Fertiliser requirements for balanced nutrition of cassava across eight locations in West Africa

• Nutrient use efficiency of cassava increases at balanced nutrition.
• Benefice:cost ratio of fertiliser use increases at balanced nutrition of cassava.
• Improved management practices substantially enhance cassava yield.
• Potassium is the most needed external nutrient to produce 8 Mg storage roots dry matter ha−1 across the study locations.
• Nitrogen and phosphorus are most needed at higher target yields.

sInsufficient and unbalanced fertiliser use widens cassava yield gaps. We assessed the spatial variability of optimal fertiliser requirements of cassava for enhanced nutrient use efficiency and increased yield using the balanced nutrition approach of the QUEFTS model. Two datasets comprised of five fertiliser experiments conducted at eight locations across Southern Togo, Southern Ghana and Northern Ghana from 2007 to 2012 were used. The ratio of storage roots dry matter yield over the sum of available N, P and K expressed in crop nutrient equivalent from the soil and nutrient inputs was used as a proxy to estimate nutrient use efficiency. Nutrient use efficiencies of 20.5 and 31.7 kg storage roots dry matter per kilo crop nutrient equivalent were achieved at balanced nutrition at harvest index (HI) values of 0.50 and 0.65, respectively. N, P and K supplies of 16.2, 2.7 and 11.5 kg at an HI of 0.50, and 10.5, 1.9 and 8.4 kg at an HI of 0.65 were required to produce 1000 kg of storage roots dry matter. The corresponding optimal NPK supply ratios are 6.0–1.0–4.2 and 5.3–1.0–4.2. Nutrient use efficiencies decreased above yields of 77–93% of the maximum. Evaluation of the performance of blanket fertiliser rates recommended by national research services for cassava production resulted in average benefit:cost ratios of 2.4 ± 0.9, which will be unattractive to many farmers compared to 3.8 ± 1.1 for the balanced fertiliser rates. The indigenous soil supply of nutrients revealed that, at balanced nutrition, K was the most limiting nutrient to achieve storage roots yields up to 8 Mg dry matter ha−1 at most sites, whereas N and P were needed at greater yields. Dry weight of storage roots measured on the control plots in our researcher managed experiment ranged from 5.6 to 12.2 Mg ha−1, and were larger than the average weight in farmers’ fields in West Africa of 4 Mg ha−1. Substantial yield increase could be attained in the region with improved crop management and fertiliser requirements formulation on the basis of balanced nutrition.