Low-Cost and Convenient Experimental Methods for Research on the Physical Characteristics of Green Manure Seeds

To improve the performance of green manure sowing and processing equipment, it is necessary to investigate the physical properties of seeds, including their dimensions, bulk density, and frictional characteristics. Focusing on the main cultivated varieties of green manure in China—milk vetch, hairy vetch, and sesbania—this study proposes three low-cost, convenient, and high-precision experimental methods to measure the geometric size of small seeds. The measured results show that the milk vetch seeds were 2.60–3.40 mm in length, 1.90–2.40 mm in width, and 0.71–0.97 mm in height. The hairy vetch pods were 23.03–32.83 mm in length, 7.39–9.74 mm in width, and 4.06–6.15 mm in height, and the seeds were 3.04–4.10 mm in diameter. The sesbania seeds were 3.81–4.29 mm in length, 1.98–2.37 mm in width, and 1.85–2.08 mm in height. The thousand-seed weights of milk vetch, hairy vetch, and sesbania seeds were 3.40 g, 26.50 g, and 15.58 g, with moisture contents of 7.18%, 9.81%, and 8.73%, respectively. The bulk densities of milk vetch, hairy vetch, and sesbania seeds were 732 g/L, 761 g/L, and 845 g/L. The angles of repose of milk vetch, hairy vetch, and sesbania seeds were 31.66°, 28.15°, and 29.82°, measured using an angle-of-repose tester. Sliding friction angles of 25.85°, 23.55°, and 24.03° for milk vetch, hairy vetch, and sesbania seeds were obtained using a dividing head. These methods and results provide valuable references for the measurement of the physical properties of small-seeded crops (e.g., green manures) as well as other small or irregularly shaped particles.