1. Ard og Plov i Nordens Oldtid;Glob,1951
2. Entstehung und Verbreitung des Pfluges;Leser,1931
3. Studies in soil cultivation 1–3;Keen;J. agric. Sci., Camb.,1925
4. Proc. 8th Conf;International Soil Tillage Research Organization,1979
5. Proc. 9th Conf.;International Soil Tillage Research Organization,1982