Fertilizer Numbers: What Are They and How Do I Use Them to Grow Better Plants?

Most plants I grow need fertilizer. However, there are thousands of different fertilizers available, and finding the right one for my plants can be confusing. Reading the labels, I wondered, “What do these numbers on the fertilizer label mean?”

Buying fertilizer is confusing enough when you are trying to find something to make your plants grow better. This guide will explain what the numbers mean and how to use them.

What Do the Numbers Mean?

The three numbers on the fertilizer label stand for the percentage of the fertilizer that is nitrogen, phosphorous, and potassium, in that order. Plants need different combinations of those nutrients depending on their growth stage and the nutrients available in the soil the plants will grow in.

How N-P-K Affects Plants?

All plants need generous amounts of nitrogen, phosphorous, and potassium. Each is essential to a different part of the plant.

Nitrogen

Nitrogen is found everywhere in the plant. Without nitrogen, plants cannot grow or put down roots. They use nitrogen in cell walls, proteins, and chlorophyll.

All plants use large amounts of nitrogen. Vegetables and fruits are considered heavy feeders because it takes a lot of nutrients to make fruits and vegetables, so they use even more nitrogen.

A lack of nitrogen will result in stunted plants, few blooms, and few fruits, vegetables, or seeds. Too much nitrogen will result in lots of foliage and few blooms. An extreme amount of nitrogen will burn the plants and can kill them.

Phosphorous

Phosphorous helps plants transfer energy from sunlight to the rest of the plant. It also helps to grow good roots and stimulates blooms. Finally, phosphorous helps plants mature.

Too little phosphorous has much the same effect as too little nitrogen. Leaves can become dark green or purplish without phosphorous. Too much phosphorous can burn or kill a plant. It can also cause problems with the plant’s ability to use iron, zinc, or manganese.

Potassium

Potassium helps form and move starches, oils, and sugars around plants. It is also important in making plants vigorous and resistant to disease.

Potassium helps regulate the opening and closing of the stoma, or pores, on the plant leaves. The stomas affect how the plant intakes carbon dioxide and exhales oxygen and water vapor.

Potassium deficiency stunts plant growth and reduces fruit and vegetable yield. Symptoms of potassium deficiency are yellow edges on the leaves. Too much potassium can inhibit the uptake of nitrogen, phosphorous, and magnesium.

Organic Versus Synthetic Fertilizers

Organic fertilizers have only materials found in nature in them. Some natural compounds, such as nicotine, are banned in organic materials.

Synthetic fertilizers have ingredients that are made in a laboratory.

To claim a product is organic, a producer must submit documentation to an agency like the USDA or another certifying agency that all the ingredients are allowed in organic products.

Different Forms of Fertilizer

The form of fertilizer is something of a personal choice. Here are some things to consider when choosing a fertilizer.

Liquid

I use liquid fertilizers mixed at half-strength when fertilizing seedlings. I use full-strength fertilizers on plants in the ground or in a container that need nutrients immediately.

I prefer to buy concentrates instead of ready-to-use liquid fertilizers. Concentrates cost more initially but are less expensive in the long run.

In addition, buying concentrates produces less plastic waste than buying ready-to-use liquids. One bottle of concentrate equals many bottles of ready-to-use mixes.

Dry

Dry fertilizers come in granules and powder. I use granules of nitrogen fertilizer to spread around my transplants when I plant them. When I use blood or bone meal, I sprinkle them around my plants or where I plant my seeds.

Most granules and powders must be watered in to start working. Some powders and granules can also be mixed with water to form a liquid fertilizer.

Dry fertilizer is usually easier to store and does not ruin as fast as liquid fertilizer.

Spikes

Spikes are granular fertilizers that are formed into a spike. I put a spike about six inches from each plant. Five-gallon containers get one spike per container. The fertilizer is gradually released over about eight weeks. I find spikes easier than having to fertilize a container several times.

Spikes do sometimes arrive broken or crumbled. When that happens, I treat the spikes like granular fertilizers.

Slow Release

Slow-release fertilizer comes in granules or pellets. The pellets are coated with a coating that gradually breaks down, releasing nutrients as it does so.

I use slow-release pellets around transplants to give them a good boost until they are ready to bloom. The disadvantage of slow-release fertilizer is that it takes about seven days for the first nutrients to be released.

Fast Release

Fast-release fertilizer can be liquid, granules, or powder. The fertilizer is usually mixed with water so that when the plant roots draw in the water, they get the nutrients.

Fast-release fertilizer can perk up a plant within 24 hours.

The Importance of Soil Tests

Performing a soil test in late winter is probably the most important thing you can do for your plants all year.

Every state in the United States has a soil lab, and every county has an Extension Agent. The Extension Agent can give you instructions for doing a soil test and tell you where to send the samples. In other countries, consult your agricultural advisor.

I do a soil test in late December. The results tell me how much nitrogen, phosphorous, and potassium to add to my soil to feed my plants.

Currently, the basic soil test is $15 where I am. For a little more, I can get micronutrient levels. I save more money putting out just what my plants need instead of over-fertilizing than the soil test costs.

Once I have followed my soil test recommendations, I know that my plants will start off right. I just need to fertilize the heavy feeders in my landscape like tomatoes as they grow.

One note of caution: you need to do a separate soil test for each type of plant. Take one for your front lawn, one for your back lawn, one for each flower bed, and one for your vegetable garden. I make a list with the sample number and where it came from. I send them all off, and when I get the results back, I get one result for each sample.

The samples are numbered but do not have where you took them on the results. If you do not keep a list, you will not know what sample came from what area. In my state, if I mark what I am growing in each sample, the results tailor the recommendations to the plants in that area.

Tailored recommendations are important because grass needs different amounts of nutrients than ornamentals or vegetables.

Choosing the Right Fertilizer for Your Plants

The right fertilizer for your plants changes depending on the plant, your soil type, and the growth stage of the plant. Plants growing in sandy soil need more fertilizer because it is easily leached out when the plants are watered. Plants in clay soil do not need as much fertilizer because the heavy clay particles hold the nutrients in the fertilizer.

All plants need nitrogen in large quantities. I spread a fertilizer high in nitrogen in the ground before I transplant any plant. In the spring, I give my plants the amount of nitrogen the soil test recommends.

Some soils are already high in phosphorous or potassium, so the soil test results may not recommend adding any of those nutrients. Too much of any nutrient can make your plants sick or even kill them. If you are looking for specific fertilizers for your plants, check out some of my works below:

Balanced versus Unbalanced Ratios

Balanced fertilizers are ones where all the numbers on the fertilizer are the same. For example, 10-10-10 is a common balanced fertilizer. My guides for some of the common balanced fertilizers are below:

I use it when my plants need the same amount of nitrogen, phosphorus, and potassium.

I recommend balanced fertilizers in cases where you do not have a soil test and are guessing what your plant needs.

Unbalanced ratios, such as 3-4-1 or 12-0-0, contain more of one nutrient than another one. Blood meal has nitrogen and nothing else. It has 12-0-0 on the front to show it has twelve percent nitrogen and no phosphorous or potassium. My guides for some of the common unbalanced fertilizers are below:

When plants start to bloom, they need more phosphorus than nitrogen, as well as some potassium. In this case, I might use a 3-4-1. Too much nitrogen will make the plant grow lots of foliage but not have many blooms.

Shopping for Fertilizer

Here are some tips to make shopping for the right fertilizer easier.

Cheap Fertilizer Isn’t

Cheap fertilizer might look like a bargain from the outside. When I look at cheap fertilizer, I see low-quality ingredients.

Many times, sewage sludge or other ingredients that may have unwanted heavy metals or other chemicals are listed.

I have also found that most of the fertilizer is filler, so I have to use more of the fertilizer than I would a higher quality fertilizer.

Do yourself and your plants a favor and buy the best fertilizer you can afford. The higher quality ingredients will help your plants more without adding unwanted chemicals to your landscape.

Fertilizer Is Heavy

I buy a lot of things through the mail. However, I have found that dry fertilizer weighs a lot, and I can save money by going to my local nursery.

Nurseries may charge a little more than a mail-order place, but the shipping from mail-order fertilizer adds a lot to the price. Compare prices before you buy online.

Make Sure You Get Enough Fertilizer

A common mistake when buying fertilizer is not buying enough. Every fertilizer, even a high-quality one, has fillers in it to make the fertilizer easier to use.

Some ingredients keep the fertilizer from spoiling, and others regulate how fast the nutrients are released. For whatever reason, no fertilizer bag is one hundred percent fertilizer.

If your soil test says you need one pound of nitrogen, you can’t just buy a pound of fertilizer.

You need to divide one by the percentage of the nutrient in the fertilizer (in decimal form) to get how much you need to spread.

For example, if you need one pound of nitrogen and want to use blood meal, which is twelve percent nitrogen, you will need to solve this equation: 1÷0.12=8.333. You will need to apply 8.33 pounds of blood meal to get one pound of nitrogen on your plants.

A ten-pound bag of blood meal should have enough in it to apply one pound of nitrogen, while a one-pound bag will not.

Final Verdict

In summary, the numbers on the fertilizer bag mean the percentage of nitrogen, phosphorous, and potassium in the bag, in that order. These nutrients are needed in large quantities by every plant.

My soil does not have the right amounts of these nutrients, so I have to spread fertilizer to give my plants the nutrients they need. I do a soil test every year to find out what nutrients I need to add to my soil. I buy the best quality fertilizer I can and apply the correct amount based on the soil test.

Fertilizing in this way means the soil starts off fertile each spring. As my plants draw nutrients out of the soil, I replace them, making sure I give the plant the nutrients it needs at that growth stage.

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Stephanie Suesan Smith

Stephanie Suesan Smith has a Ph.D. in psychology that she mainly uses to train her dog. She has been a freelance writer since 1991. She has been writing for the web since 2010. Dr. Smith has been a master gardener since 2001 and writes extensively on gardening. She has advanced training in vegetables and entomology but learned to garden from her father.

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