Pretransplant fertilization of containerized Picea mariana seedlings: calibration and bioassay growth response

The role of the root plug as a nutrient source for newly planted seedlings was evaluated for one growing season on soil bioassays retrieved from a boreal forest site. Intact (control) and bare-rooted (peat plug removed) black spruce (Picea mariana (Mill.) B.S.P.) seedlings reared in Jiffy pellets, some fertilized before ("spiked" with 60 mg N) or after (topdressed with 300 mg N) planting, were transplanted to potted soil blocks (bioassays) under greenhouse conditions. Compared with the intact control, bare-rooting alone reduced plant dry mass (16%) and N, P, and K (15%-25%) uptake, but increased these parameters (62%-101%) when combined with topdressing, suggesting that the root plug served as a crucial nutrient reserve soon after transplanting. Nutrient spiking or topdressing alone stimulated growth and nutrient uptake as well (35%-118%), but generated the largest response (81%-205%) when applied together. Mortality (7%-18%) occurred only with bare-rooting treatments. The responses reflected the sensitivity of seedlings to nutrient supply changes both in root plugs and in field soils. Nutrient spiking was more efficient in improving seedling performance than traditional topdressing because of reduced fertilizer requirements and closer availability of added nutrients for early root development.