By Binh Le in Gardening • Sun Mar 21 2021

Gardening 101: Fertilizers

What Do Plants Eat?

For an embarrassing long time, I just assumed that if I stuck plants in the ground and sprinkled some water, plants would just do their thing. That’s Florida education for you. But what nutrients do plants use to grow? What do they eat, so to speak?

For that matter, what do we humans eat? Why can't we eat dirt? Well besides tasting awful, dirt doesn’t contain much of the nutrients our bodies use. Almost everything we eat is organic in the biological sense, meaning they are related to or derived from other living things. It’s easy to forget that while feasting on spam and jello but animals have evolved to eat other plants and animals.

What do plants use for food? Many of us learned as kids that plants grow better using animal manure as fertilizer. 7th grade history class also taught me that Indians show the Pilgrims how to grow corn by burying fish in the ground. Feels a bit like voodoo magic. Why does it work?

It turns out plants don't eat dirt either. Plants have also evolved to make use of nutrients broken down from other living things.

Growing plants require soil as opposed to simply dirt. Dirt is the sand, clay, and inorganic earth that doesn’t contain much nutrients plants use. Soil is the organic rich mixture of dirt combined with nutrients broken down from other living things. These soil nutrients along with minerals dissolved in rainwater are absorbed by plants via their roots.

Animals consume some plants for nutrients, and plants also benefit from absorbing some nutrients broken down from animals. Thumbs up for the cycle of life. A consequence of this continual recycling of organic materials is that plants have evolved over millions of years to thrive in the soils created from their ecosystem.

Nature vs Garden

As loving plant parents, we try to create an environment that our plants will thrive in. First, we need to realize that different plants have evolved to favor different conditions. Therefore, we should try to customize soil conditions for their individual needs. Second, since our garden is an isolated system, it is important to replenish nutrients that our plants use up.

While it is possible to feed our plants naturally through composting, I opted to use commercial fertilizers as an easier and more consistent alternative. I was hesitant at first because fertilizer feels unnatural. Where's the plant love after all? Well, I’m sure Fluffy would much rather feast on steak and prime ribs too, but nope, he gets dog food.

I don't have an issue using commercial fertilizers because they're different from compounds like pesticides. They are more or less an optimized blend of nutrients our garden plants use as food. They're not toxic to plants, and we're not taking in toxic chemicals by eating them.

I recommend using an all-purpose fertilizer to start. It makes fertilizing as simple as possible since the nutrient demands for many plants are very similar.

However, some plants will grow noticeably better if we optimize their nutrients as needed. Just as extra protein powder may help athletes maximize muscle growth and supplemental vitamin C keeps pirates from getting scurvy, a little bit of knowledge will allow us to feed our plants the right things at the right time for the best quality yield.

I’ve tried to be as concise as possible in the following sections but there’s a lot to cover… and it’s about fertilizers. Not the most exciting topic. Nevertheless, bear with me for the next ten minutes and you'll gain enough fertilizer knowledge to grow big round tomatoes and large cucumbers that will make your friends jealous.

Primary Macronutrients - NPK Fertilizers

Nutrients that plants require in large amounts are called macronutrients. The big three are nitrogen, phosphorus, and potassium. They are like nutrient rock stars identified simply by their initials N-P-K. These hyphenated numbers are usually displayed prominently on fertilizer packaging without any explanation. An example would be something like 20-20-20 to represent N-P-K in that order. The three digits represent the ratio of each nutrient in relation to each other. This ratio outweighs most other considerations when picking a fertilizer.

Knowing everything that NPK does is not that important to us. Broadly speaking:

  • Nitrogen (N) - Grows leaves.
  • Phosphorus (P) - Grows roots, flower and fruit.
  • Potassium (K) - Promotes plant resilience, improves flowering and fruiting, converts sugar to starches.

An example of a common all-purpose fertilizer is one with something like 10-10-10 or 20-20-20 as its NPK. In contrast, fertilizers for flowers might have an NPK such as 1-34-32 while lawn fertilizers might have an NPK of 16-4-8.

Fertilizer companies customize their products’ NPK to the ratios that are most beneficial for the intended purpose. In the above case, switching to a fertilizer higher in P will benefit flowering plants once they start blooming. Fertilizers with high N and K improves the lawn since grass consists of only leaves and benefits from stronger plant resilience.

Different NPKs At Different Life Stages

As living things grow, their diets change. When we were babies, we mostly drank milk. As kids, we start to prefer simple foods. Then as adults, we eat all sorts of weird things.

A plant’s nutritional requirement also changes during different stages of its life. For young plants, growing leaves is a priority. Therefore, many starting fertilizers are nitrogen rich.

However, as some plants start to flower and bear fruits, growing leaves becomes less important than improving fruit quality and quantity. Therefore, switching to a fertilizer mixture that is higher in P and K may be beneficial.

Different plants also benefit from different nutrients because we value different aspects of the plants. A leafy vegetable such as lettuce or spinach does not require much P since they do not produce fruit. They are perfectly happy with high levels of N to keep their leaves growing.

Fruit plants such as tomatoes in contrast will produce fewer fruits if their foliage growth is left unchecked. A high N, low P diet will result in a large and healthy tomato plant but one with fewer fruits and of lower quality.

Mixing NPK Fertilizers

If I throw a chicken breast, some vegetables, and a piece of bread into a blender, it is still a healthy well balanced meal. Whether I enjoy it or not is a different matter. Our plant’s palate isn’t that discerning. They don’t really care as long as all the nutrients are there in the right ratios. Therefore, there are no issues combining different fertilizers to come up with new ratios of NPK.

Suppose that two brands of fertilizers recommend mixing one teaspoon of their fertilizer for each gallon of water. Their two NPKs are 4-18-38 and 10-52-10. If we mix half a teaspoon of each brand into the same gallon of water, we can average their NPK.

04-18-38 NPK of A
10-52-10 NPK of B
07-35-24 NPK of A & B

We can add their values together, then divide by two since we are using half the normal concentration of each. Our new mix is equivalent to a 7-35-24 fertilizer.

Another relevant example of mixing fertilizers is combining calcium nitrate to an NPK fertilizer for hydroponics. In addition to providing a source of calcium, calcium nitrate has 15% nitrogen. Its NPK is 15-0-0. If we add one part calcium nitrate to one full part of our 4-18-38 fertilizer, what we end up with is a 19-18-38 NPK fertilizer.

One reason why we may want to mix fertilizers is it allows us the flexibility of trying new NPK blends by combining what we have.

I’ve compiled a spreadsheet of commercial grade fertilizer for various crops. It’s interesting to see what ratios they use for each type of plant. I assume that their blend of NPK is fairly optimal. So if I’m not able to get good results from my all-purpose fertilizer, then I can try to adjust it closer to the professional blend to see if my results improve.

Insuring The Proper Concentration Of Fertilizers

Human foods have calories listed on the packaging to help guide us with how much we should optimally eat. If I eat only one hotdog for the whole day, I’m starving myself. But if I crush 50 hotdogs in one sitting, I’m not feeling great either.

Plants also require the right amount of fertilizers. Too little and they starve, but too much and their leaves start to die as well. The simplest way to add the correct amount of fertilizer is to always add a consistent amount every time we water our plants. Mixing the manufacturer’s recommended dose to one gallon of water each time ensures that our concentration will be fairly close to optimal.

Nevertheless, it’s useful to be able to spot check our solution’s nutrient levels directly for hydroponics.

Electrical Conductivity (EC)

EC provides a simple ballpark of how much fertilizer is in the water by measuring the amount of dissolved material in it. Young plants require less food than adult plants. A solution showing 1300 for EC is typical for many young plants and 1500 for many mature plants. If the value is too low, then we need to add more fertilizer to our solution. Conversely, if significant evaporation increases our fertilizer’s concentration, we may need to add more water. My spreadsheet of NPK fertilizers ratios also provides optimal EC values for each type of plant.

EC measuring devices sell for as low as $10 on Amazon.


pH measures how acidic or basic a solution is. It is affected by many factors. Water and soil in different parts of the country may differ in pH. Fertilizers may also change the pH level.

Proper pH levels allow plants to make use of nutrients. Plants extract their nutrients from minerals dissolved in water via osmosis. pH that is too low or too high will interfere with a plant’s ability to absorb nutrients even if the nutrients are abundant.

Most plants prefer pH between 5.5 - 6.5. if the pH is already in the optimal range, then we don't need to do anything. If it's outside that range we would need to add soil amendment to adjust its pH, or in the case of hydroponics, add pH correcting buffers.

A pH meter or testing kit also sells on Amazon starting at about $15.

Hydroponics vs Soil Fertilizers

Growing plants in soil versus in hydroponic solutions is mostly similar. Plant roots growing in soil still have to absorb nutrients that have been dissolved in water. That’s why the soil needs to be moist. The main benefit that soil provides is the rigidity for roots to anchor into so plants don't flop over. However, there’s nothing preventing plant roots from absorbing nutrients directly from the water in the absence of soil. They actually do it more efficiently.

Even though the nutrient requirement is not different for hydroponics, their fertilizer mix may be more specialized. The reason is because most soil already contains enough minerals such as calcium and magnesium that plants need, so fertilizer companies don’t include them in their mix. Therefore, we need to supplement the nutrients that may be lacking depending on our growing method.

Secondary Macronutrients - Calcium, Magnesium, Sulfur

In addition to NPK, plants require other essential nutrients in lesser amounts. We usually do not need to worry about the lesser macronutrients since most NPK fertilizers are complete, meaning they have all the nutrients plants usually require. But sometimes it makes sense to supplement our NPK fertilizers with additional nutrients that may be deficient for our intended purpose.

Instead of breaking down secondary nutrients by their individual elements, it might be easier to describe their benefits along with how we are likely to obtain them. Below is a quick rundown of popular minerals I use as supplements to my fertilizers.

It’s important to also note that some nutrients compete with each other for absorption. For example, magnesium competes with calcium so that too much magnesium in the soil may leave a plant deficient in calcium. We can detect signs of deficiencies by observing different types of unhealthy leaf growth associated with each.

Magnesium Sulfate - Epsom Salt

Magnesium is found naturally in most soil and many plants don't require much of it. Therefore, many NPK fertilizers will not have it in high concentrations. However, magnesium improves flower and fruit production. As a result, fruit plants such as tomatoes that depend on more magnesium will suffer from lower quality fruits if magnesium is deficient in its soil. Adding magnesium sulfate, commonly known as epsom salt, can correct this magnesium deficiency.

Growing plants in absence of soil means that magnesium will definitely be lacking. Therefore, epsom salt is also a very important addition to our hydroponic solutions.

Calcium Nitrate

Calcium is also naturally abundant in most soil. Therefore, most fertilizers will be deficient in calcium as well if they are used for hydroponics. Calcium nitrate is the preferred form of calcium to amend hydroponic solutions because it is the only form of calcium that dissolves readily in water.

At high concentrations, however, calcium nitrate will react with other compounds in NPK fertilizers to form solid precipitates. The reaction doesn't create anything harmful, but the solid precipitate sitting at the bottom of our reservoir means that these undissolved nutrients will not be available to our plants.

That’s the reason why many liquid hydroponic solutions sold come in two bottles, A and B. One bottle consists of the NPK fertilizer and the other consists of mostly calcium nitrate by itself. They are added to the solution one at a time to allow each to dilute enough to not react.

There’s nothing else really special about the hydroponic two solutions combo. It’s easy to mix our own from any store bought solid fertilizers. It’s much cheaper than buying the pre-mixed liquids.

Calcium Sulfate - Gypsum

Gypsum also supplements calcium but is more than 500x less soluble than calcium nitrate. It can’t be used as a source of calcium for hydroponic solutions because enough of it won’t dissolve in water.

Since Gypsum is only slightly soluble, the nutrient release will be slower. Therefore, the granules are a good choice to replenish calcium in wicking pots. I just sprinkle gypsum directly in the soil along with my NPK fertilizer when I set up my container. I add more to the top of the soil once a year.

The reason why gypsum benefits potted plants is because potting mix is used instead of normal soil to maximize water drainage in pots. Potting soil contains lightweight fillers such as perlite and coco coir to keep the soil loose and improve aeration, but the fillers do not contain calcium or sulfur. Adding gypsum amends calcium and does so without affecting the soil pH.

Calcium Carbonate - Lime

Lime can be used instead of gypsum as an alternate for amending calcium to the soil. The main difference is that lime lowers the acidity of the soil while gypsum does not. Therefore, lime is an important soil pH balancer in parts of the country where the soil is too acidic for optimal plant growth. You can Google your local soil acidity.

In addition to using coco coir as a potting mix filler, peat moss may be used instead. Peat moss is usually acidic and lime may be required to sweeten the soil if peat moss is used.

Sea Minerals

Trace minerals serve a similar role for plants that vitamins do for humans. We just need a little bit of it to stay healthy.

As many as 45 lesser nutrients seem to benefit plants in some way even if they are not strictly essential.

Minerals may improve the taste and quality of fruits. Plants also require minerals to make vitamins. One reason why organic produce is healthier is because organic soil contains many complex minerals. Sea minerals contain as many as 90 minerals in trace amounts, which may also benefit our plants.

In Conclusion

Knowing fertilizer basics will allow us to experiment and tweak our fertilizer’s nutrients. We can simply grow the same plant using two different fertilizer mixes and compare to see which one performs better. We can also diagnose nutrient deficiencies and adjust the fertilizer accordingly.

There was a lot to take in but hopefully it was informative enough to demystify the many offerings of fertilizers. There are so many ways to combine different fertilizers other than how I do it. But to give you a starting point, I will provide the exact fertilizer blend I use in each of my growing guides.

Fertilizer Recap

  1. Proper NPK ratio is more important than the actual fertilizer brand used.
  • Higher N promote foliage growth
  • Higher P to promote flower and fruits
  • Higher K to promote general health and many other functions
  • Most secondary nutrients are usually included in NPK fertilizers
  1. The following fertilizer amendments fix specific nutrient deficiencies:
  • Epsom salt supplement magnesium and sulfur
  • Calcium nitrate supplement calcium for hydroponics
  • Gypsum supplement calcium for neutral soil
  • Lime supplement calcium for acidic soil
  • Sea minerals supplement trace minerals

My hydroponic solutions always include epsom salt and calcium nitrate plus some NPK fertilizer. My fertilizer used in wicking pots adds either gypsum or lime depending on whether I mix in peat moss as a filler.

  1. EC estimates how much fertilizer is in the water, typical value needs to be around 1300 and 1500 for young and mature plants, respectively.

  2. A pH between 5.5 and 6.5 is preferred by most plants to be able to uptake nutrients.

Additional Resource: Coming Soon

Commercial Fertilizer Blends

Symptoms of Mineral Deficiencies In Plant

How To Mix Hydroponic Solutions From Any All-Purpose Fertilizer

← Home