Each of the nutrients mentioned (below) are used in a different place for different essential functions.

• Carbon

Carbon is what most of the plant is made of. It forms the backbone of many plant biomolecules, including starches and cellulose. Carbon is fixed through photosynthesis from the carbon dioxide in the air and is a part of the carbohydrates that store energy in the plant.

• Hydrogen

Hydrogen also is necessary for building sugars and building the plant. It is obtained from air and liquid water.

• Oxygen

Oxygen is necessary for cellular respiration. Cellular respiration is the process of generating energy-rich adenosine triphosphate (ATP) via the consumption of sugars made in photosynthesis. It is obtained from the air.

• Phosphorus

Phosphorus is important in plant bioenergetics. As a component of ATP, phosphorus is needed for the conversion of light energy to chemical energy (ATP) during photosynthesis. Phosphorus can also be modify the activity of various enzymes by phosphorylation, and can be used for cell signalling. Since ATP can be used for the biosynthesis of many plant biomolecules, phosphorus is important for plant growth and flower/seed formation.

• Potassium

Potassium regulates the opening and closing of the stoma by a potassium ion pump. Since stomata are important in water regulation, potassium reduces water loss from the leaves and increases drought tolerance. Potassium deficiency may cause necrosis or interveinal chlorosis.

• Nitrogen

Nitrogen is an essential component of all proteins, and as a part of DNA, it is essential for growth and reproduction as well. Nitrogen deficiency most often results in stunting.

• Sulfur

Sulfur is another important component of amino acids and proteins, and is therefore important in plant growth.

• Calcium

Calcium a part of cell walls. It also regulates transport of other nutrients into the plant. Calcium deficiency results in stunting.

• Magnesium

Magnesium is an important part of chlorophyll, a critical plant pigment important in photosynthesis. It is important in the production of ATP through its role as an enzyme cofactor. There are many other biological roles for magnesium-- see Magnesium in biological systems for more information. Magnesium deficiency can result in interveinal chlorosis.

• Iron

Iron is necessary for photosynthesis and is present as an enzyme cofactor in plants. Iron deficiency can result in interveinal chlorosis and necrosis.

• Molybdenum

Molybdenum is a cofactor to enzymes important in building amino acids.

• Boron

Boron is important in sugar transport, cell division, and synthesizing certain enzymes. Boron deficiency causes necrosis in young leaves and stunting.

• Copper

Copper is important for photosynthesis. Symptoms for copper deficiency include chlorosis.

• Manganese

Manganese is necessary for building the chloroplasts. Manganese deficiency may result in coloration abnormalities, such as discolored spots on the foliage.

Additional elements include nickel and silicon, whose requirements are vague for all but a very few select plants. Cobalt has proven to be beneficial to at least some plants, but is essential in others, such as legumes where it is required for nitrogen fixation. Vanadium may be required by some plants, but at very low concentrations. It may also be substituting for molybdenum. Selenium and sodium may also be beneficial. Sodium can replace potassium's regulation of stomatal opening and closing.

Plant nutrition is a difficult subject to understand completely, partially because of the variation between different plants and even between different species or individuals of a given clone. Elements present at low levels may demonstrate deficiency, and toxicity is possible at levels that are too high. Further, deficiency of one element may present as symptoms of toxicity from another element, and vice-versa. Carbon and oxygen are absorbed from the air, while other nutrients are absorbed from the soil. Green plants obtain their carbohydrate supply from the carbon dioxide in the air by the process of photosynthesis.

Cite: Wikipedia