In the natural world, soil replenishes itself.
As seeds grow into plants, they absorb nutrients from the soil to produce leaves, roots, and fruits. When they die naturally, their bodies decompose and return nutrients to the soil. Sometimes, a herbivorous animal like a goat or a cow comes along and eats the plant. The animal absorbs the nutrients, excretes whatever’s left as poo, and the poo becomes food for the next generation of plants.
When we farm, we break this cycle. We harvest the plants to eat and sell so the nutrients are exported far away from the farm and usually never return. When we farm on a large scale, thousands of plants absorb immeasurable quantities of nutrients from the soil. If we continue farming on that soil, more and more nutrients are absorbed until there’s no more, leaving our vegetables weak with yellow leaves, small fruits, and spindly stems.