Some orchids, such as Neottia and Corallorhiza, lack chlorophyll, so are unable to photosynthesise. Instead, these species obtain energy and nutrients by parasitising soil fungi through the formation of orchid mycorrhizas. The fungi involved include those that form ectomycorrhizas with trees and other woody plants, parasites such as Armillaria, and saprotrophs. These orchids are known as myco-heterotrophs, but were formerly (incorrectly) described as saprophytes as it was believed they gained their nutrition by breaking down organic matter. While only a few species are achlorophyllous holoparasites, all orchids are myco-heterotrophic during germination and seedling growth, and even photosynthetic adult plants may continue to obtain carbon from their mycorrhizal fungi.
^ Zhang, Guo-Qiang; Liu, Ke-Wei; Li, Zhen; Lohaus, Rolf; Hsiao, Yu-Yun; Niu, Shan-Ce; Wang, Jie-Yu; Lin, Yao-Cheng; Xu, Qing; Chen, Li-Jun; Yoshida, Kouki; Fujiwara, Sumire; Wang, Zhi-Wen; Zhang, Yong-Qiang; Mitsuda, Nobutaka; Wang, Meina; Liu, Guo-Hui; Pecoraro, Lorenzo; Huang, Hui-Xia; Xiao, Xin-Ju; Lin, Min; Wu, Xin-Yi; Wu, Wan-Lin; Chen, You-Yi; Chang, Song-Bin; Sakamoto, Shingo; Ohme-Takagi, Masaru; Yagi, Masafumi; Zeng, Si-Jin; et al. (2017). "The Apostasia genome and the evolution of orchids" (PDF). Nature. 549 (7672): 379–383. doi:10.1038/nature23897. PMID 28902843.
Most orchids are not heavy feeders. Many orchids bloom year after year with no fertilizer at all. During active growth, when new leaves are being produced, you may fertilize every other time you water at half the strength recommended on the fertilizer package. However, it's important to deliver water without fertilizer at least once a month to flush excess fertilizer salts from the bark mix and avoid fertilizer burn to the roots.