One sign of a healthy plant is the bright green of its leaves, though each type has its typical shade. The color comes from chlorophyll, a chemical that is vital to the process of photosynthesis by which plants produce the food they need to grow. Nutrients needed to produce chlorophyll are usually present in the soil and absorbed by the roots, but may need to be supplemented by fertilizer applications.
The element that has the greatest impact on leaf color is nitrogen. Not only is it needed in substantial quantities, but it is often in short supply in the soil, especially in cold spring ground. Nitrogen is essential to the production of proteins, enzymes and chlorophyll and is usually absorbed by roots in the form of nitrate or ammonium. The conversion of atmospheric nitrogen to forms that plants can use is dependent on microorganisms in the soil that are also active in the decay of organic matter, which releases nitrates. Nitrogen is readily lost by leaching, the movement of water downward carrying dissolved elements.
Phosphorous is mainly used in flower, fruit and seed production, but a deficiency will cause slow growth and a purplish hue to the leaves. It is fairly stable in the soil and does not leach out of the root zone. Sources of phosphorous include bonemeal and rock phosphate.
This nutrient is also less important for leaf production, but a deficiency will be noticed as a yellowing of the edges of mature leaves. Gardeners might be tempted to add nitrogen, but, in this case, that would have little effect. Good sources of potassium include kelp meal and greensand.
Magnesium is considered to be a secondary nutrient needed by plants in large quantities but usually available in the soil. It is essential for the production of chlorophyll and plant enzymes. One of the symptoms of a magnesium deficiency, however, is chlorosis, the yellowing of the leaves between the veins, leaving a network of green across the leaf. Dolomite limestone is a good source.
A deficiency of iron also causes chlorosis. Iron is a micronutrient, needed in small quantities, but often occurs when the pH of the soil is too low or too high for roots to absorb the minerals present. Test for pH level and adjust to suit the plant you are growing.