Fertilizer – Selecting The Right NPK Ratio

We eat 3 times a day and the food we eat provides all of the nutrients we need to live. The chemicals in food are used by our bodies to grow and maintain our body parts. The protein you eat helps build muscles. Carbohydrates provide energy that is used for all kinds of things such as moving, breathing, thinking and eating more food.

Most of the food we eat is in the form of large molecules such as protein, carbohydrates, DNA, and fats. Our digestive system takes these large molecules and breaks them down into smaller molecules like simple sugars and nitrates. Our bodies then use these small molecules as building blocks to build new large molecules. Our bodies are actually a very efficient recycling plant. We take food in, break it down into basic building blocks, and then use these building blocks to create complex body parts.

Plants differ from humans and other animals in that they do not have a digestive system. Therefore, they are not able to break down large molecules. Plants must start with small molecules like nitrate and phosphate and build the large molecules they need.

Plants Use Fertilizer to Build Large Molecules

Plants take small molecules such as carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium and dozens of other minor nutrients, and use them to build large molecules such as sugars, carbohydrates, oils, protein, and DNA. These large molecules are used for everything that happens in the plant. Carbohydrates are used to build cell walls, which in trees, eventually turn into wood. Enzymes are proteins that make all of the chemical reactions in a plant work. Sugars and carbohydrates are the energy source that allows the plant to grow. The production of flowers and fruit require many different types of large molecules and all of these are made by the plant using the small molecules we call nutrients.

The figure below shows a list of some of the large molecules found in plants along with the nutrients used to make the molecules. All of the molecules contain carbon, hydrogen and oxygen. Nitrogen and phosphorus are also common. Other nutrients are used less often, and in smaller amounts, but they are critical for building the large molecules.

If a single nutrient is missing the plant will not be able to produce all of the large molecules it needs, and plant growth will slow down or even stop. A gardener’s job is to make sure that plants always have access to the nutrients they need. That seems like a daunting task, but it is easier than you think. There are two important concepts to understand:

• Plants need much more than just the nitrogen, phosphorous and potassium found on fertilizer labels.
• They need to eat all of the time—not just once or twice a year when you fertilize.

What is NPK?

It is time to feed your plants and you go to the store to pick up some plant food which is called fertilizer. Most packages of fertilizer show the letters NPK followed by some numbers, for example: NPK 10-5-5. NPK stands for nitrogen, phosphorous and potassium which are three of the most important nutrients required by plants. The numbers following NPK are the percent amounts of each nutrient. An NPK value of 10-5-5 means that the fertilizer contains 10% nitrogen, 5% phosphorus and 5% potassium.

NPK is the common way to describe fertilizer in North America but some countries use an NPKS value where the S stands for the amount of sulphur. Sulphur can be as important as the other three nutrients.

For my techi readers: The above statement “10-5-5 means that the fertilizer contains 10% nitrogen, 5% phosphorus and 5% potassium” is not totally correct.  For a detailed explanation see my Garden Myths blog post called Fertilizer NPK Ratios – What Do They Really Mean?

The letters N, P and K are the symbols used by chemists as a short hand to describe the elements. N is used for nitrogen and P for phosphorus. The letter K is used for potassium and stands for kalium, the original Latin name for potassium. If you have trouble remembering whether P is for phosphorus or potassium, remember that the three nutrients are listed in alphabetical order. Phosphorus comes before potassium alphabetically and so the last letter in the list, K, is short for potassium.

A bag of plant food that contains 10% nitrogen, 5% phosphorus and 5% potassium, also has 80% of some other material, but you can ignore it since it has no effect on your plants. If the fertilizer is in a dry form like lawn fertilizer the 80% may be small stones or other dry inert material. If the fertilizer is in liquid form the extra material is water.

The History of NPK Ratios

To understand fertilizer it is worth while looking at the history of the NPK ratios. How did they develop? Who decided that a formula of 10-5-5 was the right formula? Is it the right formula for every plant?

Quite a few years ago a company providing fertilizer decided to market the concept of a balanced fertilizer and decided, arbitrarily, that a 10-10-10 formula would work well. This was a marketing tactic that was not based on any kind of science. Customers bought into the idea and so you now see a lot of references using the term balanced fertilizer and recommending a 10-10-10 formulation. On the surface it seems to make sense. Plants need nitrogen, phosphorus and potassium so why not give it to them in equal amounts.

To be competitive, other companies decided that they would start promoting a fertilizer that was based on the actual nutrients required by plants. Scientists took various plants and analyzed their content. For example, lawn grass is high in nitrogen relative to P and K, and the company developed an 18-10-10 fertilizer for grass. A new marketing approach was born. Make fertilizer specific to the needs of the plant.

Over time, more research showed that plants used a lot of phosphorous to make blooms and roots, and so manufacturers developed bloom boosters and starter fertilizers with formulas like 10-52-10. This idea also seemed to make sense. Give plants the fertilizer they need, at the time they need it. Starter fertilizer is used when transplanting a new plant so that new roots grow quickly. Once established, a plant could be given a fertilizer higher in nitrogen since the plant is now focused on growing new leaves. At bloom time go back to a 10-52-10 to give it another phosphorous boost.

Start Fertilizers and Bloom Boosters are a myth – they don’t work. For more details on this see Bloom Booster – Nonsense #5 on my GardenMyths.com blog.

These advances in fertilizer development resulted in an avalanche of different fertilizer formulations and gardening experts were eager to buy into the idea. They started recommending the right fertilizer for each type of plant and for various times of year. These recommendations are still popular in today’s books and websites, and they are extremely confusing.

Don’t worry – I will make fertilizer simple for you in this and the next couple of posts.

How Does a 10-10-10 Fertilizer Compare to a 5-5-5 Fertilizer?

This is a very common question. Should I use a 10-10-10 fertilizer or a 5-5-5? What is the difference? The answer is simple. It doesn’t matter which you use because they provide the same relative amount of nutrients. It is however important that you use the correct amount of fertilizer.

When dealing with fertilizer there are two things that are important: the actual amount (the weight) and the ratio of nutrients.

The ratio is the relative amount of each nutrient. In the two examples above, there is an equal amount of each nutrient and so the ratio is 1:1:1 for both of them. In comparison, an NPK value of 20-10-10 has twice as much nitrogen as phosphorus or potassium and so the ratio is 2:1:1.

A 10-10-10 fertilizer has 10% of each nutrient and a 5-5-5 has 5% of each nutrient so a bag of 10-10-10 contains twice as much fertilizer as the same sized bag of 5-5-5. The 10-10-10 is more concentrated, but both have the same ratio.

Which one is best for your garden? It doesn’t matter since they have the same ratio. If you need to add equal amounts of N, P and K, either one works just as well. However, you will have to use twice as much of the 5-5-5 to provide the same level of nutrients as the 10-10-10.

The most important thing when buying fertilizer is to buy the correct ratio so that you get the correct relative amounts of nutrients. In general, a fertilizer with higher numbers is cheaper. Many of the liquid fertilizers on the market are very dilute, in the range of 1-1-1, and they are also some of the most expensive fertilizers you can buy. From a price point of view, always buy the one with the higher numbers, provided it has the correct ratio.

What is the Correct NPK Ratio For My Plant?

Fertilizer is now available in many different NPK ratios and knowing which one to use can be very confusing. As you begin to understand fertilizers and plants you will start to realize that the answer is actually very simple. It is however very instructive to look at the question in more detail.

A million web sites and thousands of books give you recommendations for fertilizer. Lawns need one formula, tomatoes need a different one. Tress and perennials are different again. It all gets very confusing and it is all bad advice.

Statements such as the following are always wrong:

• Use 5-10-5 fertilizer for tulips.
• Use 34-10-10 on grass in spring and a 15-5-5 in fall.
• Use 5-7-3 for vegetables.

Let me repeat the last statement—the above advice is always wrong. The advice is wrong for several reasons.

Tulips might in fact need more phosphorus—the middle number—but it is just as likely that this advice is based on old myths and not on actual science. At best these recommendations are based on plant tissue analysis and not on nutrient levels in your soil.

The main reason the above recommendations are wrong is because you don’t add fertilizer to plants – you add it to soil. Read on and this will make sense shortly.

You Don’t Add Fertilizer to Plants

The idea that we feed the plant what they need seems to make perfect sense but it ignores one very important point. Plants get their food from the soil. You don’t add fertilizer to plants—you add it to soil. This is an extremely important concept that is not well understood and is best explained by a simple example. Assume that your soil is naturally very high in phosphorous. The fertilizer you use at transplant time, or for tulips, or to get more blooms does not need a high amount of phosphorous because your soil already has more than your plants can use. The fertilizer you add to the garden should be a supplement to what is lacking in your soil.

If your soil has high phosphorus levels, any fertilizer you add – for any type of plant – should contain no phosphorus because you already have too much.

Plants absorb the nutrients they need from the soil. If grass needs more nitrogen, it takes more nitrogen from the soil than a plant that needs less nitrogen. If a plant is ready to make flowers and it needs more phosphorus, it takes more phosphorus from the soil.

Matching fertilizer to what a plant needs does not make sense. Instead you should match the fertilizer to what the soil needs. If your soil is low in nitrogen and high in phosphorus you should use something like a 20-0-5 fertilizer for all of your plants no matter what type they are.

The author that recommends a 5-10-5 fertilizer for tulips has no knowledge about your soil. So they can’t make correct fertilizer recommendations for your soil.

Consider this example. For years the common lawn fertilizer recommended for North America has been some thing like 34-10-10, high nitrogen and fairly high P and K. Over the years we learned to understand two facts. First, most urban garden soil in North America has lots of phosphorus – it does not need any more to grow grass. Secondly, the excess phosphorus ends up polluting rivers and lakes. So now many states have banned phosphorus in lawn fertilizer. In Ontario, I noticed 2-3 years ago, that many brands have now eliminated P from lawn fertilizer.

But people still recommend adding extra phosphorus for their bulbs! That is dumb. If our soil has lots of phosphorus for grass, then it also has lots of phosphorus for bulbs. I grow over 200 different kinds of spring bulbs and I have never added phosphorus – they grow and flower just fine.

In my next post I’ll have a closer look at specific plant nutrients.