Basil needs fertilizer to maximize its growth and harvest. It is a very leafy plant with no fruits that requires a specific type of fertilizer, different from vegetables or ornamental plants like roses. Which one to choose?
The best fertilizer for basil plants is given by an equal mix of nitrogen, potassium, and phosphorus like a 10-10-10 or a 4-4-4. Liquid fertilizer is preferred as easier to apply. Basil should be fertilized during late spring every month if indoor and every 2-3 weeks if outdoor due to their different rate of growth.
However, you might wonder what do those 3 important numbers stand for in order to understand how this will affect your plant. You might wonder why you should use liquid fertilizer and what’s happened if you use different types. The following might answer your questions.
What To Look For in The Fertilizer For Your Basil?
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The most important factor to identify the best basil fertilizer is the so-called “grade”. This is a three-sequence number that you can find on the label of the fertilizer. Each number represents the percentage of Nitrogen (N), Phosphorus (P), and Potassium (this starting with a K in Latin) respectively.
These are the key nutrients that each plant needs for its development.
The grade is associated with the letter N-P-K (or N:P: K) on the fertilizer label. Hence, a N-P-K (or N:P:K) ratio of 10-10-10 (or 10:10:10) simply means that 10% of the fertilizer weight is Nitrogen, 10% is Potassium and 10% is Phosphorus. The remaining is typically inert material like limestone and/or sand or water.
Basil will thrive best with a 10-10-10 grade fertilizer able to provide a balance of nutrients with a relatively light (and, in my opinion, safer) concentration of nutrients This will boost the size and flavor of your basil leaves!
Here are good options for liquid and granular fertilizer for basil.
Why not a Stronger Fertilizer For Basil?
I recommend not going above the 10-10-10 grade for basil fertilization. Indeed, a higher ratio (such as 20-20-20) implies a higher nutrients concentration that might negatively affect the basil plant if there is no reason to go so heavy with it.
Solid Or Liquid Fertilizer?
Once you found your 10-10-10 fertilizer how to add it to your plants? Fertilizer in the supermarket comes in two forms: liquid and granular.
For basil, liquid fertilizer is often preferred. Indeed, Liquid fertilizer is easier to use and dose compared to a granular one. It is easier to apply and beginner-friendly.
If you go to any store for indoor herb planting (even an online one if you do not want to move!) you will notice that for domestic use the most common/popular options are liquids.
This type of fertilizer is sold in small plastic bottles (from as little as 30ml) containing either powder or (concentrated) liquid fertilizer that, before being deployed, must be diluted in water. Never use pure concentrated fertilizer if you do not want to kill your plant.
The main advantages of a liquid form are the ease in its use (just spread the mix of fertilizer and water, in the proportion indicated on the label, adopting a sprayer if possible for better results) and the uniformity by which the ingredients are in the liquid. Moreover, such fertilizers are quite inexpensive (4-5 dollars/pound for a small bottle that can last several months).
This type of fertilizer needs to be applied fairly frequently (once a month if indoor, every 2 weeks if outdoor) as it is rapidly absorbed by the plant. Remember to avoid any direct contact of the concentrated fertilizer with the plant. Such chemical can literally burn your plant (that explains why for a teaspoon of fertilizer you might need to add 1 liter of water)!
Indoor basil requires less fertilizer due to its slower pace of growth.
If you are willing to step up your gardener knowledge and learn more about fertilizer here is a tip for you: you can test the effect of your fertilizer and learn the right amount that you need to make your plant strive at its best (as depends on the initial soil conditions) by adding a “personalized” amount of water (I would start with a higher amount than the one recommended in the label for a safe soft-start).
Then, after 2-3 weeks check what’s happened to your basil. Are the leaves larger, greener and more importantly, tastier? If so then in the next fertilization try to reduce a bit the amount of water and monitor the effect on your plant again! That’s a fun and interesting learning process!
Regarding solid (better called granular) fertilizers, these are more for an advanced gardener or mass production in fields. Indeed, granular fertilizer is typically sold in larger volume and used in a blender to create a specific mix of nutrient important to increase their revenue as for them, is a business!
Moreover, such granules have the ability to slowly release the nutrients into the soil, a great advantage as allows to greatly reduce the number of times you need to fertilize (only once or twice a year!), key in mass production fields where time and cost can be drastically reduced.
Can I Use All-Purpose Fertiliser?
Yes, you can but that does not mean you should! Although many people claimed decent success with them I would recommend sticking with the 10-10-10 ratio or going for an organic one (like the 4-4-4 as the Dr. Earth one). Indeed, all-purpose fertilizer can be more prone to massive mistakes that can cause serious problems to your plant. What do I mean?
Plants, similarly to animals are complex living organisms, each one with its ideal nutrients requirement. Think about your dog. It is classified as an animal, like me and you, but would you feed him with a bar of chocolate? No, as they are unable to digest it. The same applies to plants. It is hard to believe that exists a formula that fits all. Your basil needs a specific balance of nutrients.
In the supermarket, you might be tempted by the so-called “all-purpose” fertilizers. This type of fertilizer claimed to be good for all plants, has an NPK ratio that varies greatly from brand to brand. Online I saw 12-4-8 as well as 16-20-24. So, at a first glance, such an “all-purpose” fertilizer does not look so universal if its composition can vary that much. However, you might have noticed that they all have one common feature: a higher percentage of nitrogen.
Nitrogen is a big deal! It is true and widely accepted that nitrogen allows plants, of any type, to grow larger and greener leaves (fruits) and look, overall, healthier. This is definitely true for your basil as well as reported by several studies where a higher concentration of nitrogen was associated with improved taste and fragrance. Hence, you should definitely use them to step up the game in your gardening. But in the right way!
Increasing N (nitrogen) doses resulted in higher total leaf area.From Scientific Paper
Indeed, although “all-purpose” fertilizers, can improve the overall health of your basil thanks to their high nitrogen content I do not recommend them as it is way easier (especially for people approaching fertilizer for the first time) to exaggerate the amount of nitrogen introduced in the soil. This is a problem as high nitrogen concentration negatively affects the flavor of the basil leaves that lose their fragrance, oils and change their taste for the worst (they get bitter!). That might be ok for an ornamental plant, but not for an aromatic herb that you are planning to eat in some nice dish.
Data […] also indicates that the lower rate of fertilizer produced basil with the most desirable flavour. Therefore, lower rates of N fertilizer may increase fresh basil quality.From Scientific Resource
If you want a bit of science I can tell you that, a plant, a bit like a person, can absorb only a limited amount of nutrients. Hence, if your basil absorbs more of one nutrient (nitrogen in this case as plenty in a saturated soil) is then unable to absorb, in the same amount, the others. This ultimately affects the production of all the 200 chemicals that drives its flavor that, of course, depends not on only nitrogen. Indeed, if this was not the case, why bother in developing N-P-K fertilizer?
Excess nitrogen has also other detrimental effects. It weakens the basil stem that becomes so unable to withstand the weight of the larger amount of leaves produced.
Another problem might be salt. Indeed, a large amount of nitrogen causes salt accumulation in the soil causing the root to dye as their water is sucked up out of them due to the high salinity of the soil as backed by research
The applied fertilizers amounts should be controlled to avoid additional salt accumulation …  and thus inhibit plant growthFrom Scientific Resource
Are coffee grounds recommended to fertilize basil? It is not recommended as they are acidic, with a high ratio of nitrogen, and potential cause of fungus growth in indoor plants if used moisty.
Is fertilizer ingestion dangerous for dogs and cats? Yes, fertilizer ingestion can cause hypersalivation, vomiting, diarrhea, pruritus, constipation, and muscle stiffness.
Can I fertilize before it rains? It is not recommended as the fertilizer and its nutrients might be washed away before the plant has the time to absorb them.