Christmas tree cultivation and oxygen. Nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, boron, copper, chlorine, manganese, molybdenum, iron, and zinc are obtained from the soil. If the necessary elements are not available in the local soil, nutritious fertilizers are used. Other important soil considerations include pH and drainage. Certain types of soil are preferable, depending on the type of tree. Pine trees are usually better adapted to a sandy or sandy loam soil, while white spruce trees and fir trees, such as the Douglas-fir, prefer fine-texture loams and clay loam soils. Some trees grow well in all types of soil, but in any case,
Tree walking automaton Tree walking automaton A tree walking automaton (TWA) is a type of finite automaton that deals with tree structures rather than strings. The concept was originally proposed by Aho and Ullman. The following article deals with tree walking automata. For a different notion of tree automaton, closely related to regular tree languages, see branching automaton. All trees are assumed to be binary, with labels from a fixed alphabet Σ. Informally, a tree walking automaton "A" (TWA) is a finite state device which walks over the tree in a sequential manner. At each moment "A" visits a node "v" in state