Green‐manure impacts on nitrogen availability to organic sweetcorn (Zea mays)

Abstract

Two field experiments were conducted in the Gisborne region of New Zealand to assess the effectiveness of four winter green‐manure crops (lupin, mustard/lupin mix, mustard, and annual ryegrass) for improving the short‐term nitrogen (N) availability of soils growing organic sweetcorn (Zea mays). Average soil mineral (0–15 cm) N in the control treatment (bare soil winter fallow) plots, measured at the time of sweetcorn emergence (late November 1997), was 50.4 kg N ha‐1 at Site A and 81.3 kg N ha–1 at Site B. Compared with the control (bare soil fallow) treatment, soil incorporation of the lupin and mustard/lupin mix treatments significantly increased soil mineral N by 30–45% at both trial sites at sweetcorn emergence. In contrast, the ryegrass treatment reduced soil mineral N levels by 33–43% at both sites. These treatment effects were related to green‐manure crop N concentrations just before soil incorporation. Sweetcorn N accumulation at final harvest was also significantly increased by soil incorporation of the lupin and mustard/lupin mix treatments and significantly reduced by the ryegrass treatment. Average sweetcorn ear yields in the control treatment plots at maturity were 16t ha–1 at Site A and 18 t ha–1 at Site B. However, ryegrass treatment significantly reduced sweetcorn ear yields by 64% at Site A and 48% at Site B, which was likely to have been caused by the lower soil mineral N levels attributed to soil incorporation of the ryegrass treatment. Although the lupin and mustard/lupin mix treatments increased soil N availability and uptake by sweetcorn, these treatments did not significantly improve sweetcorn ear yield. The lack of yield response was attributed to soil moisture limitations, which occurred in the latter part of the season and were likely to have restricted yield potential.