Organic broccoli production on transition soils: Comparing cover crops, tillage and sidedress N

Abstract

Little information is available about how farmers in transition to organic practices should manage short-and long-term N fertility. The objectives of this research were (1) to evaluate the leguminous cover crops lablab (Dolichos lablab L.), soybean (Glycine max L.), sunn hemp (Crotalaria juncea L.) and a mixture of sunn hemp and cowpea (Vigna sinensis Endl.) as N sources; (2) to compare N availability and broccoli yield when cover crops were incorporated with conventional tillage (CT) or retained as a surface mulch using no-tillage (NT) practices; and (3) to quantify the amount of supplemental sidedress nitrogen required to maximize the yield of organic broccoli (Brassica oleracea Group Italica) on transition soils. Broccoli was grown during the first year after organic transition in the spring and fall of 2006 at the Kentland Agricultural Research Farm near Blacksburg, VA. Spring (P < 0.001) and fall (P < 0.001) broccoli yield increased as the rate of sidedress N was increased up to 112 kg N ha -1 , and showed a quadratic correlation with leaf N (P = < 0.001, R 2 = 0.80 and P = < 0.001, R 2 = 0.38, respectively). There was no difference in spring broccoli yield between CT and NT; however, CT produced the highest yield in the fall crop. At low sidedress N rates, leaf N was highest in CT plots, but tillage had no effect on N uptake at high N rates. This indicates that early season and perhaps total plant-available mineralized N was greater in CT than NT; however, potential N deficiency in NT soil may be compensated by sidedress N. Broccoli yield was not affected by leguminous cover crop, even though the quantity of cover crop biomass and N contribution was different among species. This suggests that N availability from leguminous cover crops may be impacted by other ecological processes such as soil microbial activity. This study shows that organic CT and NT growers can maximize broccoli yield in transition soils low in N availability, by using leguminous cover crops in combination with moderate amounts of sidedress N.

Nitrogen Contribution of Legume/Cereal Mixed Cover Crops and Organic Fertilizers to an Organic Broccoli Crop

“…as a cover crop for fall broccoli in Oklahoma was not recommended as a result of N tie-up by cowpea residues and a tendency of increased mortality of broccoli transplants in cowpea plots. In a transition field in Vermont, Schellenberg et al (2009) found that broccoli yield was unaffected by various leguminous cover crop applications and a moderate rate of in-season N was recommended. N immobilization could be avoided by incorporating the cover crop at an earlier growth stage when the C:N ratio is lower (Diniz et al, 2007); however, this may not always be an option for growers, especially in central coastal California because spring rains and wet soil conditions can prohibit this practice.…”

“…Work has shown that the rate of mineralization of available N from a low C:N (less than 20) cover crop typically increases over a 3-to 6-week period after incorporation and then returns to pre-incorporation levels by Week 6 to 8 in this region, the central coast of California (Gaskell, 2004). Therefore, a cover crop can be a valuable source of short-term N but longer season vegetable crops after a cover crop rotation may require supplemental applications of fertilizer N to supply late-season N (Delate et al, 2008;Schellenberg et al, 2009;Schonbeck et al, 1993;Scow et al, 1994;Smith, 1992;Temple et al, 1994).…”

No Tillage and Strip Tillage Effects on Plant Performance, Weed Suppression, and Profitability in Transitional Organic Broccoli Production

“…This would indicate that N may not have been limiting for NT and ST treatments in this study. However, because leaf N measurements were taken at only one point in time (7-8 weeks after planting), it is possible that plants in NT and ST treatments experienced N deficiency early in the 2014 season (Schellenberg et al, 2009).…”

“…Some studies found that NT treatments yielded as much or more than CT in production of tomatoes (Creamer et al, 1996a;Delate et al, 2012), bell pepper (Delate et al, 2008), spinach (Lounsbury and Weil, 2015), and onion (Vollmer et al, 2010), while others found that NT treatments yielded poorly (Díaz-P erez et al, 2008;Leavitt et al, 2011). Schellenberg et al (2009) found that an NT system using flail mowed warmseason legume cover crops [lablab (Dolichos lablab L.), soybean (Glycine max L.), sunn hemp (Crotalaria juncea L.), and a mixture of sunn hemp and cowpea (Vigna sinensis Endl.)] had similar yields for spring broccoli, but reduced yields for fall broccoli, compared with CT.…”

Contribution of Cover Crops and Reduced Tillage Systems for Weed Management in Organic Vegetable Production

“…Similarly, hairy vetch was determined to be the best cover crop for no-till tomato production without compromising yield (Abdul-Baki et al 1996). Schellenberg et al (2009) found that a no-tillage system using flail mowed warm-season legume cover crops Egyptian bean, soybean, sunn hemp, and a mixture of sunn hemp and cowpea had similar yields for spring broccoli (Brassica oleracea L. var. italic), but reduced yields for fall broccoli, compared with conventional tillage.…”