The purpose of this study was to evaluate the effectiveness of an instructional unit on soil fertility and fertilizers in Iowa vocational agriculture classes. A pretest-posttest control group design was used in the investigation. Fifteen teachers and their classes comprised a randomly assigned experimental treatment group. Similarly, twelve randomly assigned teachers and their classes provided data from a control group. The "experimental treatment" was an instructional unit on soil fertility and fertilizers designed for use by vocational agriculture teachers. Experimental group teachers used the unit in their instruction, whereas control group teachers were asked to focus their teaching on a list of problem areas and study questions identified in the unit using their "normal" materials;Five instruments were developed and used to collect data for assessing the effectiveness of the unit. Soil fertility knowledge and the attitude inventory instruments were administered to students as both pretests and posttests. Experimental units were considered to be classes and class means were used in the analyses. The main statistical procedure used was analysis of covariance with one factor and one covariate (pretest scores);Major findings of the study were: (1) posttest knowledge scores were significantly (P < .01) higher than pretest scores for both groups, (2) posttest knowledge scores for the experimental group classes were significantly (P < .05) higher than those for the control group classes adjusting for the covariate, (3) overall posttest attitude scores were significantly (P < .05) higher than pretest attitude scores for the experimental treatment group, (4) experimental group posttest attitude scores related to the concept of fertilizer decision-making was significantly higher than those of the control group using pretest scores as covariate. These findings revealed that the unit was effective in selected Iowa vocational agriculture classes.