Nutrient Recycling
It has always been obvious that, unless nutrients are recycled back to the land, sooner or later the land will become barren.
The failure to return macronutrients like phosphate and potassium, as well as trace minerals, back to the land has distressed observers for many centuries. Carolyn Steele, in her excellent book Hungry City, notes that the a number of ancient city states appear to have collapsed as a result of a failure to maintain the fertility of their soils. Cities like London and Paris up to about 100 years ago were acutely aware of the value of human and animal waste, which was carefully collected by market gardeners who needed it to grow food for the city. Paris in the late 19th century grew much of its fruit and vegetables at the sewage farm at Gennevilliers. When London decided to build a sewage system which instead flushed sewage out to see, Justus von Liebig, the "father of fertiliser", wrote to the Prime Minister urging him not to make such a mistake:
The cause of the exhaustion of soil is sought in the customs and habits of the townspeople, ie in the construction of water-closets, which do not admit of a collection and preservation of the liquid and solid excrement. They do not return in Britain to the fields, but are carried off by the rivers into the sea. The equilibrium in the fertility of the soil is destroyed by this incessant removal of phosphates and can only be restored by an equivalent supply."
In modern agriculture, there has been a perception that flushing valuable nitrate, phosphate and potassium down the drain (where it becomes dispersed in the sea or otherwise lost to the land) is not a problem, because phosphate and postassium can always be mined, and more nitrate can always be manufactured from natural gas. As we are seeing, however, it is beginning to be clear that the "always" in that sentence is not true. We have been able to practice this linear agriculture for a while, but our ability to do so was always bound to be time-limited, and that limit may now be upon us.
By contrast, the Permaculture movement, whose principle is to design systems that are sustainable in the true sense of the word (ie they can be continued indefinitely), have always been aware of the need to recycle nutrients. Many permaculturalists have tended to assume that this implies an agrarian lifestyle where the consumer lives so close to the field that all nutrients can be returned very directly. Modern composting toilets provide one means of doing this, but more simple methods have been practiced in China and other Asian countries for thousands of years (thus proving their long-term effectiveness).
A key question for our future food system is whether techniques can be developed to return urban waste back to the land at a reasonable cost. This will require either the use of composting toilets and urine collection, or of downstream sewage processing. At present the latter is already used to return much of the "sludge" to the land, but urine (which contains the greater part of the nitrate and phosphate) may still be lost.
It is important to note that it is not good enough simply to return these nutrients to the land somewhere, such as to market gardens around the cities. The land that grows wheat for the city, whereever it is, needs to get its share of the nutrients back. Otherwise wheat yields will steadily reduce as phosphate, potassium and trace mineral levels reduce.
For these reasons it may make more sense to talk about food supply cycles rather than supply chains.