As living organisms, plants have a basic set of requirements in order to grow and thrive. Most people might guess correctly that plants need sunlight, soil and water to be healthy, but several other ingredients are also key: space, air and protection. However, even the obvious requirements are only beneficial if they're provided in the proper amounts---too much water, for instance, will kill a plant as easily as one that is under watered.
Sunlight requirements vary widely from plant to plant, depending on its natural growth habit, native habitat and function. The vast majority of vegetable plants, for instance, require a minimum of six to eight hours of sunlight daily; ferns, on the other hand, typically require far less, perhaps four to six hours of light, dappled sunlight each day. Energy from the sun is the primary driver of a plant's powerhouse. Without solar photons or their equivalent, the photosynthesis that produces sugars for energy cannot take place.
Soil provides the foundation for roots to grow and take up nutrients and minerals necessary for growth and reproduction. All soils are made of particles consisting of decomposed organic matter, pulverized minerals and elemental nutrients, including those elements necessary for proper plant growth: nitrogen, potassium, phosphorous, carbon, oxygen, calcium, magnesium and sulfur. Plants also require a cocktail of other nutrients and minerals in tiny amounts, including boron, chlorine, copper, iron, manganese, molybdenum and zinc, all of which are available in most soils in adequate amounts.
Plants need room, not only for air to flow properly around foliage but also so roots have the right environment in which to spread. Outdoor plants should be spaced so that at their mature size, they are just barely touching; indoor plants need extra room, since indoor airflow lacks cross breezes.
Plants are known for their reverse respiration process. While animals inhale oxygen and exhale carbon dioxide, plants do just the opposite. Plants growing in environments lacking adequate fresh air, such as those areas contaminated by airborne pollutants, exhibit symptoms such as stunting, wilting and deformed leaves or fruit. Plants need good levels of both oxygen and carbon dioxide for respiration and the photosynthesis process, respectively.
Plants growing in potentially difficult conditions need protection from too much sun, wind, trampling feet or marauding pests. Gardeners in areas overrun by deer are constantly hunting for ways to deter the animals from eating their plants. Trees, shrubs and plants in urban areas need protection from people; soil compaction can be a major problem, creating dense soils through which roots can barely penetrate. Vegetable gardeners are always aware of nighttime temperatures in the spring and fall, as a freak late frost will kill tender plants like tomatoes and cucumbers.