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Deep‐rooted cover crops may help alleviate effects of soil compaction, especially in no‐till systems. We evaluate compaction‐alleviating ability of three Brassica cover crops and cereal rye (Secale cereale L.). Using a minirhizotron camera, we observed soybean [Glycine Max (L.) Merr.] roots growing through compacted plowpan soil using channels made by decomposing cover crop roots. Soybean yield response to the preceding cover crops was most pronounced at the site with most severe drought and soil compaction. At this location, with or without deep tillage, soybean yields were significantly greater following a “forage radish + rye” combination cover crop. Rye left a thick mulch, resulting in conservation of soil water early in the season. Root channels left by forage radish (Raphanus sativus L. ‘Diachon’) may have provided soybean roots with low resistance paths to subsoil water. Due to lower than normal winter precipitation, this study was a conservative test of the cover crops' ability to alleviate the effects of soil compaction.

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.

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Abstract:The primary cause of soil degradation in sub-Saharan Africa (SSA) is expansion and intensification of agriculture in efforts to feed its growing population. Effective solutions will support resilient systems, and must cut across agricultural, environmental, and socioeconomic objectives. While many studies compare and contrast the effects of different management practices on soil properties, soil degradation can only be evaluated within a specific temporal and spatial context using multiple indicators. The extent and rate of soil degradation in SSA is still under debate as there are no reliable data, just gross estimates. Nevertheless, certain soils are losing their ability to provide food and essential ecosystem services, and we know that soil fertility depletion is the primary cause. We synthesize data from studies that examined degradation in SSA at broad spatial and temporal scales and quantified multiple soil degradation indicators, and we found clear indications of degradation across multiple indicators. However, different indicators have different trajectories-pH and cation exchange capacity tend to decline linearly, and soil organic carbon and yields non-linearly. Future research should focus on how soil degradation in SSA leads to changes in ecosystem services, and how to manage these soils now and in the future. OPEN ACCESS

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