Agronomist Andrew McGuire explains why there is not enough manure or compost available – and can never be enough – to replenish soil fertility at a systems level. Studies that show improved outcomes on single farms are not measuring improvement at a systems level.
Returning manure to the soil that produced it can be a sustainable way of restoring soil organic matter. Too often the organic matter of many farms is pushed through the bottleneck of intensive livestock production to be applied just a few farms. Robbing Peter to pay Paul.
Can we produce enough manure to serve as a major source of soil nutrients? Agronomist Andrew McGuire shows the limitations due to entropy make it impossible for manure to make up more than a portion of nutrient replenishment.
Farmers around the world have come to depend on manufactured inorganic fertilizers containing key plant nutrients phosphorus, nitrogen and potassium to enhance soil fertility, especially in the otherwise poor soils of most tropical settings. But while all three are relatively abundant in nature, commercially viable sources of phosphorus to make these fertilizers could be exhausted just a few decades from now. That prospect, which remains a source of heated debate, has spurred a drive to recover the significant quantities of this element that disappear in the waste streams of cities and farms.