1. Alberta Farm Machinery Research Centre (1994) Evaluation report 713. Kee Ultrasonic depth control system. Prairie Agricultural Machinery Institute, Humboldt, pp 1–10
2. Auernhammer H (1989) Electronics in tractors and machines (in German). BLV Verlagsgesellschaft, Munich
3. Bowers CG, Bowen HD (1975) Drying front sensing and signal evaluation for planters. Trans Am Soc Agric Eng 18(6):1051–1058
4. Bowers CG, Collins CA, Harris EP (2006) Low soil moisture planting of cotton for optimum emergence. Appl Eng Agric 22(6):801–808
5. Carter LM, Chesson JH (1993) A soil moisture seeking planter control. ASAE Winter Meeting, Chicago, Paper no. 931553