Land required for legumes restricts the contribution of organic agriculture to global food security

Commercial extraction of nitrogen (N) from the atmosphere began soon after World War II and has provided N fertilizer that has transformed agriculture to meet, through greater crop areas and yields although with some regional shortfalls, the increasing food demand of a world population that has increased from 2 billion then to 7.6 billion in 2018. N fertilizer now provides more N input to agriculture (113 Mt N/year) than biological N fixation by legumes (33–46 Mt N/year) on which earlier agriculture relied entirely. Persistent claims over the last decade for return to organic methods, which include rejection of fertilizer N, are based on studies that erroneously claim adequate productivity to feed the world. Previous analyses, by contrast, have estimated that organic agriculture (OA) could at best support a world population of three to four billion. The problem is two-fold. First, organic crops grown in sequences with legumes or treated with N manures mostly yield less than crops grown with N fertilizer. Second, substantial areas of legumes are required to provide adequate N for required yields of non-legume crops. Recent analyses have overestimated the yield of organic crops by omitting the effect of weeds, pests and diseases, and by ignoring the land required for legumes. The result is a large overestimation of the relative productivity of OA. The effect of area is critical because, since there is little opportunity to increase cropping area beyond the current 1400 Mha, land for legumes means less land for, and consequently lower total production from, non-legume food crops. To replace 100 Mt N fertilizer/year with legumes at a net fixation of 100 kg N/ha/year would leave just 30% of cropland available for non-legumes producing a similar proportion of current yield. Even with major gains in yield, organic systems cannot feed our populous world and less so as the population increases to an expected 9.8 billion by 2050