What is Fertilizer?

Fertilizer is often referred to as “plant food.” Plants actually make food for themselves through a process called photosynthesis. The product of that process is carbohydrates, or sugars, that the plant can use for growth.

Fertilizer is more of a supplement that provides essential nutrients that plants can’t make for themselves. The nutrients are normally present in soil and are taken up through the roots of the plant. Fertilizer can be used to add nutrients when a soil is deficient in them.

What does Fertilizer provide Plants?

Nitrogen 

Nitrogen is a common nutrient in most fertilizers. Nitrogen is vital for plant growth. It is part of some of the key components of photosynthesis, like chlorophyll, the pigment that makes leaves green.

When nitrogen is available to a plant, the plant will have a good green color and will be able to make food for itself. When nitrogen is deficient, a plant may grow poorly and have a pale green or yellow green color.

Too much nitrogen can also be a problem. Applying too much nitrogen at one time can damage plant roots. Excessive nitrogen can also make a plant grow faster than normal and have softer, more succulent leaves. This faster growth can be more susceptible to infection by fungi, leading to disease.

Phosphorus

Phosphorus is often included in fertilizers. Phosphorus aids in production of new roots, as well as flowers and fruit.

Products known as “bulb boosters” have a higher phosphorus content because when spring-flowering bulbs are planted in fall, they need to produce roots right away. Products that contain more phosphorus help them do that. Fertilizers made for flowering trees and shrubs also have a higher phosphorus content to help those plants flower well. Plants that are not flowering well may not be getting enough phosphorus.

It is wise to have a soil test run if a phosphorous deficiency is suspected.

Excessive applications of phosphorus can lead to problems when phosphorus gets into water sources. It can lead to excess growth of algae which can affect the fish and wildlife that live in and around those bodies of water.

Potassium

Potassium is also a common component of many fertilizers. Plants use potassium for many internal processes that we can’t see. These include activation of enzymes involved in internal growth processes, as well as improved resistance to drought, insects, diseases, and other stresses.

A deficiency of potassium can lead to reduced growth or stunted plants. Excess potassium can interfere with the uptake of other nutrients from the soil.

How and When to Use Fertilizer

Fertilizers are labeled to indicate proportions of available nutrients. A label showing a 20-5-5 (N-P-K) formulation indicates 20% nitrogen (N), 5% phosphorus (P), and 5% potassium (K). All fertilizers carry this information on their labels.

For Trees and Shrubs

Trees and shrubs need to be fertilized only once a year. The addition of any soil nutrient is recommended only if soil or plant foliage tests indicate a deficiency.

If needed, fertilizer can be applied in late fall, once plants are dormant. The roots of the tree remain active longer into the fall than the above-ground part of the tree. The roots can absorb and store the applied fertilizer, strengthening the whole tree and preparing it to leaf out in spring.

As an alternative, fertilizer can be applied in spring as new growth resumes. Evergreens tend to go into dormancy more slowly, so spring can be the better time to fertilize them.

Shade trees will be served well by a fertilizer with equal amounts of nitrogen, phosphorus, and potassium (NPK), such as 10-10-10 or 12-12-12.

Flowering trees can benefit from a fertilizer with slightly more phosphorus, like 5-10-5 or 12-6-12.

For evergreens, look for fertilizers formulated specifically for evergreen trees and shrubs. These products often contain additional nutrients needed by evergreens.

The fertilizer selected should be applied evenly across the soil surface. Do not use fertilizer containing herbicides, such as those formulated for use on lawns. There are many types of fertilizer on the market. Be sure to read and follow the label directions to get the most out of the product you choose.

For Other Garden Plants

Annual and perennial flowers should be fertilized with a fertilizer that has slightly more phosphorus. This will help with flowering.

Perennials, with their limited flowering time, may need to be fertilized only once in spring.

Annual flowers may need more regular applications, since they will flower continuously throughout the season. Follow the label directions of the product you choose for best results.

Ornamental grasses—especially native species—may need little to no fertilizer. These plants often perform well without the addition of fertilizers.

Vegetable gardens can need regular fertilizer to help them produce crops regularly. There are fertilizers made specifically for vegetable gardens. Because many vegetable plants produce a flower before they produce a crop, they can benefit from a fertilizer that contains more phosphorus.