What do the NPK Numbers Mean on a Bag of Fertilizer?

For the average organic gardener, little fertilizer is ever needed unless a soil test indicates it. Generally, if enough compost and organic material are added to your garden each season, vegetables, fruit, and annuals have few additional needs. Rarely if ever do established perennials like trees and shrubs require fertilizer in the home landscape if compost and mulch are used every season. Supporting the soil with amendments is usually sufficient.

But at times, vegetables and fruit need a little help. After years of mining your garden soil for nutrients, certain elements may be slightly depleted, especially for heavy feeders like tomatoes. But once again, the only way to know for sure which elements are lacking is to get a soil test from your local extension service or a soil lab.

There are visual cues for nutrient deficiencies, but in many cases, these same symptoms may also be signs of disease or soil pH imbalance which no amount of fertilizer will cure. One should always proceed with caution and add only what’s necessary – adding too much fertilizer can not only compromise the health of plants, but also contribute to problems in local waterways.

What do the NPK fertilizer numbers mean?

Bags of garden fertilizer have three numbers on the label in bold, referred to as the fertilizer analysis or “grade”

• the first number refers to the percentage of nitrogen (N) in the mix;
• the second number is the percentage of Phosphate (P);
• the third number is the amount of Potassium (sometimes referred to as potash) (K).

These three numbers, expressed as N-P-K, are the primary nutrients necessary for plant health. For instance, a bag of 10-10-10 fertilizer contains 10% nitrogen, 10% phosphate and 10% potassium by volume. A bag of 5-10-5 is 5% nitrogen, 10% phosphate and 5% potassium. The remaining ingredients are inert substances like sand, sawdust, sterile dirt, peat moss, ground corn cobs, perlite, rice hulls, limestone or other ingredients. High percentages of NPK generally indicate synthetic substances were used to manufacture the fertilizer. Organic fertilizers tend to have lower percentages and a slower release of nutrients.

Inert ingredients – those that do not react with the fertilizer or feed the plants – are intended to dilute or reduce the concentration of the fertilizer’s active ingredients. High concentrations of fertilizer can burn delicate roots and stems; keep the fertilizer from drying out or hardening and clumping; or make the distribution of the fertilizer easier.

What does NPK do for plants?

Nitrogen, Phosphorous, and Potassium (NPK) are the 3 most important nutrients for plant growth, sometimes referred to as macronutrients.

Nitrogen is vital for a plant’s growth, development, and defense against diseases and stress. 

Every part of a plant needs nitrogen – it helps plants photosynthesize, gives them their green color, and make the proteins they need to produce new tissues. But if too much nitrogen is added, the plant may grow lots of foliage and stems but few fruits, vegetables, roots or flowers. In extreme cases, too much nitrogen can actually kill a plant.

When nitrogen is too low, the lower leaves turn yellow and the whole plant may turn pale green (but there are also other reasons for discoloration which are not related to nitrogen). Plants like legumes, however (beans, peas, and others), do not take up nitrogen from the soil, but from the air. They return nitrogen to the soil when the legume dies and decays.

Phosphorus is a vital component for many processes in plants.

Phosphorous stimulates root development, improves flower and seed formation, increases stalk and stem strength, helps the plant resist disease, and is critical in photosynthesis. Phosphorus deficiency causes stunted growth and poor flowering and fruiting. Too much phosphorous – applying when it is not needed – can increase chlorosis, a yellowing of the leaves caused by insufficient chlorophyll.

Potassium is important for many plant processes

Potassium supports the yield and quality of the edible parts of the plant. Signs of potassium deficiency are stunted growth and yellowish lower leaves.

Nitrogen is the one element that frequently needs replenishment, as it’s easily lost to rain and irrigation. Plants need this primarily just before growing and during the early stages of growth when they produce as many new cells and foliage as possible. Adding Nitrogen late in the season does little for the plant as it’s difficult for the plant to utilize. When added late, nitrogen remains in the soil until next season, moves below the root zone, or is leached away by rain and weather. Ultimately, plants will only use what they need of any element, regardless of how much garden fertilizer you add.

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The pH of your soil plays a role in how plants use fertilizer

The pH of your soil – the measure of acidity or alkalinity – is important to monitor when growing any crop in your garden. For instance, acid-loving crops like blueberries struggle when pH levels are not low enough (acidic). When the soil is too alkaline, they can’t take up certain nutrients like Iron, regardless of how much fertilizer is added to the soil. The natural acidity or alkalinity of soils varies by region and the only way to determine pH is with a soil test.

Can I use animal manures as fertilizer?

Manures from dogs and cats should never be used to fertilize edible crops as they may contain parasites, bacteria, and viruses harmful to human health. Composted horse manure, cow manure, and manures from bats, chickens, and others are generally safe. In fact, these manures are a excellent sources of organic matter and nutrients for any soil.

Manures contain a lower percentage of nutrients by weight and release them slower than prepared fertilizers. But manures are highly effective when used regularly throughout the growing season. One note of caution, however: horse manures from horse stables will also contain “bedding” like straw which may contain lots of weed seeds, adding to your weed problems. Manures also include urine. See more about using manures in your garden from the University of Kentucky.

Ways to apply garden fertilizer

Broadcasting

When a large area needs to be fertilized – for instance, a new lawn – a recommended rate of fertilizer is spread over the growing area and incorporated into the soil. Broadcast spreaders are usually used for this purpose. Water in the fertilizer after application.

Banding

When fertilizers are broadcast and worked into the soil, much of the phosphorus is locked up and is not immediately available to the plant. Banding is one way to make phosphorous available as the first roots develop. Narrow bands of fertilizer are applied in furrows 2 to 3 inches from the garden seeds and 1 to 2 inches deeper than the seeds or plants (applying the fertilizer band too close to the seeds will burn the roots of the seedlings).

To apply fertilizer by banding, run a string between 2 stakes where the row of seeds is to be planted. With a corner of a hoe or a trowel, dig a furrow parallel with the string 3 inches deep and 3 inches to one side of the string. Spread 1/2 the suggested rate of fertilizer in the furrow and cover it with soil. Repeat the banding technique on the other side of the string. Then sow your seeds underneath the string. For widely spaced plants, such as tomatoes, create bands 6 inches long for each plant or in a circle around the plant, 4 inches from the base of the plant. Do not permit fertilizer to contact seeds or the base of the plant.

Side-Dressing

During the growing season, fertilizer may need to be applied for heavy feeders which require a lot of nitrogen. Scatter the fertilizer near the plant, being careful not to contact leaves or stems, and water in.

Liquid solutions

Liquid fertilizers applied at the time of planting and throughout the growing season are an immediate source of nutrients. Follow the label directions carefully for dilution and application rates, keeping in mind that too much liquid fertilizer can burn plants.

Side-Dressing

Dry fertilizer is applied when plants are growing throughout the season. Scatter the fertilizer 6-8 inches from the plants on both sides. Work it into the soil and water thoroughly (dry fertilizer requires moisture to break down).

Foliar Feeding

Foliar feeding is used when a quick growth response is needed; when micronutrients such as iron or zinc are locked in the soil; or when the soil is too cold for the plants to uptake fertilizer from the soil.

Nutrients applied to the foliage are absorbed and used by the plant quite rapidly, beginning within minutes after application and completed within 1 to 2 days. Foliar nutrition can be a supplement to soil nutrition, but never a substitute.

For instance,

• an application of phosphorus spray will help at transplant time to help establish seedlings in cold soils.
• for perennial plants, early spring growth is usually limited by cold soil, even when the air is warm.
• in cold soil, microorganisms are not active enough to convert fertilizer into nutrient forms the roots can absorb, even though the plants can utilize them.

Liquid feeding is more of a one-time procedure, either as a transplant starter or as a foliar feeding to correct a deficiency in a major or trace element. Follow the directions carefully, as using too much foliar fertilizer can quickly burn the foliage.