Christmas tree cultivation has shifted toward the production of higher-quality trees, increasing land quality expectations as well. Indeed, some species of tree, such as the Fraser fir, are unable to grow on low-quality, marginal farmland. Flat or gently rolling land is preferred to that with steep slopes and inclines, which is prone to erosion and fluctuations in fertility. Noticeable obstructions, such as rocks, fences or significant underbrush, are also undesirable. Like all crops and plants, Christmas trees require a specific set of nutrients to thrive. There are 16 elements crucial for growth; of those, three are obtained through air and water: hydrogen, carbon,
Noble polyhedron 'dual' covers both kinds. The dual of a noble polyhedron is also noble. Many are also self-dual: Noble polyhedron A noble polyhedron is one which is isohedral (all faces the same) and isogonal (all vertices the same). They were first studied in any depth by Hess and Bruckner in late 19th century, and later by Grünbaum. There are four main classes of noble polyhedra: If we allow some of Grünbaum's stranger constructions as polyhedra, then we have two more infinite series of toroids: We can distinguish between dual structural forms (topologies) on the one hand, and dual geometrical arrangements when