The three main nutrients that have been identified as absolutely necessary for plants are nitrogen (N), phosphorus (P) and potassium (K). These three are also known as macronutrients, and are the source of the three numbers commonly found on organic fertilizer labels.

Nitrogen (N) is responsible for above-ground vegetative growth of plants, and for overall size and vigor. It is probably best known for its ability to "green up" lawns. That's because nitrogen is a major component of chlorophyll, the green substance in plants responsible for photosynthesis. Nitrogen can be added to your soil through composted manure, blood meal, canola meal, and fish powder. Too much nitrogen and your plants will grow extremely fast, resulting in long, spindly, weak shoots with dark green leaves. Too little nitrogen and your plants will slow or even stop their growth, and have leaves turning yellow and dropping sooner than they should.

Phosphorus (P) promotes healthy growth, strong roots, fruit and flower development, and greater resistance to disease. Rock phosphate, bone meal and some guanos are sources of phosphorous. A phosphorus deficiency is recognized by dull green leaves and purplish stems. Plants are generally unhealthy, sometimes yellowing. Lack of blooming with lush green foliage may also indicated a lack of phosphorus.

Potassium (K), also known as potash, is essential for the development of strong plants. It helps plants to resist diseases and protects them from the cold. Because potassium plays a supporting role, it can be hard to spot deficiencies. Generally, leaves will show blue, yellow or purple tints with brown blotches or discoloration within or at the edges. Plants will lack growth and have small fruit and sickly blooms. Sources of potassium include greensand, sul-po-mag (sulfate of potash magnesia, quick release) and many liquid fertilizers.