The elements essential to plant health are classified as macronutrients, which are needed in the largest quantities, or micronutrients, required in minute amounts. Both are necessary. The primary macronutrients are nitrogen (N), phosphorus (P) and potassium (K). The relative proportions of these nutrients are listed as N-P-K on fertilizer labels.
Stimulates plant root growth and the uptake of other nutrients. Plants deficient in nitrogen tend to be chlorotic– pale yellowish green– and stunted with thin, spindly stems. Most nitrogen is derived from the decomposition of organic matter and nitrogen fixation by bacteria. To maintain nitrogen levels in fertile soils, mimic natural processes by mulching with compost or other organic matter. Alfalfa, blood meal or other natural fertilizers, or nitrogen-fixing green manures can increase the nitrogen levels of infertile soils.
Phosphorus enhances photosynthesis, nitrogen fixation, flowering, fruiting and seed production. It also encourages root development. Symptoms of phosphorus deficiency include delayed flowering or fruit set and a purplish cast on leaves and stems. Bone meal is a natural fertilizer high in phosphorous that also contains calcium. It is commonly used early in the season during planting. Bone meal is a slow-acting and long-lasting fertilizer, but it can attract animals so it is commonly mixed with compost before adding to the soil.
Potassium is known to activate 80 enzymes responsible for basic plant processes such as carbohydrate metabolism and photosynthesis. It is critical to reducing the loss of water from leaves and increases the ability of the roots to take up water. Adequate soil potassium is linked to improved drought tolerance, improved winter hardiness, better resistance to some fungal disease and greater tolerance of insect pests. When plants suffer from potassium deficiency, the tips and edges of the oldest leaves yellow and die and appear burned around the edges. Compost can help maintain good potassium levels in fertile soil, while kelp meal is a renewable source that can help raise potassium levels in deficient soils. Composted wood ash is another source of potassium, but should be used only on acidic soils.
Other nutrients are considered secondary because they are typically found in sufficient quantities in the soil and do not require amendments. Secondary nutrients include calcium, magnesium and sulfur. Micronutrients or trace elements include iron, boron, copper, manganese, zinc, chlorine and molybdenum. Except in highly acidic or alkaline soils, micronutrients deficiencies are uncommon, and a balanced supply can be maintained with regular applications of organic matter.