FERTILIZERS 101- by Pam Sinclair

Pam Sinclair is a certified professional horticulturalist. A gardening coach, writer, and designer. Pam has a degree in architecture for the University of Washington and a degree in Ornamental Horticulture and Landscape Design from Edmonds Community College. She has a landscape design consultation business, Gardengal Designs that has been active for about 15 years.


To many gardeners, the basics of fertilizing – when, how and with what – are a bit of a mystery. What are all those strange ingredients, what to they do for our plants and gardens and do we really need them? And what do those numbers mean and is there a difference between organic and synthetic fertilizers and if so, what is it? To answer these questions, it is helpful to understand some basic plant biology, know what sort of soil you are working with and demystify some of the technical aspects of fertilizers and their application.

Plants and “Plant Food”

To many folks, the terms ‘plant food’ and ‘fertilizer’ are used interchangeably and this notion is often promoted by fertilizer manufacturers. However, this is incorrect. Fertilizers are NOT plant food. Plants use carbon dioxide, water, energy from the sun and elements from the soil to manufacture their own food through the process of photosynthesis. These soil elements – or nutrients – are often already present in sufficient quantities to address plant needs. But when they are missing or concentrations deficient, fertilizers are used to supplement these nutrients. Fertilizers are just combinations of essential plant nutrients.

The act of applying these supplemental nutrients – fertilizing – has its drawbacks as well as benefits. While the judicious application of fertilizers, when needed, can promote healthy foliar growth and flower and fruit production, unnecessary or over-application of fertilizers can decrease plant health and can lead to decline or even death. Excess fertilization can increase the incidence of some plant diseases and often stimulates rapid, lush growth that is attractive to insect predation, tends to be less drought tolerant, more susceptible to cold damage and is more easily damaged by heavy winds and rains. Improper fertilizing can also encourage leaf growth at the expense of flowers and fruits or seeds. And finally, unnecessary or over-application of fertilizers contributes greatly to the pollution of our streams and waterways.

What is a Fertilizer?

A fertilizer is any material of natural or synthetic origin that is applied to soils or to plant tissues (usually leaves) to supply one or more plant nutrients essential to the growth of plants. Plants need a range of nutrients in varying concentrations to achieve optimum growth. Typically these are broken down into 2 classifications: macro-nutrients or those essential for plant growth and required in the largest concentrations; and micro-nutrients or trace minerals that enhance various aspects of plant growth but are utilized in very low concentrations and not uniformly by all plants.

The macro-nutrients consist of nitrogen (N), necessary for healthy leaves and stems; phosphorus (P) for flowering, fruiting and seed production; and potassium (K) for strong root development. These are known as the ‘big three’ and are essential for healthy plant growth. The micro-nutrients include calcium (Ca), magnesium (Mg) and sulfur (S) – sometimes referred to as secondary macro-nutrients as they are needed in higher quantities than the other micro-nutrients – and iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), boron (B) and molybdenum (Mo), necessary in much smaller concentrations than the secondary nutrients. Micro-nutrients often are necessary to facilitate the absorption of the major nutrients as well as to enhance the process of photosynthesis in addition to supplying biological support (reproduction, fertilization, cell development, chlorophyll production), assisting the formation of proteins and sugars, enhancing flavor and promoting maturity.

Prepared, packaged fertilizers (as opposed to composts or manures) will always come labeled as to contents and their proportions The big three macro-nutrients will always be displayed clearly on the packaging as a ratio, often referred to as the NPK ratio, and will be expressed as three numbers (10-10-10, 3-2-1, etc.) and reflect the percentage by weight of the concentrations of nitrogen, phosphorus and potassium, always in that order. Therefore a 10-10-10 formulation will consist of 10% nitrogen, 10% phosphorus and 10% potassium. The remaining material will be any trace elements or micro-nutrients and fillers. This ratio is known as the “guaranteed analysis” and this labeling is required by law and is also extremely helpful in determining how much fertilizer, especially granular synthetics, to apply. For example, if the recommendations or instructions are to apply 10 pounds of nitrogen, 1 pound of phosphorus and 5 pounds of potassium per 1000 square feet, you would need a 100 pound bag of 10-1-5 fertilizer. Liquid and organic products will have a different rate of application, so read the labels carefully.

If or When to Fertilize

This seems to be the area that causes the most confusion for home gardeners. The marketing campaigns of big fertilizer manufacturers, especially those that produce synthetic fertilizers, would have you believe that unless you use their product regularly and repeatedly, you will be unable to maintain a lush green lawn, your trees and shrubs won’t grow and your garden will not produce an abundance of flowers or fruit. This is simply not true. For centuries plants have grown successfully without any help from humans and many still do, so it is obvious that they can grow without additional fertilization, especially in environments to which they are adapted. Most of what they need to thrive is already present in the soil and available through the atmosphere. And some plants actually contribute nutrients to the soil, either through the shedding of their parts (leaves, needles and other plant tissues) or through their roots in a process known as nitrogen fixation. It is only when soils become depleted or deficient of required nutrients that fertilizing becomes necessary.

So how does one determine when or if fertilizing is necessary? The first and easiest way of determining what plant nutrients are present and in what concentrations is to have a professional soil test done. This will generate a written report that outlines the existing nutrient concentrations in your soil as well as the chemistry of the soil, which can affect how specific nutrients are accessed and metabolized. This report will make specific recommendations for what nutrients to add and in what amounts and if you need to make other alterations to soil conditions. Easy-peasy!!

A second, less specific method and one that requires a bit of experience to master is simply to look at the plants themselves. Lackluster or stunted growth, sparse foliage, lack of flowers or fruit production or discoloring can all be symptoms of nutrient deficiency but can also be symptomatic of other plant problems including water issues or insect and disease problems. The trick is knowing the difference but in general, cultural issues like these can be rather quickly ruled out by close examination, leaving some sort of nutrient deficiency as the problem.

The single most common nutrient deficiency will be that of nitrogen (N). It is the most mobile of the basic plant nutrients and is easily lost to the atmosphere in a process known as volatization as well as being leached out of the soil profile by rainfall and irrigation. Nitrogen deficiency is most often characterized by slow, stunted growth, and pale or yellowed foliage. Deficiency symptoms
generally appear on the bottom leaves first. In severe cases, the lower leaves have a “fired” appearance on the tips, turn brown, usually disintegrate, and fall off. Nitrogen is also the easiest of the required macro-nutrients to be replaced, as it is plentiful in compost, manures or other organic matter used as soil amendments or mulches and through a wide variety of nitrogen-rich fertilizers. And some plants, primarily those in the pea family, will contribute nitrogen to the soil through a process known as nitrogen fixation.

A very cursory fertilizing rule of thumb is to apply supplemental fertilizers only when necessary. Larger plants – trees and shrubs – when well-established in the landscape seldom require additional fertilization. Smaller plants, because of a far less extensive root system, may require only an annual application. Annuals and harvestable plants like vegetables often benefit from periodic fertilizing as they are heavy nutrient users and often pull nutrients from the soil. And many gardens can benefit from nutrient supplementation without using fertilizers at all via the use of nutrient-rich mulches, applications of compost or manures or by grass-cycling or using mulching lawn mowers.

Synthetic versus Organic

Prepared, packaged fertilizers can be either synthetic – created in a laboratory or manufactured – or organic – derived from naturally occurring animal and plant products or mined minerals. Both will achieve the same result of supplying the necessary nutrients for healthy plant growth but how they deliver these nutrients is quite different.

Manufactured synthetic fertilizers are typically derived from byproducts of the petroleum industry. They all tend to be water soluble salts that can be immediately accessed by plant roots unless treated or coated to be released slowly over a period of time. Because they are usually fast acting, they can easily burn plant roots and foliage if applied incorrectly. And because they are water soluble, they leach readily into groundwater and contribute heavily to pollution of streams, rivers and other bodies of water. Repeated usage can cause build up of the salts in the soil, affecting the ability of the plant to take up water through its roots, can discourage healthy populations of necessary and very beneficial soil organisms and alter the soil structure or friability.

Organic or natural fertilizers require the activities of soil organisms to break them down into their elemental forms so they can be accessed by the plant roots. As this process takes time to occur, organic fertilizers are slow acting therefore, by definition, slow release. This same characteristic makes them much less likely to leach into groundwater, reducing the potential for pollution. Because they offer plant nutrients in low concentrations, they are difficult (but not impossible) to over-apply so seldom result in rapid, lush, vulnerable growth but offer a more sustained nutrient delivery. And they also encourage healthy populations of the beneficial soil organisms needed to digest them, resulting in a healthy, biologically active soil.

Fertilizing Containers

Container plants, whether grown as houseplants or outdoors, require a different approach to fertilizing. Since container potting soil is virtually devoid of any plant nutrients, including the required basic 3 macro-nutrients, they must be provided by the gardener. And because of the frequent watering this type of growing condition demands which tends to wash out any existing nutrients, these need to be applied often. Permanent container plantings can be addressed annually with a application of a slow release fertilizer. More seasonal containers, like annuals or veggies, can be fertilized with a water soluble liquid fertilizer of your choice. The common practice is “weekly, weakly” or once a week with a half strength dilution. Make sure your fertilizer of choice also offers all necessary micro-nutrients or trace elements as well.

It is difficult to recommend organic fertilizers for container plants because in addition to being pretty much devoid of nutrients, potting soil also lacks any significant populations of the soil microorganisms necessary to process the organic fertilizer. Recent developments in this area have resulted in a number of relatively new water soluble organic products that bypass this important step and deliver the nutrients in an immediately available form.

Fertilizing Do’s and Don’ts

Don’t fertilize when plants are stressed, as in the heat and dryness of midsummer (except for annuals and container plants), when plants are dormant or diseased or immediately after planting or transplanting.

Do learn to assess your plants’ health before needing to fertilize. A soil test can help to determine the need or learn to examine poorly performing plants for signs of nutrient deficiencies.

Do avoid “weed & feed” products, as these contain pesticides that can be harmful to pets and small children and contribute heavily to water pollution.

Do apply only as the label suggests, regardless if using organic or synthetic products. If a little is good, more is not better! Over-applying fertilizers is not only harmful to the environment but can lead to the build up of nutrients in levels that can be toxic to the plants.

Do look for granular type fertilizers than be spread evenly throughout the root zone and watered in. Fertilizer spikes tend to concentrate the fertilizer to specific limited areas not easily accessed by the plant roots. Liquid fertilizers are most appropriate for potted plants and can be unnecessarily expensive if broadcast throughout the garden as well as hard to deliver evenly. And remember that very few nutrients are absorbed through the foliage, so direct any liquids at the base of the plant so they can be taken up by the roots.

Basic Fertilizer Terms

Fertilizer – any material used to supply one or more of the essential plant nutrients.

Fertilizer ratio – the relative proportions of nitrogen (N), phosphorous (P) and potassium (K). Stated as a “guaranteed analysis” based on percentage per volume weight.

Synthetic fertilizer – a fertilizer that is man-made or manufactured, often from petroleum byproducts.

Organic fertilizer – a fertilizer that is derived from naturally occurring materials, such as plant parts (alfalfa or cottonseed meal, kelp meal), animal products (bat guano, feather or bone meal) or mined minerals (greens and, rock phosphate) or a combination of these materials.

Complete fertilizer – a fertilizer that contains some of each of the three macro-nutrients; nitrogen, phosphorous and potassium.

Incomplete fertilizer – a fertilizer missing one or two of the 3 macro-nutrients, eg. 3-15-0, 0-2-0.

Balanced fertilizer – a fertilizer containing equal proportions of the 3 macro-nutrients, eg. 10-10-10.

CRF – Controlled Release Fertilizer. A synthetic fertilizer that is treated or coated so that it releases nutrients gradually over an extended period of time, eg. Osmocote.

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