Anyone who has grown a garden, maintained a lawn, or kept house plants knows that it is necessary to
apply a fertilizer to the soil to keep cultivated plants healthy. Asthey grow, plants extract nutrientsthey need from
the soil. Unless these nutrients are replenished, plants will eventually cease to grow. In nature, nutrients are
returned to the soil when plants die and decay. However, this does not occur with cultivated plants. Humans
cultivate plants mainly for food, either for themselves or for livestock. When cultivated plants are harvested, the
nutrients that the plants extracted from the soil are taken away. To keep the soil productive, it is necessary to
replace these nutrients artificially. The kinds and amounts of nutrientsthat plants need have been determined and
can be supplied by applying to the soil substances that contain these nutrients.
A plant contains a great number of chemical compounds. The major compound in all plants is water. The
percent of the plant's weight that is water varies greatly from one kind of plant to another, from less than 20% to
more than 90%. After the water isremoved, the bulk of the dry plant material consists of carbohydrate compounds
containing the elements carbon, hydrogen, and oxygen. Using the energy of sunlight in a process called
photosynthesis, plants make carbohydrates in their leaves. The carbon and oxygen in carbohydrates come from
carbon dioxide, which the plant absorbs from the air, and the hydrogen comes from water absorbed both through
the roots and through the leaves. About 90% of the weight of carbohydrates is carbon and oxygen. Therefore, a
plant obtains around 90% of its dry weight from the air.
Although carbohydrates account for most of the dry weight of a plant, the plant contains smaller amounts
of other compounds that are necessary for its growth. Plants contain proteins, which are essential in the chemical
reactions of photosynthesis. Proteins contain the element nitrogen in addition to carbon, oxygen, and hydrogen.
Some proteins contain sulfur as well. Plants also contain DNA, which carriesthe genetic information that controls
how a plant grows. DNA containsthe element phosphorus, in addition to nitrogen, carbon, oxygen, and hydrogen.