My calculation is based upon the population of the Earth when we had to start mass scale deployment of chemical fertilizer (mined Phosphorus and Potassium, captured Nitrogen) to keep feeding the industrialized world and reduce famine elsewhere. This indicated that the soil depletion caused by separation of populations from the crops became a crisis.
There was a secondary crisis when more soil elements (Calcium, Magnesium, Iron, Sulfer, etc.) depleted in the same way. We are now in the third wave, where trace elements' depletion are causing a breakdown in immune systems and defenses against crop antagonists. This is why we now need insecticides, herbicides, vermicides, anti-fungals, and antibiotics to bring food to market, cancer has gone from rare to the biggest killer, and why various flu types have become pandemic.
The ONLY sustainable food chain is where all of the elements are returned to the soil, instead of flushed to the bottom of the ocean. Over the last 80,000 years humans killed half of all life on Earth using spears, fire, and metal tools; and now the twin pillars of civilization, food imports and water borne sanitation, are killing the rest FAST.