What is Hydroponic Fertilizer?

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Robert Pavlis

Hydroponics has become more popular especially with the invention of the aerogarden systems, like AeroGarden. DIY home hydroponic grow systems are also popular, as are Kratky methods. All of these systems require hydroponic fertilizer which is quite a bit more expensive than regular soluble fertilizer. It begs the question, can you use regular fertilizer in place of hydroponic fertilizer? What makes hydroponic fertilizer special and what should you be looking for when you buy them?

hands holding lettuce plant showing roots
Lettuce grown hydroponically, source: Farm

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What is Hydroponic Fertilizer?

The marketing hype around these products would lead you to believe that hydroponic fertilizer is much superior and quite a bit different than regular soluble fertilizer, but that is not really true. Both products contain similar ingredients. Both use simple salts such as calcium nitrate, magnesium sulfate, potassium phosphate. When these salts are dissolved in water they form ions that are the same in both types of products and include things like iron, magnesium, nitrate, ammonium, sulfate etc.

Hydroponic fertilizer and regular fertilizer are so similar that analytical laboratories use the exact same test for both.

The various products contain different amounts of each ingredient and therefore have different NPK values and different amounts of micronutrients. But fertilizer for hydroponics is not that different from regular fertilizer. There are some differences that will be discussed below.

I am using the terms “regular fertilizer” and “soluble fertilizer” to refer to products that are synthetic and water soluble, or are already provided in a liquid form. Organic fertilizers are a bit different and are discussed below.

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Aeroponic Gardening

Aeroponic garden systems, such as the AeroGarden, are normally supplied with aeroponic fertilizer which is no different than hydroponic fertilizer. Everything in this post also applies to aeroponic gardens.

Claims for Hydroponics Fertilizer

The marketing material for hydroponics and many comments online would lead you to believe these products are special. Here are some of the claims.

Hydroponic Nutrients are Purer

I doubt that is true, but even if it is – it does not matter. In fact when it comes to fertilizer, purity can be a negative quality. When the compounds are dissolved in water, the salts break apart and form ions. Once they are ions they all have the same purity. A potassium ion from a dirty source is identical to a potassium ion from a clean source.

Impurities in fertilizer are usually other compounds and some of these will be the other micronutrients that are not put into fertilizer. For example, plants do need a very small amount of sodium and sodium is usually one of the impurities in fertilizer. It is not added, because plants need very little and a bit of impurity adds enough.

Hydroponic Fertilizer is More Balanced for Hydroponic Systems

By balanced, I assume they mean that the ratio of nutrients is better suited for hydroponics. That is really not the case. First of all, different brands of hydroponics fertilizer have different ratios of nutrients. There is clearly no one “perfect” balance. Secondly, regular fertilizer can be selected to have the same range of nutrients, so they are just as balanced.

Formulated to Provide Nutrients in a Form that is Available to Plants Grown in a Soilless System

Almost all indoor potted plants grow in a soilless system – usually peat moss or coir. Outdoor containers are also soilless. Even some raised beds use a soilless system, e.g. Mel’s square foot mix.

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Clearly regular fertilizer is also formulated for these soilless systems. In fact, all peat-based and coir- based media are really hydroponic systems, just like hydroponic systems using sand or clay pebbles.

Soil Fertilizers Must be Broken Down by Soil Microorganisms

Organic fertilizers do have this requirement, but synthetic ones don’t. Synthetic soil fertilizers dissolve in water as easily as hydroponic fertilizers and the nutrients in both are immediately available.

Use the Right Nitrogen for Hydroponics

The form of nitrogen used in fertilizer can be important. Nitrogen can be supplied as nitrate, ammonium or urea. Contrary to a lot of online information, plants can use all three of these forms of nitrogen. Their preference is nitrate and they tend to use urea the least, but they can use all three. Urea is broken down quite quickly into ammonium and nitrate by microbes. This is one reason urea is used more in fertilizer for gardens; the second reason is price.

Which form of nitrogen is best? Hydroponic studies show mixed results on this question. In one case they found plants did not grow as well with urea. In another study by the same group, the difference in tomato growth was minor between nitrate and urea and they concluded that, “urea should be a suitable hydroponic N source for tomato plants”. Tomato seedlings are less able to use urea. Nitrate is the preferred source for hydroponics and most commercial products use it. However, if a synthetic water soluble fertilizer uses nitrate, it is just as suitable for hydroponics and mixtures of nitrate and urea also work.

Organic Hydroponic Fertilizer

Is organic better? Many people are convinced that organic is always the better option but that is not true in this case. Organic is great in soil systems if it adds carbon to help microbes build soil structure. Many liquid forms of organic fertilizer don’t add much carbon. Organic is also a good choice when you want a slow steady feed for plants. That is great in a garden but not in potted containers, or in hydroponics.

Organic fertilizer provides some nutrients in a plant available form that plants can use right away, but most of the nutrients are tied up in large molecules that need to be degraded by microbes before they become available. In hydroponics you don’t want to put plants on hold until this happens. Organic fertilizer is not a good option for growing in either pots using a soilless mix or hydroponics.

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Essential Micro-Nutrients

Some synthetic fertilizer only provides the macronutrients; the NPK. They don’t include micronutrients because soil can provide them. Since a soilless environment requires the addition of micronutrients, most hydroponic fertilizer and some general purpose soluble fertilizer does add them.

Comparison of micronutrients found in hydroponic and regular fertilizer:

NutrientMaromalife A + B Hydroponic FertilizerPeters Profession 24-8-16 Fertilizer
Iron0.03%0.015%
Manganese0.06%0.03%
Copper0.05%0.015%
Zinc0.01%0.03%
Boron0.15%0.015%
Molybdenumnone0.006%

Why Does Hydroponics Use “A” and “B” Mixtures?

A and B mixtures are used to prepare concentrated stock solutions or to ship liquid fertilizer in a more concentrated form. The reason A and B are kept separate until they are diluted is because some ions are not very soluble and will precipitate out if they are too concentrated. This happens mostly with calcium, sulfate and phosphate, which form calcium sulfate and calcium phosphate. Plants can no longer use them once they form a solid.

Some hydroponic fertilizer does not use A and B mixtures. If the product is shipped with enough water, the nutrients stay in solution. A and B is also not needed when the fertilizer is supplied in a dry form, which is becoming more popular.

Dry vs Liquid Hydroponic Fertilizer

Hydroponics fertilizer is sold in both liquid and dry (solid) forms. Which is better?

The liquid form may be a bit easier to use, but it comes with a price, both financial and environmental. Shipping water around the world is bad for the environment and costs you extra money. Dry is a better option.

Grow vs Bloom Stage

As plants progress through different stages of life they have different requirements for nutrients. Seedlings tend to use more nitrogen and less potassium. As they mature, lower nitrogen levels may help initiate blooming. To meet these needs, commercial fertilizer is provided for both a growth and a bloom stage. Cornell has done a good job presenting the required nutrient levels for various stages of growth in tomatoes. You will notice that the difference between stages is small and most home growers simply don’t have enough control over levels to worry about the differences. Several manufacturers of hydroponic fertilizer agree and offer a one stage solution.

Bloom Boosters don’t work, either in hydroponics or the garden.

table of nutrients showing values for seedlings, growth phase and bloom phase
Nutrients used to grow tomatoes hydroponically, in different stages of growth, source: Cornell University

Best NPK for Hydroponics

Most plants grow best with a standard NPK and there is no need to try and match it to the type of plant. From the above table you can see that tomatoes grow well with an NPK of 4-1-6. Penn State Extension recommends an NPK of 3-0.5-2. A value somewhere between these two, such as 3-1-3, would work well.

To learn how to calculate NPK from the above values have a look at Fertilizer NPK Ratios – What Do They Really Mean

The Quality of Your Tap Water

Almost all fertilizer, both solid and liquid forms, require you to dilute the product in water before it is used for plants. Some products ask you to do this with distilled or RO water, both of which have few chemicals in it. Some products don’t mention the type of water, and still others offer two products, one for distilled water and one for hard tap water.

To better understand how water quality affects plants see: Solving Water Problems For Indoor Plants: Hardness, pH, Salts, Alkalinity & Chlorine

Why is the quality of your tap water important?

  • Alkalinity
  • pH
  • Cal-Mag

Each of these water properties affects how plants grow in the final hydroponic solution.

Alkalinity

Alkalinity is a measure of the total carbonates (CO3), bicarbonates (HCO3) and hydroxyl ions (OH). These ions usually get in our water when it dissolves limestone which also contributes calcium and magnesium. Water that has high alkalinity usually has high calcium and magnesium levels. Alkalinity neutralizes acids and tends to keep the solution alkaline.

Learn more about alkalinity in What is Alkalinity – It May Not Be What You Think?

pH

The pH of the final hydroponic solution is determined by both the added fertilizer and the water used for dissolving it. Urea and ammonium makes the water more acidic, while nitrates make it more alkaline. Plants prefer growing in a pH between 6 and 7.

Cal-Mag

The calcium and magnesium levels in water, known as cal-mag, can be quite high in tap water which is the reason many common fertilizers do not include calcium or magnesium – your water already has enough of these nutrients. You can buy a cal-mag fertilizer that only contains these nutrients if it is needed. Hydroponic fertilizer tends to include calcium and magnesium because they assume you will be mixing it in distilled water.

If you are using a hydroponic fertilizer that contains calcium and magnesium, and you have hard tap water (high cal-mag) you should mix the fertilizer with distilled, RO or rain water. That prevents having the level of cal-mag too high.

Alternatively, you can use a different fertilizer that does not include calcium and magnesium. There are some hydroponic fertilizers formulated for hard water.

If you are using a soluble fertilizer that does NOT include calcium and magnesium, and your water is hard, you don’t have to add cal-mag fertilizer because your tap water is adding enough on its own. But if your tap water is soft, then you should add some cal-mag.

This table summarizes the above to help you match the type of water to your type of fertilizer:

Type of WaterFertilizer that contains calcium and magnesiumFertilizer with no calcium or magnesium
Soft water (soft tap water, RO, rain, distilled)SuitableNot suitable, unless you add Cal-Mag
Hard water (contains calcium and magnesium)Not suitableSuitable

My tap water is very hard, so I mix it with rain water in a ratio of 1:3 (tap:rain) and I don’t add any extra cal-mag.

Learn more about cal-mag and how to use it in Cal-Mag for Plants – What Is It and Do You Need It?

Can you Use Regular Fertilizer Instead of Hydroponic Fertilizer?

As explained above there is nothing special about hydroponic fertilizer. Any fertilizer that easily dissolves in water can be used with the following caveats:

  • The fertilizer should contain micronutrients
  • At least half of the nitrogen should be in a nitrate form
  • You use the right water for mixing so that calcium and magnesium levels are neither too high, nor too low.

Even though many people on social media are convinced that hydroponic fertilizer is special, and marketing companies perpetuate this myth, it simply is not true. The nutrients in them are the same as any other fertilizer. The amount of nutrients does vary from product to product.

FAQ

Q1: What is the difference between hydroponic fertilizer and regular fertilizer?

There really is no difference. Both types have various amounts of micronutrients. Both use the same salts to make the fertilizer. Hydroponic tends to have more nitrate as a nitrogen source while regular uses urea because it is less expensive.

Q2: Can you use hydroponic fertilizer on regular plants?

Yes, they work just fine. However, it tends to be more expensive.

Q3: What kind of fertilizer do you use for hydroponics and aeroponics?

You can use hydroponics fertilizer or regular fertilizer provided it is water soluble and non-organic.

Q4: What is the best fertilizer for hydroponics?

Use a soluble fertilizer that is sold dry. This type is less expensive and better for the environment since you are not shipping water. An NPK of 3-1-3 is ideal.

Q5: Can I make my own hydroponic nutrient solution?

You can, but since this requires numerous micronutrients, it is not worth the effort. If you want to save some money use regular soluble fertilizer that provides at least 50% of the nitrogen in a nitrate form.

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

2 thoughts on “What is Hydroponic Fertilizer?”

  1. For the home gardener hydroponics seems a step to hard, however it is promoted by those that have a vested interest & want to sell you a hydroponic system

    I think I will stick to regular gardening, which can be hard enough at times, although I appreciate those without a garden may wish to go down the hydroponics route

    Thank you for this interesting article Robert

    Reply
  2. Dear Plantaholic. Thank you for your commentary opening the hype on various fertilization formats. This really helps me to stay on track with the fundamentals of plant growth supplements learned from an early age. I enjoy and trust your education and thank you heartily for your altruistic knowledge.
    Rick Chase

    Reply

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