Garden Fertilizers Broken Down: What Does “N P K” have to do with Gardening?

Last Updated on December 31, 2018 by Kimberly Crawford

Nitrogen, phosphorus and potassium are the major ingredients in garden fertilizers. Many elements including nutrients are present in soil and needed for plant growth.

What does “N P K” have to do with gardening? Healthy, robust lawns, gardens and houseplants depend on a number of factors including soil characteristics, location in the U.S., nutrients present in the soil naturally or added.

Many elements including nutrients are present in our soil and needed for plant growth. Three – nitrogen, phosphorus and potassium are the major ingredients in fertilizers for lawns, gardens and farm crops.

There are many fertilizers available for use from many different sources. All fertilizers work on the same principle: add nutrients to the soil to assist in plant growth.

Nitrogen, Phosphorus and Potassium: N, P, K

• Nitrogen (N) is necessary for normal vegetative growth and green color. Nitrogen is often naturally in short supply in soil. A deficiency results in stunted, yellowish-green plants.
• Phosphorus (P) is essential for flower, fruit and seed production, and good root development. Phosphorus is particularly important during early seedling growth stages.
• Potassium (K) is needed for healthy roots and stems, and aids plants with the respiration process. It is sometimes called potash.

Fertilizer Labels

Fertilizer labels indicate nutrient quantities listed as percentages of N, P and K. The first number is the percentage of nitrogen (N), followed by phosphorus (P), then potassium (K). A 10-10-10 fertilizer is 10% nitrogen, 10% phosphorus, 10% potassium; the remainder is an inert material for even distribution. These ingredients may be organic or inorganic, natural or manmade.

Often fertilizers are marketed by use such as for a lawn, vegetables, or general purpose. The problem with general categories is that they are not specific for individual soils.

Continuous use of a 10-10-10 fertilizer will eventually build up very high levels of phosphorus in the soil, which may not be a problem for growing plants. But, it could potentially result in environmental problems such as algae blooms, excessive lake weeds and fish kills due to high levels of phosphorus in run-off.

Obtain a Soil Test Before Buying Fertilizers

Have a soil test before purchasing fertilizers. This will save money – only buying what is needed, and will make sure only what is needed is applied, an environmentally sound approach. The Cooperative Extension Service in the county of residence will help get soil tested and assess the results.

Next, select between inorganic and organic fertilizers. Both have merits and problems and are much debated. The real bottom line is that either can be misused and over-used, resulting in negative environmental effects. The importance of regular soil tests cannot be stressed enough.

Inorganic and Organic Fertilizers

Inorganic fertilizers (manufactured) are usually less expensive. The nutrients are in a readily available form, meaning plants respond faster. Slow-release formulas are marketed for house plants and flowers. Look for these as sticks pushed into the soil.

Inorganic manufactured nitrogen fertilizers are usually made from petroleum. With a high nutrient content, less is required. Phosphorus and potassium are mined.

Organic fertilizers contain nutrients that must be chemically changed by soil bacteria before the plants can utilize, and are available over a longer period of time. These often contain several micronutrients in addition to N, P and K.

Natural organic fertilizers are made from waste products ranging from chicken feathers to manures to treated sewage sludge. Many of these are familiar to gardeners as blood meal, bone meal or fish emulsions.

Natural organic fertilizers have lower nutrient levels. Usually, an analysis is not available, making it difficult to know how much to apply.

This combined with slow absorption rates often result in over-application, adding to an already higher cost. Livestock manure has been an inexpensive organic for food production since the beginning of organized agriculture. Urea is an example of a manufactured organic fertilizer.

Phosphorus and Water Quality

While phosphorus is an important nutrient for plants, it has an impact on lakes. A small supply of phosphorus in lakes limits growth of algae and weeds, a desirable condition. 

Phosphorus run-off into waters increases the phosphorus supply and can cause undesirable algae blooms and weeds. This results in fish kills and loss of water recreation.

Much concern has been raised about the contribution of lawn fertilizers to lake pollution. Minnesota, Manitoba and several cities and counties have banned phosphorus in lawn fertilizers (except for establishment of new lawns and golf courses).

After Ann Arbor, Michigan prohibited the use of phosphorus on lawns in 2006, University of Michigan researchers conducted a study to determine the ban’s impact. Phosphorus levels in the local river dropped by an average of 28 percent in the two years following the ban, compared to amounts recorded in prior years.

Know What Soils Need Before Purchasing Fertilizers

Fertilizers can be expensive, and can be expensive to the natural environment. For many reasons, care should be taken before purchasing and applying fertilizers. Test so the soil needs are known, research the types of fertilizer to be selected.