Mechanical and biological characteristics analysis of mechanically transplanted rapeseed substrate block seedlings

Abstract: The physical characteristics of rapeseed substrate block seedlings could affect its mechanical transplanting. In this paper, we measured the mechanical and biological characteristics of cubic rapeseed substrate block and link them to the safety in collecting, transporting and transplanting rapeseed substrate block seedlings. Orthogonal experiments were designed with substrate components ratio of earthworm dung, yellow brown soil, vermiculite mixture in the substrate at 1∶2∶1, 1∶1∶1 and 2∶1∶1 respectively, their associated compactness at 1.0, 1.2 and 1.4 respectively, and the concentration of boron and selenium in the solution at 0.24%, 0.27% and 0.30% respectively. We measured the mechanical characteristics of the rapeseed substrate block seeding (including its yield strength and drop loss rate) and the biological characteristics of the seedlings (including strong seedling index, substrate encapsulation degree and root robustness). The results showed that both substrate composition and compaction affected the mechanical characteristics of the rapeseed substrate block seedling, the seedling growth and root robustness at significant level or above; the concentration of boron and selenium also had a significant impact on the mechanical characteristic of the rapeseed substrate block seedlings. Substrate encapsulation degree and root robustness were both significantly affected by substrate composition ratio, compactness and concentration of the boron and selenium. The yield strength and drop loss rate of the rapeseed substrate block seedlings increased with both content and compactness of the yellow brown soil. With earthworm feces content increasing, the strong seedling index, substrate encapsulation degree and root robustness increased first followed by a decline. An increase in compaction degree and concentration of boron and selenium also resulted in an increase followed by decline in substrate encapsulation degree and root robustness. We used a comprehensive scoring method to determine the optimal combination of the seedling-raising factors by taking yield strength, strong seedling index, substrate encapsulation degree, root robustness and drop loss rate as evaluating factors with each having a weighting coefficient of 0.2. The results showed that the mechanical characteristics and the growth quality of the seedlings peaked when the ratio of all substrate components was 1∶1∶1, the compactness was 1.2 and the concentration of boron and selenium was 0.30%. A bench test results revealed that the requirements for mechanical transplanting were met when the transverse seedling pushing-speed was 20 mm/s, synchronous conveyor belt speed was 30 mm/s, the seedling picking frequency was 40 plants/min, the substrate loss rate was 2.57%, in which the feeding seedling successful rate was 90.93%. The failure of sending and collecting the seedlings could occur 1) in the process of mechanical transplanting where seedlings intertwined, resulting in an instability of the gravitational center of the seedlings, 2) in that the home-made transplanter might not be accurate enough in the bench test as there was a gap between the feeding tray and the synchronous conveyor belt, and that the installation angle of the row feeding device might bear errors. All these could result in the feeding-seedlings rate dropping to 90%.