Plants require seventeen essential nutrients to grow and thrive. Three of these- carbon, hydrogen, and oxygen- are absorbed from the air and water. The remaining fourteen are obtained from the soil, but sometimes they must be supplemented with fertilizers to ensure healthy plant development. Fertilizer Are are substances that contain nutrients designed to promote and support plant growth. Of the seventeen essential nutrients, three are needed in the largest amounts: nitrogen (N), phosphorus (P), and potassium (K). These are known as primary macronutrients. The nutrient content of a fertilizer is guaranteed by the three numbers displayed on the packaging, which indicate the percentage by weight of N, P, and K. For example, a 10 pound bag of 10-5-5 fertilizer contains 1 pound of N, 0.5 pounds of P, and 0.5 K. The remaining weight consists of filler materials that help prevent caking, reduce dust, and allow for even application. 

In addition to the primary macronutrients, there are three secondary macronutrients: calcium, magnesium, and sulfur. The remaining eight essential elements are classified as micronutrients: boron, copper, chlorine, iron, manganese, molybdenum, nickel, and zinc. While micronutrients are needed in much smaller quantities, they are just as essential to plant health as macronutrients. 

Nitrogen is required in the largest amount after carbon, oxygen, and hydrogen. It is highly soluble, making it susceptible to leaching and runoff. Nitrogen levels in the soil can fluctuate rapidly due to environmental and biological factors, which is why most soil testing labs do not include nitrogen in routine soil analyses. N is critical for chlorophyll production, which gives plants their green color, and is essential for protein synthesis. Plants that are N deficient may look pale green to yellowish green.

Phosphorus is relatively insoluble and highly sensitive to soil pH. P can accumulate in soils and pose a risk of polluting waterways through runoff. It plays a key role in cell division and is important for root development, as well as flower and fruit production. Visually, P deficiency can be identified in early vegetative stages as abnormally dark green or reddish purple color along the edge of lower plant leaves. Often P deficiency is observed in early spring in low pH soils and during dry soil conditions. Where P is sufficient in soils deficiency symptoms can occur due to cold temperatures because root growth is slowed and therefore phosphorus up is limited. As temperatures warm, symptoms diminish. 

Potassium is moderately mobile in soil. It is more stable than nitrogen but more mobile than phosphorus. It is involved in plant metabolism, regulates water uptake, supports cellulose production, and strengthens cell walls. Potassium deficiency often results in ‘lodging’, where plants fall over due to weakened stems and inadequate internal water pressure. Other common visual symptoms of potassium deficiency are yellowing leaf margins of older leaves. If severe, leaf edges may brown and die.

Soil pH plays a crucial role in nutrient availability. It measures the acidity or alkalinity of the soil and affects the chemical reactions that control nutrient uptake by plants. Most fruits and vegetables grow best in slightly acidic to neutral soils, with a pH range of 6.0 to 6.5. However, some plants, such as blueberries, prefer more acidic conditions, between pH 4.2 and 5.2. Soil pH can be adjusted using amendments such as ground limestone or elemental sulfur. Ground limestone is used to raise pH and make soil more alkaline, while elemental sulfur can be applied to lower pH increasing acidity.

Fertilization should be based on a soil analysis, as nutrient needs vary depending on the plant, type of soil, season and other factors. General guideline for applying fertilizers include:

• If a large quantity of fertilizer is needed, it’s best to split-apply it over a few weeks rather than all at once.
• Avoid direct contact with fertilizer and plant roots or stems, as this can cause damage or “burning”.
• Be cautious with late summer or early fall fertilization, which can stimulate new growth and interfere with the plant’s natural dormancy or winter hardening process.
• In addition to the environmental pollution over applying fertilizer can cause, it can also cause  nutrient imbalances and toxicities that negatively affect plant growth and development.

Soil tests are free to North Carolina residents through the North Carolina Department of Agriculture from April 1st to Thanksgiving and $4 per sample between Thanksgiving and March 31st. For more information about plant fertility or soil testing, contact your local Cooperative Extension Office