Abstract An inexpensive piston‐action ball mill for the rapid preparation of plant and soil material for the automated analysis of 15N and 13C contents is described. A sample throughput of 150 samples per hour is possible with at least 50% of the particle sizes below 105 μm. The range of particle sizes obtained with the grinder allows a level of precision of less than 1% for the analysis of 15N isotope enrichment using an automated continuous flow nitrogen and carbon isotope ratio mass spectrometer. Considerable savings in labor and costs are possible compared with other sample preparation systems currently available. The use of the mill is illustrated by a study of isotope enrichment of different plant organs of tropical forage species.