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Every living thing—and a plant is no exception—must obtain raw materials from its environment. These nutrients include the major ingredients of macromolecules: carbon, hydrogen, oxygen, and nitrogen. Plants are autotrophs, and obtain both carbon and oxygen from the atmosphere through the reactions of photosynthesis and cellular respiration (see Chapters 9 and 10). Hydrogen comes mainly from water, so it is plentiful when there is an adequate water supply. Nitrogen, as you will see later in this chapter, enters most plants from the soil. The activities of microorganisms are important in converting organic nitrogen and nitrogen gas into inorganic forms that are usable by plants.

In addition to nitrogen, organisms require other mineral nutrients: inorganic elements that are used for various cellular processes. For example, proteins contain sulfur (S), nucleic acids contain phosphorus (P), chlorophyll contains magnesium (Mg), cytochromes contain iron (Fe), and cellular signaling can involve calcium (Ca). Most plants obtain these nutrients from the soil. Within the soil, minerals dissolve in water as ions, forming a solution—called the soil solution—that contacts the roots of plants.