Maintenance of ecosystem services from forests, such as C sequestration and wood production, is dependent upon an adequate supply of mineral nutrients. Unlike agricultural systems, forest ecosystems are rarely deliberately fertilised. Thus, the cycling of nutrients between the soil and the plants is one of the defining aspects of forest ecosystem functioning. The availability of nutrients for plant uptake can limit the productivity and even the very survival of ecosystems; many forest ecosystems are limited by N and/or P. Crucial for plant metabolism and growth, the accessibility of nutrients in the soil is the result of several biogeochemical processes, often involving complex feedbacks. Only 17 elements have been identified as essential to higher plants, but inaccessibility of any of them makes it impossible for plants to complete their life cycle. Based on the amount of each element required for normal plant growth, nutrients are commonly classified as macronutrients and micronutrients.
Macronutrients are required in large amounts and usually constitute more than 0.1% dry mass. They are divided into two groups: non-mineral macronutrients (C, H, O) and mineral nutrients (N, P, K, Ca, Mg, S). Macronutrients are required in such high amounts as they are often in structural elements in plants such as cell walls, membranes, nucleic material or proteins (N, P, Ca), or required in high amounts in the cytoplasm and vacuole for regulating enzyme activity or processes such as osmoregulation (K, Mg).