Penetration of cover crop roots through compacted soils

Tap-rooted species may penetrate compacted soils better than fibrous-rooted species and therefore be better adapted for use in "biological tillage". We evaluated penetration of compacted soils by roots of three cover crops: FR (forage radish: Raphanus sativus var. longipinnatus, cv. 'Daikon'), rapeseed (Brassica napus, cv. 'Essex'), two tap-rooted species in the Brassica family, and rye (cereal rye: Secale cereale L., cv. 'Wheeler'), a fibrous-rooted species. Three compaction levels (high, medium and no compaction) were created by wheel trafficking. Cover crop roots were counted by the core-break method. At 15-50 cm depth under high compaction, FR had more than twice and rapeseed had about twice as many roots as rye in experiment 1; FR had 1.5 times as many roots as rye in experiment 2. Under no compaction, little difference in root vertical penetration among three cover crops existed. Rapeseed and rye root counts were negatively related to soil strength by linear and power functions respectively, while FR roots showed either no (Exp.1) or positive (Exp. 2) relationship with soil strength. We conclude that soil penetration capabilities of three cover crops were in the order of FR > rapeseed > rye.