Rice-husk biochar effects on organic carbon, aggregate stability and nitrogen-fertility of coarse-textured Ultisols evaluated using Celosia argentea growth

There are insufficient data supporting the enormous potential of biochar in highly weathered tropical soils. This glasshouse study assessed rice-husk biochar (RHB) effects on soil organic carbon, aggregate stability and nitrogen fertility of sandy-loam Ultisols which were evaluated using spinach (<em>Celosia argentea</em>) growth. Five RHB rates 0, 5, 10, 20, and 40 g per two-kg-soil (0, 7.5, 15, 30 and 60 t ha⁻¹, respectively) were studied under 0, 4, 8, and 12 weeks of incubation (WOI). Batched potting of treatments enabled sowing on one date. Treatment effects on soil quality were assessed at sowing and spinach growth six weeks later. Soil organic carbon generally increased with RHB rate, with the greatest increments (37%) in maximum rate relative to no-biochar control for 8 WOI. Aggregate stability also generally increased with RHB rate, the range being 7.21%-17.21% for 8 WOI, beyond which it decreased in 10 and 20 but not 40 g pot^–1. Total nitrogen was always highest in maximum rate, increasing with rate only for 8 WOI. Treatment affected plant height more clearly than leaf count. Optimum rates were 5 or 10 g pot^–1 for 8 and 4 WOI, respectively (plant height) and 10 g pot^–1 for 8 WOI (leaf count). Soil organic carbon influenced soil aggregate stability (R² = 0.505) which in turn was quadratically related to plant height (R² = 0.517), indicating stability threshold for spinach. Adding RHB at 40 g pot^–1 (≈ 60 t ha⁻¹) to coarse-textured tropical soils is suggested to sustain its soil aggregating effect beyond the growth phase of short-cycle leafy vegetables which require a lower rate (10 g pot^–1) 8 weeks before sowing. The observed role of soil aggregate stability in spinach growth rather than the overall effects of RHB should guide further search for edapho-agronomic optimum rate of RHB.

Sweet Corn Ontogeny in Response to Irrigation and Nitrogen Fertilization

To achieve optimum quality, sweet corn should be harvested at the milking stage, therefore understanding of plant phenology could be the most important aspects for economic return in this crop. Phenological sensitivity to the environment could be especially important in the management of water and nitrogen. In the current research, sweet corn ontogeny in two years was monitored in response to irrigation and nitrogen fertility: three water regimes and five nitrogen levels. The results showed that nitrogen and water application significantly affected duration in sweet corn between emergence and silking. As nitrogen and water level was increased, the days and cumulative temperature units (TU, °C) from sowing to silking significantly increased. In 2014, sowing to silking ranged from 66 days equal to 1035 TU with deficit water and nitrogen treatment to 72 days equal to 1140 TU at full irrigation and highest nitrogen treatment. In 2015, the range of sowing to silking was from 67 days equal to 1090 TU, to 73 days equal to 1180 TU. In contrast, neither nitrogen nor water treatments had a large influence on the duration of the silking to milking period. Across the two years the duration of silking to milking was approximately 506 TU. Therefore, once silking date had been resolved harvest date of sweet corn could be readily predicted independent of water or nitrogen treatment as occurring about 506 TU following silking.

Challenges, strategies and research priorities in legume-based nitrogen management for organic small grain producers in the Northeastern US

High yields and crop quality in organic small grain production can only be achieved through successful management of nitrogen. Experienced farmers and advisors in the Northeastern U.S. were asked to discuss the most pressing challenges in organic nitrogen management for organic small grain systems, with a particular focus on legume green manures (LGMs). Eighteen semi-structured interviews with 12 farmers and eight advisors were conducted between December 2017 and March 2018. The farmers employed a range of materials and practices for fulfilling the nitrogen fertility needs of their crops, including LGMs, animal manures and organic fertilizers. Farmers and advisors identified cost, overdependence on external nitrogen sources, nitrogen source access, diversifying rotations, weed management and predicting nitrogen mineralization of organic residues as major challenges in nitrogen management. Results indicated that cost-effectiveness in nitrogen management is essential, but that farmers consider additional factors, such as weed pressure and long-term soil health effects, when choosing nitrogen sources or practices. Legume-based nitrogen fertility is promising for small grain systems in this region, but structural challenges, such as the lack of animal agriculture proximal to grain operations, and limited cash crop markets, impede the development of diverse rotations that feature long-term legume sods. Recommendations include additional field-based research, including on-farm, participatory LGM studies as one avenue. Programming and educational outreach should focus on bolstering farmers' understanding of nitrogen mineralization of incorporated LGM residues, as well as their ability to anticipate and respond to sources of variability in LGM systems.