Effects of green manure crops on short-term nitrogen availability in organic sweet corn systems : a thesis presented in partial fulfilment of the requirements for the degree of Master of Applied Science in Soil Science

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Massey University
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In the Gisborne Region of New Zealand (NZ) many organic sweet corn growers use a range of winter green manure crops as a means of maintaining and improving soil fertility, particularly the availability of soil N. Some debate exists as to the most suitable green manure crops and their effectiveness at improving short-term N availability for subsequent sweet corn crops. Two field trials were conducted in the Gisborne Region to assess the effectiveness of four winter green manure crops using a subsequent sweet corn crop to evaluate N availability. Two sites, Site-A at Tekaraka and Site-B at Tolaga Bay, with BIO-GROW NZ organic certification were used in this study. A Latin Square trial design was used at each site consisting of 25 plots made up of five replicates of each of the following five treatments: control (bare soil), blue lupin (Lupinus angustifolus), mustard (Brassica sp.), mustard/blue lupin mix and annual ryegrass (Lolium multiflorum). Just prior to the soil incorporation of green manure treatments (early-mid September 1997), the lupin crop had the highest N concentration and N accumulation levels of 2.1% N and 156 kg N ha-1, respectively, at Site-A and 2.1% N and 173 kg N ha-1, respectively at Site-B. Soil incorporation of green manure treatments significantly influenced soil (0-150 mm) mineral N (nitrate and ammonium) levels measured at sweet corn emergence (30 November 1997) and at 51/2 weeks post emergence. At sweet corn emergence the lupin, mustard/lupin mix, mustard, control and ryegrass treatments resulted in soil mineral N values of 68, 66, 57, 51 and 29 kg.N.ha-1, respectively, at Site-A and 118, 118, 91, 81 and 54 kg.N.ha-1, respectively, at Site B. At both sites, the lupin and mustard/lupin mix treatments resulted in soil mineral N levels significantly higher than the control treatment. In contrast, the ryegrass treatment resulted in soil mineral N levels significantly lower than the control treatment. These treatment effects were related to green manure crop N concentrations just prior to soil incorporation. On average over both sites, the lupin and mustard/lupin mix treatments, which had high DM yields (7900 kg and 6500 kg.DM.ha-1 respectively), had the highest N concentrations (2.0% and 2.1% N respectively). The ryegrass treatment, which also accumulated a high average DM yield (6200 kg.DM.ha-1), contained the lowest average N concentration of only 1.1% N. Sweet corn N accumulation at harvest was also significantly influenced by green manure treatments. At both sites, ryegrass significantly reduced sweet corn N accumulation compared with all other treatments, being 44% and 36% lower than control treatment value of 117 kg.N.ha-1. At Site-A, the lupin, mustard/lupin and mustard treatment effects on sweet corn N accumulation were not different from that of the control treatment at final harvest. However, at Site-B the lupin and mustard/lupin mix treatments did produce sweet corn N accumulation levels significantly higher than the control treatment; being 21% and 18% higher than the control value of 102 kg.N.ha-1, respectively. Compared to the control treatment sweet corn yield (17.3 t ha-1 averaged over both sites), none of the four green manure treatments improved sweet corn yield even though the lupin and mustard/lupin mix treatments both increased soil N availability and sweet corn N accumulation. Soil moisture limitations probably restricted yield potentials. However, the ryegrass treatment detrimentally affected sweet corn yields at both sites. When compared to the control treatment reductions of 64% and 48% at Site-A and Site-B, respectively, were measured. Soil mineral N (0-150 mm) tested early in the sweet corn growing season gave a better relationship with sweet corn N accumulation and yield compared with the incubation tests used. Short-term soil incubation tests, conducted under aerobic and anaerobic conditions, were not useful as indicators of net N mineralisation as they did not relate well to actual soil N mineralisation or crop response. Although both the lupin and the mustard/lupin mix treatments had similar effects on soil N availability and sweet corn N accumulation, of the two the lupin treatment achieved a higher level of estimated N fixation. On average the estimated N fixation in the lupin treatment (98 kg N ha-1 averaged over both sites) was higher than N losses in harvested sweet corn ears (77 kg N ha-1 averaged over both sites). This positive N balance would help compensate for other possible N losses from the soil-plant system (ie. ammonia volatilisation or nitrate leaching). Overall, the lupin green manure treatment appears be the best crop in terms of improving short-term N availability for the subsequent sweet corn crop and for maintaining an N balance in the soil-plant system. But ultimately, the benefit of lupin as a green manure crop will also depend on environmental conditions and management practices.
Fertilizers, Sweet corn, Green manuring