What Is NPK in Soil Testing?


Soil tests are done to give a farmer or homeowner an accurate picture of the current state of his soil, including nutrient levels, pH and percentage of organic matter. Recommendations for the amount of fertilizer to use are based on this analysis. Nitrogen, phosphorous and potassium, with the symbols N, P and K, are the major nutrients needed by plants and the ones most likely to be lacking in optimum quantities in the soil.

Soil Testing and its Benefits

To get an accurate reading of the quality of a homeowner's soil, five to 10 samples at a depth of 6 to 8 inches need to be taken from the area. These are blended together and then a single sample from that mixture is used to get a representative reading. A soil testing service will usually ask what crop you plan to grow so it can make a specific recommendation for nutrient supplementation. This keeps guesswork to a minimum when deciding on materials to add to the soil when you are growing beets, for example, versus potatoes or lettuce.

Nitrogen and its Effects

Nitrogen, represented by the symbol "N," is necessary for photosynthesis and other processes of growth and development. Too little nitrogen leaves plants stunted and yellow, while too much causes dark green growth with few flowers and fruit. Some forms of nitrogen are rapidly depleted and often unavailable to plants in the soil. It is renewed from the breakdown of green organic matter by microorganisms but may not be accessible in cold spring soils.

Phosphorous and its Effects

Phosphorous is a stable element that does not move through soil easily. It is necessary for proper flower, fruit and seed formation and deficiencies may cause problems in these structures.

Potassium and its Effects

Potassium is also necessary for flower, fruit and seed production and, like phosphorous, does not move readily through soil. Deficiencies may include weak stems as well as smaller flowers and fruit.

Using N-P-K Recommendations

The need nitrogen, phosphorous and potassium in a plant depends on the type of plant and the reason you're growing it. Lettuce, for example, is used for its leaves, so lots of nitrogen is a plus. On the other hand, broccoli needs to put up substantial flower stalks, so phosphorous and potassium are important for a good crop. You can fine tune the fertilizer to your vegetables or flowers by using fast-acting fertilizers, such as liquid formulas. If your soil is high in potassium but low in phosphorous, you can use a slow-acting, high phosphorous fertilizer, such as bone meal, for long-term balance. Then, use a regular vegetable or flower fertilizer such as 5-10-10 for short-term needs.

Keywords: major plant nutrients, N-P-K plant growth, soil testing benefits

About this Author

Over the past 30 years, Mara Grey has sold plants in nurseries, designed gardens and volunteered as a Master Gardener. She is the author of "The Lazy Gardener" and "The Complete Idiot's Guide to Flower Gardening" and has a Bachelor of Science in botany.