Plant growth and nutrient uptake response to increased fertilization can be conceptually described by cur vi linear relationships depicting phases of nutrient deficiency, sufficiency, luxury consumption, and toxicity to rationalize fertilizer prescriptions and improve nutrient diagnosis. We validated this model to determine optimum nitrogen storage of young black spruce (Picea mariana (Mill.) BSP). Container seedlings were supplied with a mixed nitrogen–phosphorus–potassium (N–P–K) fertilizer at rates ranging from 0 to 80 mg N/seedling and reared in a greenhouse for one growing season. Plant growth and nutritional parameters of the plants exhibited classic responses of N deficiency, luxury consumption, and toxicity that were corroborated by vector diagnosis and appeared consistent with the conceptual model. Seedling biomass production was maximized at sufficiency (30 mg N/seedling), whereas N content of tissues peaked at the optimum loading rate (64 mg N/seedling). Toxicity occurred at the 80 mg N/seedling dose rate that increased plant N concentration (5%) but reduced growth (17%) and N content (14%) relative to the optimum level. Plant N content was raised 150% by optimum loading, exemplifying the effectiveness of this practice for building internal N reserves prior to planting. The newly validated model will help refine fertilizer recommendations and nutrient diagnosis for other species or cultural systems.

Access to knowledge on farm management practices is essential for the maintenance of productive agroforestry systems. Farmers who lack the means to acquire farming knowledge from formal sources often rely on information within their informal social networks. However, little research has explored the explicit structure of farmer communication patterns. We examined advice network structures by using farmer attributes, i.e., kin relationships, community involvement, and imitation, to characterize structural positions and investigated the consequences of such structure on farming practices in cocoa agroforestry systems in Ghana, West Africa. Furthermore, we used a multicommunity approach; we constructed networks for four communities to increase replication and enhance the generality of our conclusions. A high density of advice ties occurred among a small group of farmers, indicating a coreperiphery structure. Settler farmers composed 73% of core position members, suggesting that social proximity did not control the formation of informal advice structures. Because core farmers were highly participative in community activities, the promotion of community involvement may facilitate the movement of knowledge and social exchange to strengthen informal networks. Farmers in both core and peripheral structural positions indicated that they observed fellow farmers and subsequently adopted their practices. Of highly sought farmers, 84% used external information, predominately from government institutions, thus functioning as bridging links between formal and informal networks. Both external and farmer-derived sources of knowledge of agroforestry practices were transferred through informal advice networks, providing available information throughout the farming community, as well as a foundation for community-based adaptive management.

Nutrient loading stimulates N retranslocation, an important mechanism of N reuse in plants to support new growth. We quantified N retranslocation in young black spruce [Picea mariana (Mill.) BSP] using tracer and nontracer techniques to examine enhanced field performance after nutrient loading. Nursery reared seedlings were transplanted to sand‐filled pots fertilized with 15NH4 15NO3 at rates equivalent to 0 and 200 kg N ha−1 simulating poor and rich soils. After one growing season (120 d), biomass increased (118%) on the poor soil without N gain demonstrating the significance of internal N reserves for retranslocation to new growth. Nutrient loading improved retranslocation (218%) and new biomass (156%) after planting confirming the advantage of higher preplant N reserves (175%) for later nutrient demand. Enhanced N availability in the rich soil accelerated growth (236%), N uptake (258%), and retranslocation (23%) in seedlings. Retranslocation increased with time reflecting higher N demand as seedlings become larger and suggest the process is driven by sink strength. Nontracer estimates of N retranslocation in seedlings fell short of isotopic determinations because of inability to discriminate between soil and plant derived N in tree components. Although fertilization promoted N uptake (125–258%), 15N recovery in plants averaged 12 to 19% indicating low fertilizer efficiency in young trees. Total reliance of unfertilized plants on internal N reserves for growth on the poor soil affirms the importance of retranslocation to meet plant N demands, and also exemplifies initial short‐term independence on soil N for newly planted seedlings that can be prolonged by nutrient loading.

Foliar analyses of individually fertilized balsam fir trees [Abies balsamea (L.) Mill.] were examined to explain significant growth responses and nutrient status changes. Treatments consisted of factorial combinations of N, P, and K, and one separate treatment of dolomitic lime. Simultaneous comparisons of first year changes in foliar nutrient concentration, content, and dry weight simplified assessment of deficiency, dilution, and luxury consumption. N fertilization increased both needle dry weight and N composition, but lowered foliar P, K, Ca, and Mg concentrations by dilution. Additions of P and/or K did not change needle weight and led to luxury consumption of P and K. Lime alone had no measurable effect in the first year. Overall correlations between foliar concentration and content of individual nutrients accounted for 68–78% of the total variability for foliar P, K, Ca, and Mg, but 99% for foliar N which was unaffected by dilution. A concentration of 2.1–2.3% N in current foliage is the approximate upper limit for growth response for these balsam fir trees.

Farm product diversification, shade provision and low access to fertilizers often result in the purposeful integration of upper canopy trees in cocoa (Theobroma cacao) plantations. Subsequent modification to light and soil conditions presumably affects nutrient availability and cocoa tree nutrition. However, the level of complementarity between species requires investigation to minimize interspecific competition and improve resource availability. We hypothesized beneficial effects of upper canopy trees on cocoa biomass, light regulation, soil fertility and nutrient uptake. We measured cocoa standing biomass and soil nutrient stocks under no shade (monoculture) and under three structurally and functionally distinct shade trees: Albizia zygia (D.C.) Macbr, a nitrogen fixer; Milicia excelsa (Welw.), a native timber species; and Newbouldia laevis (Seem.), a native small stature species. Vector analysis was employed to diagnosis tree nutrition. Cocoa biomass was higher under shade (22.8 for sole cocoa versus 41.1 Mg ha −1 for cocoa under Milicia), and declined along a spatial gradient from the shade tree (P < 0.05). Percent canopy openness differed between the three shade species (P = 0.0136), although light infiltration was within the optimal range for cocoa production under all three species. Soil exchangeable K was increased under Newbouldia, while available P decreased and total N status was unaffected under all shade treatments. Nutrient uptake by cocoa increased under shade (43-80% and 22-45% for N and P, respectively), with K (96-140%) as the most responsive nutrient in these multistrata systems. Addition of low-density shade trees positively affected cocoa biomass close to the shade tree, however proper management of upper stratum trees is required for optimum cocoa productivity and sustainability.