AbstractBased on Mandel's additive plus multipicative model for ANOVA, this paper presents a procedure to represent fertility differences in field trials arranged in rectangular plans. After fitting any block and treatment effects, the residuals are assigned to their positions in the rectangular planting plan, and then decomposed into orthogonal components, of which the largest ist attributed to fertility. If the variance associated with this component is significantly larger than those remaining, the observed data are then adjusted for it; the process is repeated until there is no significant component.The procedure is illustrated by data from a variety trial.