Using fertilizers for plants is not an uncommon practice. But right now, the attention has shifted from “feeding the plants” to “feeding the soil”. Maybe you feel right now the need to buy either a chemical or natural material to boost your plant growth and keep it healthy. That’s great! You just have to keep in mind that buying the wrong fertilizer for your plants may actually cause more harm than good. If you are not sure what to look for in a fertilizer and how to choose the right one, read this quick guide on fertilizers.

Product Label and the numbers on it

Choosing fertilizers is not only about which has the best packaging or which has the cheapest price. Doing so may worsen your plant’s health. If you examine carefully the label of the product, you will notice that there are three numbers written. It could be 5-10-10, 10-10-10 or 10-6-4, etc. These numbers may be located either below or above the product name. These three numbers are essential in deciding whether that fertilizer is appropriate for your plant’s needs or not.

What are these three numbers for?

These three numbers, or “the big three” are what we call N-P-K ratio. N-P-K stands for Nitrogen, Phosphorous, and Potassium respectively, its acronym derived from the chemical element symbol of the said three. The first number represents the nitrogen content, the second, the phosphorus, and the third, the potassium. Thus, a 100-pound bag of fertilizer with an N-P-K ratio of 10-10-10 means that it contains 10 pounds of Nitrogen, 10 pounds of Phosphorus, and 10 pounds of Potassium.

Interpreting the N-P-K Ratio

Applying basic mathematics, it is very easy to understand the ratio behind it. A 20-20-20 or 19-19-19 N-P-K has a 1:1:1 ratio. However, 15-30-15 has a 1:2:1 ratio, while 30-10-10 has a 3:1:1 ratio. Basically, it is like simplifying the ratio in its lowest terms. 

Does it mean that a 10-10-10 fertilizer has the same content as the 5-5-5 variant?

Nutrient-wise, this doesn’t make much of a difference because in terms of ratio, they are both 1:1:1. However, this can matter in terms of the quantity. Another way of interpreting it is by looking at it in percentages. A 10-10-10 means 10 percent of each component and 5-5-5 means 5 percent for each. Hence, a 100-pound fertilizer with 10:10:10 ratio has twice as much nitrogen, phosphorus and potassium as the 5-5-5 ratio. The most important thing is to get its ratio in lowest terms.

Once you know its ratio, next you have to determine which is appropriate for the plant kind and stage you are buying it for. 

  • 1-1-1 ratio or balanced, is ideal for matured plants
  • 2-1-1 ratio or boosted growth, is advisable only for seedlings
  • 1-2-2 ratio, or rooting, is perfect for plants after repotting
  • 1-2-1 ratio, or flowering, is best for matured plants once a month

These ratios are historically and scientifically proven. Whichever number in the ratio is higher indicates what the plants need—either growth, tissue generation, disease protection, flower and fruit production, etc.

The Importance of knowing N-P-K Ratio

Information suggesting a certain ratio for plants such as 5-7-3 for vegetables and 5-10-5 for tulips are wrong. It is because there is no-size-fits all ratio that is formulated specifically for plants, flowers, crops, etc. We have to remember that fertilizer is applied on soil and not on-plants, thus, it is more practical to do soil analysis rather than plant tissue analysis.

If after soil analysis, and it is found that there are no specific soil needs, the 5-5-5, or the all-purpose fertilizer is recommended.

That is why it is important to know the basics in N-P-K interpretation and the function of each element. Doing so enables you to choose the right product for your plants’ needs with confidence.

You might be asking, why of all the elements, why does it have to be the Nitrogen, Phosphorus, and Potassium? It is because along with 17 nutrients, these three are the primary macronutrients. In other words, plants need large quantities of these in order to survive. The plants also need other elements, such as iron, but unlike the aforementioned three, iron is only needed in smaller quantities.

Since plants get their nutrients from the soil and air, sometimes, soil can get depleted of nutrients. This is the reason why fertilizers are needed to replenish the soil’s lost nutrients, which will later on impact the plant’s health.

Digging Deeper into Nitrogen, Phosphorous, and Potassium

We already know that these three elements are considered as the primary macronutrients. However, how and why does each element actually work?


The chlorophyll, or the green-coloring pigment of plants enable plant growth. It is interesting to note that the Nitrogen (chemical symbol N), works well in promoting leaf growth and optimum shoot as this element is also a component of the chlorophyll molecule.  Without nitrogen, there will be no chlorophyll. Without chlorophyll, there will be no photosynthesis. Without photosynthesis, the plant will die.

Adding a good amount of nitrogen to plants that require huge amounts of the said element maximizes the benefit in terms of growth and health. It encourages green growth on plants. Basically, more nitrogen, more chlorophyll, and more energy absorbed. Nitrogen deficiency in plants will lead to yellowing of leaves, and a slower growth. However, too much of it or over fertilizing will also make a plant burned.

 Leafy plants and vegetables such as lettuce and spinach are examples of plants that would love a great amount of nitrogen. However, doing the same thing for a fruiting or flowering plant could have excessive green growth. For instance, adding 6-2-1 or 10-5-5 N-P-K ratio would hamper plants’ fruit or flower production.


Contrary to nitrogen which may hamper fruit and flower production, phosphorus (chemical symbol: P) does the opposite as it actually aids it. That is why, for plants that produce flowers and fruits, such as squash and tomato, having a big amount on the second number or the N-P-K, or choosing fertilizers with a high concentration of phosphorus is recommended.

Phosphorus also aids cell division, root development, and plant tissue generation, thus also recommended for root crops. It also protects plants from diseases. Phosphorous deficiency may lead to stunted growth and reddish-purple leaves. In this case, bone meal, rock phosphate and manure are good for the soil as these are rich in phosphorus.

However, too much phosphorus concentration can lead to water pollution as well as algal blooms.


This element, (with a chemical symbol K), also known as potash, controls the stomata or the pores on the surface of the leaf. In other words, this regulates carbon dioxide uptake, thereby affecting the plant’s vigor.

Aside from its role in carbon dioxide regulation, it also helps with water, sugar, and nutrient movement within the plant’s tissues. Large concentrations of potassium can be found in wood ash, compost and manure.

Potassium deficiency may lead to stunted growth, inability to withstand extreme temperatures, and lower fruit and vegetable yield. On the other hand, too much of the said element is also not good as it can lead to nitrogen deficiency.

Other essential plant nutrients

Aside from the big three nutrients mentioned above, there are other nutrients that contribute to plant health, albeit in small quantities. These are Calcium (Ca), Magnesium (Mg), and Sulfur (S).

Calcium promotes roots and shoots growth; magnesium helps in seed formation and darker green leaves for photosynthesis; while sulfur maintains the said dark green color.

To find out which of these elements you need to focus on, it is important to do soil testing. Knowing what the soil lacks and providing it will greatly benefit your plant. Because of changes in soil components, its needs will also change, hence, it is advisable to have the soil tested every two to three years.

Other facts you should know about fertilizers


The pros and cons of synthetic and organic fertilizers

N-P-K nutrients may come in synthetic or organic versions.  The former means it is laboratory-made. The downsides of the synthetic ones are that it can cause pollution and may “burn” and damage the plants. 

On the other hand, organic fertilizers come from natural materials such as feather meal and manure. This kind of fertilizer works well with soil microbes, and may even promote a healthier soil biology. You will also be getting value for your money as the nitrogen is released back into the air within 15 weeks, compared to that of the synthetic fertilizers which is 3-6 weeks, a study from the University of Massachusetts revealed. The downside is that organic fertilizers need longer time to enrich the soil.

There are also options that combine both organic and synthetic fertilizers for optimum results.

The difference between granular and soluble fertilizers

In terms of formulations, it could also be granular or soluble. What are the differences between the two?

Granular fertilizers, from the word “granules” come in solid form, thus, it needs water and a given time (could be weeks) before they dissolve and are available for plant absorption. This kind of fertilizer is appropriate for long-term plant health.

 Meanwhile, the soluble fertilizers, also known as “liquid feed”, once dissolved in water are easily available for plant absorption. This is used as a supplement for granular fertilizers in order to boost plant growth, as these are rich in nitrogen.

What if I do not want to buy a fertilizer?

If you are not into buying fertilizers and would love to improvise and make your own, you could try compost tea. This kind of fertilizer is made from liquid extraction from microbes such as fungi, bacteria, nematodes, etc. The same process as brewing beer is used in making compost tea.