Onion Response to Added N in Histosols of Contrasting C and N Contents

Adjusting the N fertilization to soil potentially mineralizable N in Histosols is required to secure high vegetable yields while mitigating nitrate contamination of surface waters. However, there is still no soil test N (STN) relating the response of Histosol-grown onion (Allium cepa L.) to added N. Compositional data analysis can integrate soil C and N composition into a STN index computed as Mahalanobis distance (M2) across isometric log ratios (ilr) of diagnosed and reference soil C and N compositions. Our objective was to calibrate onion response to added N against a compositional STN index for Histosols. Reference compositions were computed from high N-mineralizing Histosols reported in the literature. Soil analyses were total C and N, and a residual soil mass (Fv) was computed as 100%-%C-%N to close the compositional vector to 100%. The C, N, and Fv proportions were synthesized into two ilrs. We conducted thirteen onion N fertilization trials in Histosols of south-western Quebec showing contrasting C, N, and Fv proportions. Each crop received four N rates broadcast before seeding or split-applied. We derived two STN classes separating weakly to highly responsive crops about the M2 value of 5.5. Onion crops grown on soils showing M2 values >5.5 required more N and yielded less in control treatments compared with soils showing M2 values 5.5) soils responded significantly (P < 0.10) to 60 and 180 kg N ha-1, respectively. Using literature data and the results of this study, we elaborated a provisory N requirement model for Histosol-grown onions in Quebec.