AbstractThe effect of shale fragments as soil conditioner was investigated in two greenhouse experiments using alfalfa as the test plant. Four soils differing by the texture and the shale content were used, the < 2 mm material alone or in a mixture with coarser fragments, shale or gravel. In the first experiment, cuts 1–4 were taken under adequate watering before water stress was applied for cuts 5–8. Water stress was applied since the beginning in the second experiment.Soil‐shale substrates generally produced the highest yields. Soil‐gravel substrates followed next in the first experiment but sustained generally the lowest yield in the second experiment.The results were related to the effect of the coarse fragments on soil structure and water retention of the substrates.