Determining the value of legumes as soil-fertility amendments can be challenging, yet this information is required to guide public policy and to incentivise prescribed land-management practices such as conservation agriculture. We apply a directional distance function to data from Malawi, to estimate shadow prices for symbiotic nitrogen and the technical efficiency for mixed maize-legume production systems. The shadow prices reflect the trade-off between fertiliser-nitrogen and symbiotic-nitrogen required to achieve a given quantity of output. Our results reveal considerable technical inefficiency in the production system. The estimated shadow prices vary across farms and are, on average, higher than the reference price for commercial nitrogen. Our results suggest that it would be beneficial to redesign the current price-support programs that subsidise chemical fertilisers and indirectly crowd-out organic soil amendments such as legumes.