The main nutrients plants need are nitrogen, phosphorus, and potassium--NPK. Most nitrogen fertilizer is made from natural gas, while legumes such as peas, alfalfa, and soybeans can also fix nitrogen naturally. Unfortunately, we don't have the same plant-based alternate sources of phosphorus and potassium--everything we use is either mined from phosphate and potash deposits or recycled organic matter.
"The absolute worst case scenario, from a nutrient standpoint, is what we are doing today.
Producing any grain (corn, soybeans, wheat, etc) and shipping that food away from where it was grown is the largest depleter of P & K [phosphorus and potassium] from the land. By exporting food out of where it was grown your are exporting your most concentrated nutrients the plants have stored. The process essentially is mining the soil.You want to be exporting only C,H,O,and N off the land. They are all replaced via gases in the atmosphere. A century ago most of the food was recycled where it was grown via animal or human consumption and waste. Not so today. Exporting food from the "interior" to the major cities on the coasts moves P & K away from productive land, essentially into water systems"
Anaerobic digesters offer an opportunity to improve the recycling of nutrients--not only does manure become easier to handle, but food waste can be mixed in and its nutrients returned easily to farmland. It's definitely an improvement over practices other food-waste treatment practices such as composting and water treatment facilities. As long as most compost ends up as landscaping material, the nutrients might as well be gone, and wastewater treatment typically treats nutrients as a problem to be minimized rather than a resource to be recovered.
A movie called "Dirt" has recently gotten quite a bit of attention; I haven't seen it yet, and I suspect it doesn't dwell on specific nutrient issues, but hopefully it has been raising awareness over the importance of caring for the soil. A future with well-balanced agriculture needs plenty of awareness--and good stocks of phosphorus and potassium.