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Fertilizer Basics: A Tutorial

A Bag of Fertilizer Standing in the fertilizer aisle of a garden or farm store, you are faced with a dizzying array of fertilizer options, many with a series of three numbers like 10-10-10, 20-20-20, 10-8-10 or many other combinations of numbers. You may be asking yourself, “What do the numbers on fertilizer mean?” These are NPK values, which leads to the next question of, “What is NPK?”

What Do The Numbers Mean?

The three numbers on fertilizer represents the value of the three macro-nutrients used by plants. These macro-nutrients are nitrogen (N), phosphorus (P) and potassium (K) or NPK for short.

The higher the number, the more concentrated the nutrient is in the fertilizer. For example, numbers on fertilizer listed as 20-5-5 has four times more nitrogen in it than phosphorus and potassium. A 20-20-20 fertilizer has twice as much concentration of all three nutrients than 10-10-10.

Test Your Knowledge: try the Fertilizers Quiz

The fertilizer numbers can be used to calculate how much of a fertilizer needs to be applied to equal 1 pound of the nutrient you are trying to add to the soil. So if the numbers on the fertilizer are 10-10-10, you can divide 100 by 10 and this will tell you that you need 10 pounds of the fertilizer to add 1 pound of the nutrient to the soil. If the fertilizer numbers were 20-20-20, you divide 100 by 20 and you know that it will take 5 pounds of the fertilizer to add 1 pound of the nutrient to the soil.

These fertilizer numbers, also called NPK values, should appear on any fertilizer you purchase, whether it is an organic fertilizer or a chemical fertilizer.

Why Is NPK Important?

Compost Bin So now that you know what the numbers on fertilizer mean, you need to know why NPK is important to your plants. All plants need nitrogen, phosphorus and potassium to grow. Without enough of any one of these nutrients, a plant will fail.

Nitrogen (N) – nitrogen is largely responsible for the growth of leaves on the plant.

Phosphorus (P) – Phosphorus is largely responsible for root growth and flower and fruit development.

Potassium (K) – Potassium is a nutrient that helps the overall functions of the plant performs correctly.

Before you apply fertilizer to your garden beds, you should have your soil tested. This will also help you determine what balance of fertilizer numbers will be appropriate for your garden’s soil needs and deficiencies.

Understanding Nitrogen (N)

All plants require nitrogen for healthy growth and reproduction. More importantly, plants use nitrogen for photosynthesis. While native plants are better adapted to their surroundings and oftentimes less affected by nitrogen deficiency, in plants such as vegetable crops, supplemental nitrogen may be required.

Nitrogen Requirements

Yellowing leaves from nitrogen defiency.

As organic matter decomposes, nitrogen is slowly converted to ammonium, which is absorbed by plant roots. Excess ammonium is turned into nitrate, which plants also use to produce protein. However, unused nitrates remain in the groundwater, resulting in leaching of the soil.

Nitrogen Deficiency

 Dropping of leaves
 Poor growth

Excess Nitrogen

 Plants may not flower
 Plants may not fruit
 Poor growth
 Burning causing plant to shrivel and die

Raise Soil Nitrogen

There are several ways to add nitrogen to soil. Supplemental nitrogen is usually provided by using organic or chemical fertilizers. Plants obtain nitrogen through compounds containing ammonium or nitrate. Both of these can be given to plants through chemical fertilizers. Using chemical fertilizer to add nitrogen to soil is faster; however, it is more prone to leaching, which can be harmful to the environment.

Building up levels of organic matter in the soil is another way of raising soil nitrogen. This can be achieved by using organic fertilizer in the form of compost or manure. Growing legumes can also supplement soil nitrogen. Although organic fertilizer must be broken down in order to release compounds containing ammonium and nitrate, which is much slower, using organic fertilizer to add nitrogen to soil is safer for the environment.

Understanding Phosphorus (P)

The function of phosphorus in plants is very important. It helps a plant convert other nutrients into usable building blocks with which to grow. Phosphorus is one of the main three nutrients most commonly found in fertilizers and is the “P” in the NPK balance that is listed on fertilizers. Phosphorus is essential to a plant’s growth.

Phosphorus deficiency.

Phosphorus Deficiency

 Plants are small
  Few or no flowers
 Bright green or purplish cast
 Weak root systems

Excess Phosphorus

 It's very difficult for a plant to get too much phosphorus as it is difficult for them to absorb phosphorus

Raise Soil Phosphorus

There are many chemical fertilizers that can help you with replacing phosphorus and getting a good nutrient balance in your soil. When using chemical fertilizers, you will want to look for fertilizers that have a high “P” value (the second number in the fertilizer rating N-P-K).

If you would like to correct your soil’s phosphorus deficiency using organic fertilizer, try using bone meal or rock phosphate. These both can help with replacing phosphorus in the soil. Sometimes, simply adding compost to the soil can help plants be better able to take up the phosphorus that is already in the soil, so consider trying that before you add anything else.

Understanding Potassium (K)

Plants and potassium is actually a mystery to even modern science. The effects of potassium on plants is well known in that it improves how well a plant grows and produces but exactly why and how is not known. As a gardener, you do not need to know the why and how in order to be hurt by a potassium deficiency in plants

Potassium deficiency.

Potassium Deficiency

 Plant does more poorly than expected
 Older leaves, may have brown spots, yellow edges, yellow veins or brown veins.

Raise Soil Potassium

If you wish to add potassium to your soil at home, you can do so in several ways without having to use potash or other commercial potassium fertilizer. Compost made primarily from food byproducts is an excellent source of potassium. In particular, banana peels are very high in potassium.

Wood ash can also be used, but make sure that you apply wood ash only lightly, as too much can burn your plants.

Greensand, which is available from most nurseries, will also add potassium to you garden.

Organic Fertilizers

Organic fertilizer for gardens is typically made of single ingredients and can be matched to your garden’s particular nutritional needs. The types of organic fertilizer can come from plant, animal or mineral sources, depending on what chemicals your garden requires. In order to qualify as an organic fertilizer, the materials must naturally occur in nature.

Fertilizer for organic gardening isn’t the quick and instant fix that chemical fertilizers can be. With organics, you have to let moisture and beneficial organisms break down the content of the fertilizer material in order for the plants to get to the nutrients inside. In general, half of the nutrients in an organic fertilizer ingredient can be used the first year it is applied, and the rest of it is slowly released in the years to come, feeding and conditioning the soil.

Plant-based Fertilizers

 Kelp seaweed
 Compost tea
 Legume cover crops
 Green manure cover crops

Animal-based Fertilizers

 Bone meal
 Blood meal
 Fish emulsion
 Manure tea

Mineral-based Fertilizers

 Epsom salt

Chemical Fertilizers

Chemical, or conventional fertilizers, are synthetic products that appear in many forms, such as granular or liquid. Although conventional fertilizers are still widely used, they do have their downsides. For instance, conventional fertilizers can be harmful to the environment and, if overused, can burn the plants. Nonetheless, millions of gardeners prefer using conventional fertilizers over other methods, as they are less expensive and fast acting.

Granular Fertilizers

Granular fertilizers work better on lawns or other large garden areas and landscape plantings, as these are generally slow releasing. The plants take in nutrients during periods of rain and watering intervals.

Liquid Fertilizers

Liquid fertilizers are fast acting. They are great choices for container plantings or smaller garden areas. These fertilizers are the easiest to apply and most popular since you can apply them as you’re watering.

Keep In Mind

Over fertilization is the most common problem associated with using conventional fertilizers. This not only results in stunted growth and burnt foliage but can make plants more vulnerable to pests and diseases too.

Environmental problems with conventional fertilizers arise when excess nutrients find their way into and contaminate water resources. They can even threaten wildlife when they drink from these resources or feed on the garden plants. Therefore, care should always be taken whenever using conventional fertilizers.

Conventional fertilizers do not help the soil as organic types, like manure or compost, do. While organic forms may be slower, they are healthier alternatives. However, if you choose to use conventional fertilizers, follow directions carefully and use caution to avoid over fertilizing.

Further Information

 The Life Cycle of Plants: Fertilization
 The Spruce: How to Make Your Own Fertilizer
 Pollination and Fertilization
 4 Ways To Use Eggshells For Your Plants
 10 Ways to Use Banana Peels in Your Garden