Potential yield challenges to scale-up of zero budget natural farming.

Under current trends, 60% of India's population (>10% of people on Earth) will experience severe food deficiencies by 2050. Increased production is urgently needed, but high costs and volatile prices are driving farmers into debt. Zero budget natural farming (ZBNF) is a grassroots movement that aims to improve farm viability by reducing costs. In Andhra Pradesh alone, 523,000 farmers have converted 13% of productive agricultural area to ZBNF. However, sustainability of ZBNF is questioned because external nutrient inputs are limited, which could cause a crash in food production. Here, we show that ZBNF is likely to reduce soil degradation and could provide yield benefits for low-input farmers. Nitrogen fixation, either by free-living nitrogen fixers in soil or symbiotic nitrogen fixers in legumes, is likely to provide the major portion of nitrogen available to crops. However, even with maximum potential nitrogen fixation and release, only 52-80% of the national average nitrogen applied as fertilizer is expected to be supplied. Therefore, in higher-input systems, yield penalties are likely. Since biological fixation from the atmosphere is possible only with nitrogen, ZBNF could limit the supply of other nutrients. Further research is needed in higher-input systems to ensure that mass conversion to ZBNF does not limit India's capacity to feed itself.