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Effects of green manure use on sweet corn root length density under reduced tillage conditions

A green manure (GM) is a crop grown primarily as a nutrient source and soil amendment for subsequent crops. In environments such as Florida, combined use of GM and reduced tillage may improve soil water and nutrient retention and reduce potential groundwater pollution. In the first 3 years of a long-term experiment, use of GM in a reduced-tillage system on a sandy Florida soil benefited the season-long growth of sweet corn (Zea mays L. var. Rugosa) much more than final ear yields. To help understand these patterns, we evaluated response of sweet corn roots when in rotation with GM of sunn hemp (Crotalaria juncea L.; summer) and cahaba white vetch (Vicia sativa L.; winter 2002–2003) and a multi-species mixture of hairy vetch (V. villosa Roth.) and cereal rye (Secale cereale L.; winter 2003–2004). Treatments included sweet corn with combinations of 0 or 133 kg chemical N ha−1 (as NH4NO3) and with or without GM. A highly fertilized treatment (267 kg chemical N ha−1) without GM was also included. Soil cores were sampled from three depths (0–15, 15–30 and 30–60 cm) both between and within corn rows. Data from two experiments showed that use of GM increased sampled corn root length density (RLD) by 44–54%, although only within the upper 15 cm of soil in one of the two experiments. Corn following GM plus 133 kg chemical N ha−1 produced up to 44% greater RLD than corn with 267 kg chemical N ha−1. Sampled RLD decreased with distance away from corn plants (from in-row to between-row positions, and from shallow to deeper depth), with roughly 85–95% of sampled RLD existing in the top 30 cm of soil across all treatments. During the 2004 experiment, we found that broadcast, as opposed to banded (placed along corn row only), chemical N application resulted in more even distribution of corn RLD between in-row and between-row positions during late-season without regard to GM crop. Although GM permitted optimal sweet corn growth with a 50% reduction in chemical N application, ear fill during the final 1–2 weeks before harvest may have been reduced in GM treatments. GM effects on the amount and spatial distribution of sweet corn RLD may help explain these trends. Provision of greater N from GM residues and/or altered distribution of supplementary chemical N and irrigation may be required to achieve greater ear yield benefit from GM.