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K. Francis Salifu, Douglass F. Jacobs, Guillermo Pardillo, and Mary Schott

We examined growth and nitrogen (N), phosphorus (P), potassium (K), and microelement nutrition of grafted black walnut (Juglans nigra L.) seedlings exposed to increasing nutrient supply and grown in the greenhouse for 18 week. Plants were potted and grafted within the first 4 week, then fertigated once each week for a 7-week period with a varying nutrient solution of 20N–4.4P–16.6K that delivered 0, 1160, 2320, and 4620 mg N/plant. Plants were harvested at week 18. There was a positive mean growth response to increased fertilization, although trends were statistically similar across treatments. Leaf nutrient concentration ranged from 22 to 31 g · kg–1 N, 5 to 14 g · kg–1 P, and 19 to 25 g · kg–1 K. The 2320 mg N/plant treatment increased leaf nutrient content 18% to 86% for N, 33% to 303% for P, and 23% to 58% for K compared with the control. Nitrogen efficiency decreased with increased N supply. Increased nutrient retention in the growing medium at higher fertility suggests root plugs could serve as immediate critical nutrient sources for grafted black walnut seedlings after outplanting. Study results suggests nursery fertilization can be used to improve the nutritional quality of grafted black walnut as well as store nutrients in root plugs for later utilization to benefit early establishment success.

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Mindy L. Bumgarner, K. Francis Salifu, and Douglass F. Jacobs

Container seedling production systems for forest tree nurseries are challenged by the need for judicious water consumption. Subirrigation systems may provide an alternative to overhead systems by mitigating water use, yet remain relatively untested for propagation of forest tree seedlings. We evaluated effects of overhead versus subirrigation under varying media (40:60, 60:40, and 80:20 peat:perlite) and fertilization (0 or 1.2 g nitrogen/plant) regimes on nursery development and first-year field performance of northern red oak (Quercus rubra L.) seedlings. Fertilization increased aboveground biomass production and nutrient content, but decreased root dry weight. Relative to overhead-irrigated seedlings, subirrigation increased red oak seedling aboveground biomass production as well as above- and belowground nitrogen content under fertilized conditions. Media had no effect on plant response. Subirrigation increased electrical conductivity (5 dS·m−1 greater) and decreased pH in the upper media zone (0 to 5 cm from top), whereas the opposite effect was found in the lower zone. Nursery fertilization was associated with reduced field survival and growth, which may have been the result of transplant stress resulting from higher shoot:root. Subirrigated seedlings had greater field diameter growth. Our results suggest that subirrigation could serve as a viable alternative to overhead systems in container propagation of hardwood seedlings.

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R. Kasten Dumroese, Douglass F. Jacobs, and Anthony S. Davis

Restoration of Acacia koa A. Gray (koa) forests on degraded sites in Hawaii is important for conservation of rare, endemic plants and animals and is often accomplished by planting nursery-grown seedlings. To be successful after outplanting, koa seedlings must access sufficient nutrients from the soil and outcompete other vegetation. Forming symbiotic associations with nitrogen-fixing microorganisms may help alleviate inherent nutrient deficiencies found on volcanic sites, but interactions between inoculation and fertilization during production of koa seedlings are not well studied. Under operational nursery conditions, we fertilized koa seedlings at six controlled-release fertilizer (CRF) rates (ranging from 0 to 11.9 kg·m−3 Osmocote Plus 15N–9P–12K), which were also either inoculated with a locally collected source of nitrogen-fixing Bradyrhizobium spp. or not. Fertilized seedlings were significantly larger than the control; on average, fertilized seedlings were 300% taller and had 180%, 700%, and 350% more root-collar diameter, shoot biomass, and root biomass, respectively. Nitrogen (N) concentration was 23% to 119% greater in fertilized seedlings than nonfertilized seedlings. Inoculation with Bradyrhizobium had no significant affect on seedling characteristics except for root N concentration, which was 8% higher for inoculated seedlings. Fertilizer rate and inoculation interacted to significantly affect nodule number and nodule biomass/seedling. With inoculation, we observed an increase in nodule number (16.5 to 25.5) and biomass (5.8 mg to 11.7 mg) with fertilizer rate up to 4.7 kg·m−3 but a decline to nearly zero at the highest rates. Without inoculation, nodule formation was scarce (3.2 nodules/seedling or less) and only observed when CRF was 2.3 kg·m−3 or less. Our results suggest that high rates of fertilization and successful nodulation are not mutually exclusive nursery practices. Concurrent use of fertilization and inoculation in the nursery may provide opportunity to produce larger seedlings that have high potential to reduce the time needed to achieve canopy closure and thereby help achieve restoration objectives.

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Mindy L. Bumgarner, K. Francis Salifu, Michael V. Mickelbart, and Douglass F. Jacobs

Excessive fertilization may induce physiological drought and/or ion toxicity, which can reduce growth or cause mortality in cultured plants. Although nursery subirrigation produces stock of forest trees of equal or better quality to conventional overhead irrigation, detailed analyses of fertilization responses specific to these systems are lacking. We evaluated the effects of fertility applied as a 15N–9P–12K controlled-release fertilizer at rates equivalent to 0, 1.2, 1.8, 2.4, 3.0, or 3.6 g nitrogen (N) per plant on media properties and northern red oak (Quercus rubra L.) seedling development grown with subirrigation. Aboveground plant growth and nutrient content of seedlings increased up to 1.8 g N/plant but declined at higher rates and total mortality occurred for treatments of 2.4 to 3.6 g N/plant by the end of cultivation. Root biomass generally declined with increasing fertilization. Media electrical conductivity (EC) increased with increasing fertility, particularly in the upper media layers, where values exceeded 3.0 dS·m−1 at the highest rates. Fertilization had little effect on media pH. Predawn leaf water potential and osmotic potential (ψS) were reduced at high nutrient applications. Thus, increasing fertility beyond ≈1.8 g N/plant in this subirrigation system apparently resulted in accumulation of excessive fertilizer salts in media and/or ion toxicity, which caused plant mortality. Because subirrigation systems are prone to persistence of residual fertilizer salts in the medium and holding tanks, fertilization prescriptions must be carefully tailored to species and cultural systems to prevent potential for plant damage associated with overfertilization.

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K. Francis Salifu, Michael A. Nicodemus, Douglass F. Jacobs, and Anthony S. Davis

We evaluated suitability of chemical indices of three media formulations or substrates (A, B, and C) consisting of composted pine bark, coconut coir pith, sphagnum peatmoss, processed bark ash, and perlite in varied proportions for growing northern red oak (Quercus rubra L.) seedlings. These substrates were ranked according to their ability to promote seedling growth. The low-yielding substrate (A) was devoid of pine bark and perlite and the medium-yielding substrate (B) contained no peatmoss or processed bark ash. The high-yielding substrate (C) contained all components. Additionally, we tested plant response to high nitrogen (N) fertilization on each substrate. Media EC, pH, and total dissolved solids measured at transplanting explained 68%, 43%, and 66%, respectively, of the variation in plant dry weight and 39%, 54%, and 46%, respectively, of the variation in shoot height. Vector diagnosis effectively ranked nutritional limitations on seedling growth as N > P > K. High N fertilization highlighted element deficiency in seedlings grown on substrate A, but resulted in element toxicity and antagonistic interactions in plants established on substrates B and C, respectively.