Healthy flowers and plants require adequate levels of NPK from soils.

How to Adjust NPK in Soil

by A. Lynn Cochran

Along with water and carbon dioxide, nitrogen (N), phosphorus (P) and potassium (K) are the three main nutrients required for healthy plant growth. Adequate levels of these help plants produce lush, green foliage, robust blooms and good crops of vegetables and fruits. A lack of one can negatively impact plant health. Commercial fertilizers are labeled with specific NPK percentages to provide a general idea of nutrient content to help you determine the amount and rate of application. Ideally, these should be applied according to specific results and recommendations of a soil test.

Soils and Testing

Soil types and nutrient content vary greatly from one region to another across North America. Before planting a landscape, garden or lawn, conduct a thorough soil test. Many states and localities offer free or low-cost testing through extension offices and universities. Many commercial landscape services also provide soil testing services and a variety of kits can usually be found at local garden centers. Knowing your soil type, pH and nutrient content will help you determine the types and amounts of NPK fertilizers best for your situation and the plants you intend to grow.

Nutrient Properties

A quality soil test will offer specific fertilizer recommendations based on your individual results. Plants need nitrogen to produce vigorous, green growth. Levels of nitrogen can vary greatly from day to day based on rainfall, organic soil content, soil structure and pH. Most soil tests will not measure this nutrient because levels are so variable. However, recommendations for nitrogen supplements can be made according to your soil conditions. Phosphorus and potassium are needed for root growth, bloom, vegetable and fruit production. These persist in soils longer than nitrogen. Thus, they can be directly measured and supplementation rates determined accordingly.

Synthetic Fertilizers

Numerous fast-acting and long-acting synthetic NPK fertilizers and blends are available to modern gardeners. These include granular, powdered and liquid forms and may be mixed for ready use or concentrated for dilution before application. These are required to feature a package label with preparation instructions and general rates of application. Short-acting fertilizers generally require several applications at regular intervals during the growing season. Longer-acting types may only need to be applied once or twice annually.

Organic Fertilizers

Options for organic fertilizers include commercially prepared mixes, manures, compost, meals of alfalfa, fish, bone and blood, among others. These products are also required to label NPK content and recommended application rates. In general, organic fertilizers release nutrients more slowly than synthetics, and only one application per season may be necessary. Non-commercial NPK fertilizer alternatives include homemade compost, leaf mold, grass clippings and coffee grounds. These can be worked into the soil before planting or used as a top-dressing once or twice annually.


As the risks of excess crop and lawn fertilization have become better understood in recent years, the regular use of NPK fertilizers, in the absence of soil testing, is discouraged. Surplus accumulations of nitrogen and phosphorus from soils can easily contaminate waterways and aquifers, impacting drinking water quality, aquatic life and sensitive ecosystems. Your soil may contain sufficient levels of some or all NPK nutrients. Only use the amount of NPK fertilizer necessary and recommended for your soil.

About the Author

Lynn Cochran is a professional writer and contributing author to the educational website, Gardening Carolina. He also volunteers as a North Carolina Master Gardener. He is educated in environmental science, botany, health care and English literature. He is currently pursuing an accelerated master's degree in applied geography at the University of North Carolina at Greensboro.

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