The nutrients plants need for good health are typically divided into three groups: macronutrients, secondary nutrients, and micronutrients.
The chief plant nutrients are carbon, oxygen, hydrogen, nitrogen, phosphorus, and potassium. Carbon and oxygen are absorbed from the air (the carbon coming from carbon dioxide); hydrogen comes from water. The remaining three nutrients ― nitrogen, phosphorus, and potassium ― are all absorbed from the soil. Often referred to as "the three major nutrients," these are the elements that must most often be added to garden soil if plants are to thrive.
Nitrogen. Nitrogen is required in large quantities for the synthesis of proteins, chlorophyll, and enzymes. The more rapidly and actively a plant is growing, the more nitrogen it needs.
Plants get their nitrogen naturally through several sources, primarily from decomposing organic matter. Rainfall also carries atmospheric nitrogen into the soil; some ground water contains nitrogen, too. And specialized bacteria living on the roots of certain plants (legumes in particular) can extract nitrogen from the air spaces between soil particles and make it available for absorption.
Because nitrogen is water soluble, it is easily leached from the soil by rain and watering. If plants aren't receiving an adequate supply of nitrogen, they'll look spindly and grow more slowly than they should; older leaves may turn yellow and drop. You can make up a nitrogen deficit in several ways. In beds of ornamentals or vegetables, work compost or well-rotted manure into the soil, or apply nitrogen-containing fertilizers to the soil or to plant foliage (or to both). In vegetable gardens, you can also practice crop rotation, planting legumes such as peas or beans every third year to return nitrogen to the soil.
Phosphorus and potassium. While nitrogen is particularly necessary when plants are actively growing, phosphorus and potassium are important when the reproductive phase begins. Both play a role in the development of fruits and seeds. Potassium has another function as well: it is crucial for root formation. A lack of either phosphorus or potassium may result in decreased size or quantity of flowers or fruits. A plant suffering from potassium deficiency may also have an overall weak root system (and a decreased yield, if it's a root crop); above ground symptoms include weak stalks and stems and some curling of older leaves.
Unlike nitrogen, phosphorus and potassium are found in the mineral particles that make up the soil. Over time, they're released into the film of water surrounding the soil particles; from there, they are absorbed by plant roots. Because they do not move readily through the soil in solution (as nitrogen does), they must be applied near plant roots to do the most good. Simply sprinkling them on the soil surface won't have much effect.
Next: secondary nutrients