Laboratory bioassays were used to investigate plant growth‐regulating effects of three different experimental soil additives, designatedEXP95, W91,andZ96.A yeast growth test was used as a general assay of bioactivity, responses to soil additives were compared to those of known plant growth regulators [indoleacetic acid (IAA), gibberellic acid (GA3), and kinetin]. A corn coleoptile elongation test was used to assay for auxin‐like activity and a dwarf pea bioassay was used for gibberellin‐like activity. The three soil additives were tested at five solution concentrations ranging from 1 to 10,000 ppm (by volume). All three soil additives stimulated yeast growth, depending on the concentration of the test solutions. However, all three soil additives inhibited plant growth in the two plant bioassays. Although this study clearly demonstrated that the three soil additives had significant biological activity at very low concentrations, there was little evidence for auxin‐like or gibberellin‐like activity.