Does Rootstock and Fertilizer Choice Affect Apple Orchard Productivity and Soil Community Ecology?

Rootstock genotype and soil fertility management practices in apple (Malus domestica Borkh.) orchards impacts soil health and nutrient status, plant associated soil microbial communities, and tree growth and fruit yield. Growers select specific apple rootstocks for use in their orchard systems to confer beneficial traits, including size control, precocity, and pest and disease resistance. Rootstock genotypes confer some of these traits through interactions with the soil microbiome. Such interactions may alter soil microbial community structure, resulting in changes to tree growth and yield. Nitrogen (N) fertilizers may improve fruit yield and quality. However, in excess of tree requirements nitrogen fertilizers may reduce crop yield and quality, and may cause environmental problems such as the contamination of ground and surface waters. The addition of carbon-based amendments, such as compost, have been shown to reduce N and water loss, while improving soil structure and making certain mineral nutrients more available to plants and microorganisms. In 2013, a pot-in-pot study was implemented to determine the effects of different composts (yardwaste and chicken litter), calcium nitrate (CaNO₃) fertilizer, and combinations of compost and CaNO₃ fertilizer on apple tree growth and nutrient status, soil health and microbial activity. For this study, ‘Gala’ scions were grafted to five rootstocks, ‘Budagovsky 9’, ‘Malling 9’, ‘Geneva 41’, ‘G. 214’, and ‘G. 935’. Four individual tree replicates of each rootstock were treated with 40 kg ha ^-1 N from either chicken litter compost, yard waste compost, CaNO₃ or a combination of 20 kg ha^-1 N from compost and 20 kg ha^-1 from CaNO₃. There was also an unfertilized control. After one growing season, there was no significant differences in leaf N status among all fertilizer treatments. However, microbial respiration was greater in soils treated with yardwaste compost compared with other treatments and the control, indicating an increase in microbial activity in this treatment. Throughout the next two years, leaf N and microbial respiration will continue to be monitored in addition to trunk cross sectional area, soil carbon to nitrogen ratios, microbial biomass, and microbial community composition. Understanding the effects of fertilization practices on orchard productivity and edaphic factors will allow us to develop more efficient fertilizer practices for high-density apple orchards.