Soil-based, field-specific fertilizer recommendations are a pipe-dream

• Published in: Geoderma 2020-12, Vol.380, p.114680, Article 114680
• Main Authors: Schut, A.G.T., Giller, K.E.
• Format: Article
• Language: English
• Subjects: Agronomic efficiency; Laboratory errors; QUEFTS; Soil analysis; Soil nutrients
• ISSN: 0016-7061; 1872-6259
• DOI: 10.1016/j.geoderma.2020.114680

•Laboratory ring-tests showed large coefficients of variation in soil analyses.•Estimates of soil N, P and K supply for a single field varied strongly.•Field-specific recommendations based on a single soil sample were inaccurate.•Fertilizer with balanced N:P:K content strongly reduced errors in recommendations. Fertilizer recommendations are key for farmers: the investment is relatively large for smallholders and risky with unknown yield responses and variable fertilizer prices. Are agronomists able to provide useful site-specific fertilizer recommendations that reduce these uncertainties? We evaluated the influence of errors introduced due to soil sampling and chemical analysis procedures both within- and among laboratories on fertilizer recommendations. Using what we consider to be conservative estimates of the uncertainty in estimating soil supply of N, P and K in a single composite soil sample, the resulting 90% confidence interval of fertilizer recommendations ranged from 86 to 186, 0–58 and 38–114 kg N, P and K ha−1 respectively. The numerous laboratory services and digital applications providing field-specific recommendations appear to promise more accuracy than soil analysis can realistically deliver. We conclude that a field-specific fertilizer recommendation based on a single composite soil sample is indeed a pipe-dream.