Methods for reducing the tillage force of subsoiling tools: A review

Conservation tillage technology (CTT) is a promising and efficient method for farmland protection and utilization. As one of the core technologies of CTT, mechanical subsoiling has been identified as an effective practice to disrupt hardpans and reduce soil bulk density to promote better water infiltration and crop root development. The tillage force during subsoiling (TFDS) is extremely high and methods for reducing the TFDS have been developed. A comprehensive summary about typical subsoiling tools and corresponding tool characteristics and current methods for TFDS reduction were reported in this review. The TFDS reduction methods were introduced in five aspects: biomimetic, oscillatory, lubrication, experimental, and others (i.e., line element design, layered subsoiling, crop straw and root anti-blocking, high-pressure gas splitting, rotary subsoiling, and selection of working depth and time). The mechanisms and limitations of current methods were analysed. The effect of various methods on draught forces of subsoiling tools with similar working parameters were quantitatively investigated and compared using statistical analyses. This review also recommends four future research directions about further reduction of TFDS to promote the development of TFDS reduction technology and CTT.