Seed drill instrumentation for spatial coulter depth measurements

An even and correct depth placement of seeds is crucial for uniform crop germination and for obtaining the desired agricultural yield. On state-of-the-art seed drills, the coulter down pressure is set manually by static springs or heavy weights, which entails that the coulter's seeding depth reacts to variations in soil resistance. The aim of the study was to develop and test an instrumentation concept installed on a low-cost, lightweight, three meter wide, single-disc seed drill, for on-the-go measurements of spatial depth distributions of individual coulters under real field conditions. A field experiment was carried out to measure individual coulter depths at three different operational speeds. The targeted seeding depth was −30 mm but shallower mean coulter depths were obtained and the depth decreased slightly - although not significantly - with increasing speed, i.e. to −22.1, −20.9 and −19.0 mm for 4, 8, and 12 km h−1, respectively. The coulter depths ranged between −60 mm (below the surface) and even above surface at all speeds, but the variation tended to decrease with decreasing speed. However, soil resistance influenced coulter depth as indicated by a significant block effect. The mean coulter depth varied up to ±5 mm between the blocks. In addition, significant depth variations between the individual coulters were found. The mean depths varied between −14.2 and −25.9 mm for the eleven coulters. The mean shallowest coulter depth (−14.2 mm) was measured for the coulter running in the wheel track of the tractor. The power spectral densities (distribution) of the coulter depth oscillation frequencies showed that the majority of oscillations occurred below 0.5 Hz without any natural vibration frequency. The study concluded that the instrumentation concept was functional for on-the-go spatial coulter depth measurements.