Carbon, oxygen, and hydrogen are derived from air and water, but N, P, K, Ca, Mg, S, and various trace elements must be present in usable form in the soil for plants t survive and grow. The acidity or alkalinity of the soil also influences plants. Extremes of soil acidity or alkalinity are toxic in themselves, but even slight variations from neutrality affect availability of soil nutrients and must be corrected. Soil structure determines the plant's supply of water and oxygen, the ease of root growth and the availability of nutrients. In general, adding decaying organic matter to soil improves its texture.
Water is essential for growth.
It may be added to the soil through irrigation. too much water in the soil suffocates or rots plants and must be drained off. Soil is not the only medium for plant growth. In hydroponics plants are grown either in water to which all necessary nutrients have been added, or in sand or gravel with nutrients added. Plants are subject to a wide variety of injuries and diseases caused by viruses, bacteria fungi, other green plants, worms, insects, mites, birds, and animals. Pest control involves such different techniques as using chemicals toxic to pests, breeding disease resistant plants, and introducing natural predators to kill pests.