogen, so that is why farmers use fertilizers today. But "a long time ago," farmers did not have fertilizer.

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