One thing that we are not short of in our society is experts, and one would imagine that, amongst all the university professors and government scientists, there would exist a considerable body of knowledge relating to how much produce it is safe to take from a particular area of ground before it starts to suffer a loss of fertility. However, if this knowledge does exist, it has never been put in the public domain.
This may be because modern farming has evolved in such a way that its proponents do not fully acknowledge the concept of soil fertility at all: agricultural science treats a field as though it is composed of more-or-less inert matter; a soil analysis is conducted, and nutrients are added in the quantities and proportions required for a particular crop. Once the crop reaches maturity, one-hundred-per-cent of the harvested material can be sold to the food-processing industry, and, in theory, the land is left in no worse than it was at the beginning. One problem with this technique is that it leaves the farmer completely at the mercy of the global agro-chemical and agricultural-equipment industries – he or she has to have a modern tractor, modern equipment, and prairie-like fields in order to be part of the system, and they then have to use the latest crop varieties (possibly genetically modified), and the latest chemical formulations, backed up by expensive agricultural consultants, in order to operate. If they are lucky, the price they are paid for their produce will offset all these costs: but they have no personal control over whether or not their farms are profitable, or how they manage them. This is not farming in the traditional sense of the word, it is a form of industrial production; it is does not require soil fertility, but relies instead upon an annual input of raw materials.
The alternative is to return to the traditional idea of managing the land in such a way that its fertility is maintained, or even enhanced, from year to year, without the necessity of bringing in extra material from outside: but then we come back to the question of how much produce can safely be removed from a farm before fertility is reduced.
This is not a trick question (i.e. the answer isn’t nothing). Plants are able to capture the sun’s energy, and convert it into vegetable matter, using basic ingredients (water, air and minerals) which exist in abundance in nature. If all of this plant material is consumed locally, on the farm itself, then the land becomes richer and richer in organic matter, and its fertility actually increases.
The more fertile a piece of land becomes, the more it produces, and the more can be taken from it – but the reverse is also true.
There is not a simple answer to the question of how much can be taken from a piece of land without reducing its fertility. A small-scale, organic farmer, can make the judgement based on the health of the wildlife on the farm, the vigour of the crops, and the diversity of wild plants. In the case of an industrial-scale modern farm, the answer is probably that nothing should be taken off until the soil has been given a chance to recover from the treatment to which it has been subjected.