For New Growers - Evaluating Commercial Hydroponic Fertilizers

 

I like to draw the subjects of these posts from questions on the r/Hydroponics subreddit on reddit.com .

https://www.reddit.com/r/Hydroponics/new/

And a number of users ask about commercial nutrients after becoming thoroughly confused between answers and the information on the manufacturers’ web sites. I’m not surprised they get confused. Makers are anxious to present their products as unique and to get buyers to commit to particular feeding schedules using unique products that mean more future sales. The result is that they all seem very different, which is, of course, the intent.

But they are handicapped by the fact that most all manufactured fertilizers made specifically for hydroponics provide essentially the same nutrients. Their advantage is the human tendency to, once someone buys a product and has success, to believe they made the wisest of all choices, and they will promote their choice to others. Doesn’t help the new grower trying to make sense of the differences. Still, great P.R.

But the nature of plant nutrition means the makers can’t really get too far off the beaten path in terms of ingredients. They can use some alternatives, such as incorporating organics like kelp product or worm casting extracts, but that while the choice is reasoned, it’s not a big difference, because it all circulates in the plant as the same elemental nutrients. But the good news is they all work. Once you figure out how to use them.

I intend to provide the new grower with some basis for evaluating hydroponic fertilizer products and learning why makers make the kinds of recommendations they make for using them. I often use the term “nutrient set” to mean a set of nutrients complete with all the significantly useful plant nutrients and ready to use, rather than a core set that is just a basic offering of macro-nutrients that need to be supplemented by minor and micro-nutrients before all the common requirements of a large number of plants are met.

I like establishing some principles in a discussion like this, so we don't have to keep repeating them. I will call them “rules,” things that always apply and that should be remembered.

Rule #1 – The term “fertilizer” can mean anything, a single substance or an elaborate mix that can be taken up by plant tissue as nutrient and incorporated to the plant’s benefit. Don’t assume anything just because it’s labeled as fertilizer.

Rile #2 – Nutrient means a substance that can be dissolved in water and taken up by the plant roots. It can also refer to the chemical elements that, in plant tissues, are used in its growth or metabolism. So we call Nitrogen a nutrient and Calcium a nutrient. But we also call Calcium Nitrate a nutrient. And we must never forget the most essential nutrients, without which there would be no plant life, Carbon, Hydrogen and Oxygen.

Rule #3 – If a fertilizer is not labeled specifically for use in hydroponic growing, it may be compounded of substances that cannot be used without risking one kind of trouble of another. It may be compounds that are not water soluble, making them entirely unusable in hydroponics. Or it may be ingredients that inter-react in water.

Rule #4 – Soil fertilizers and hydroponic fertilizers are for fundamentally different purposes. It can be said that soil fertilizers feed the soil, not the plants. Soil contains microbes and experiences geological processes that break down many convenient or inexpensive compounds to forms the plant can take up. The definition of hydroponics is that it uses no soil, so no microbes, so our nutrients must be water-soluble and in a chemical form the plant can take in directly.

Rule #5 – Plants can only take up ionic forms of elements, so we must provide them as what are called ionic salts. Salt here doesn’t mean table salt. Salts are chemical compounds of ions. Ions are atoms or molecules, each with its total number electrons unequal to the total number of protons, leaving it with an electrical charge and an eagerness to combine with other ions of opposite charge. The original “opposites attract.” When ions combine in this way, they become ionic compounds. Ionic compounds are soluble in water, because water has an affinity for charged ions, and the charged ions that made up the compound disassociate in the water. Not all ionic compounds are soluble in water, but there are enough that are soluble that contain ionic nutrients we need that we can more or less easily find what we want to make up fertilizers to dissolve in water.

Rule #6 – The analysis provided by makers of fertilizers shows percentages of specific elements and compounds but by convention, some of them are shown as the equivalent of different forms than are actually used as ingredients. It helps make it easier to compare analyses.

So what are we buying?

 

Let’s start by looking at a very straightforward product, Masterblend™ for hydroponics, and the two additional parts usually supplied with it. Masterblend makes multiple products useful in hydroponics, but the most popular is their 4-18-38 “Tomato” Formula. Why the Tomato Formula has become something of a standard in hydro, I can’t say. I suspect it was simply that it was available, and it worked. Masterblend makes other formulas, one even labeled “Hydroponic,” but there’s little difference among the ones we might take an interest in. Thousands of growers use the “Tomato” for everything, and I’ve seen the same product labeled as “Tomato and Hydroponic” and “Tomato and Vegetable” in the past.

 

 

Nevertheless, the “4-18-38” is part of the conventional system of labeling fertilizers according to the relative amounts of the three primary nutrients, no matter what else it contains. The numbers represent nitrogen, phosphate and potash. The primary nutrients are nitrogen, phosphorus and potassium.

Don’t worry too much about these numbers, so long as the nutrient set is for hydroponics, it will do for most anything you want to grow hydroponically. And the N-P-K numbers are far from being a full description of a fertilizer and certainly do not tell you if it is an appropriate product for hydroponics.

Like all fertilizers intended as the core of a hydroponic nutrient set, Masterblend is more than just Nitrogen, Phosphate, and Potash. This is a good time to point out that N-P-K expresses the proportions of the elements Nitrogen, Phosphorus and Potassium. It says nothing else about the ingredients. There is no elemental Nitrogen, no elemental phosphorus and no elemental potassium in it. In fact there could be no elemental Nitrogen, because under Earth sea level conditions, the element Nitrogen is a gas.

But the proportions and relative amounts of those three elements are universally used to classify fertilizers because those are the major elements, the most important for plant growth. Lets look at how those three are provided as ingredients. But first, since we said we could not use those chemical elements directly, how do we provide them to the plant?

And here, at the risk of making this seem like a maze of rabbit trails, we must diverge again. Fertilizer makers usually present, on the label or on their web site or both, an analysis. The analysis is the list of ionic nutrients and the percentages of each. Note that Masterblend is an NPK 4-18-38, and there those numbers are in the analysis.

Total Nitrogen (N) 3.5% Nitrate Nitrogen 0.5% Ammoniacal Nitrogen	4%
Available Phosphate (P2O5)	18%
Soluble Potash (K2O)	38%

Nitrate is an ionic form of Nitrogen. (See how this is working?) Its chemical formula is NO₃^-. (There are other nitrates with different numbers of oxygen atoms.) Almost all nitrates are soluble in water. In Masterblend, it contributes 3.5% of the nutrients contained in the bag labeled Masterblend.

Ammoniacal nitrogen (NH₃-N) is ammonia. Ammoniacal nitrogen can be troublesome in hydroponic systems in ways that it is not in soil where it is used massively in conventional farming. So you will often see soil fertilizers using a far larger portion of it. But it is a water soluble ionic form and so is a candidate for our fertilizers, if we use it with discretion.

Phosphate is an ionic compound of phosphorus and oxygen.

Potash is an ionic Potassium salt. K₂O, Potassium Oxide, isn’t taken up by plants, but the amount of potash is commonly reported in K₂O equivalent, no matter what form of potash is used. Masterblend gets that equivalent from three different compounds, none of them actually K₂O.

Those figures address the primary macronutrients. There are three other macronutrients, but they are present in far lesser percentages and they are not part of the N-P-K system.

Masterblend contains one of those secondary macronutrients, Magnesium. It does not contain the other two secondary macronutrients, Sulphur and Calcium, in significant amounts. As we will see, they will be provided by additional components.

Continuing with the Masterblend analysis with the micro-nutrients:

Boron (B)	0.20%
Copper (Cu) 0.05% Chelated Copper (Cu)	0.05%
Iron (Fe) 0.40% Chelated Iron (Fe)	0.40%
Manganese (Mn) 0.20% Chelated Manganese (Mn)	0.20%
Molybdenum (Mo	0.01%
Zinc (Zn) 0.05% Chelated Zinc (Zn)	0.05%

We could not, of course, use powdered iron, copper, manganese, or the others of these metals. They are not soluble and cannot be taken up by the plant.

But if we look beyond the analysis, we find a list labeled, “Plant nutrients derived from.” We have now moved to the actual compounds used as ingredients in the mixture:

Potassium nitrate

Potassium phosphate

Ammonium phosphate

Potassium chloride

Magnesium sulfate

Boric acid

Copper EDTA

Iron DTPA

Manganese EDTA

Sodium molybdate

Zinc EDTA.

 

EDTA and DTPA are called chelating agents. They make metallic compounds soluble. On some labels, this is expressed with the term “chelated,” as in Chelated Iron.

 

At this point, we know that Masterblend provides known proportions of the elemental nutrients, and by examining the “derived from” list, we can determine that the ingredients are indeed water soluble, as they must be for us to use them. A fertilizer not made for hydroponics may not contain useable ingredients. It may presume the presence of the microbes and geological processes that break down the compounds in the soil fertilizer. However, some fertilizers can do double duty. You can always use hydroponic fertilizers in soil.

But we need more than is in that bag marked Masterblend. Masterblend, the branded product itself, is a sort of core set of nutrients. It is intended to be used with two other parts, Calcium Nitrate and Magnesium Sulfate (Epsom salts). Each part is supplied as a powder, and the three parts are often sold bundled. To use, each part is introduced to the reservoir individually and in order. Each part can also be dissolved in water as concentrates and held indefinitely, but the three parts should not be combined before being used. Mixing nutrients that can react to each other can cause some elements to precipitate out as solids and become unavailable to the plants. Because it is in powder form the Masterblend sets can be purchased in larger weight packages which make a very large volume of working nutrient solution. It can therefore be very economical.

Calcium is a vital element in plant growth and is not included in Masterblend. Calcium Nitrate is the common means of providing it. Calcium Nitrate has an N-P-K or 15.5-0-0 . The 15.5 is for nitrogen. Calcium is not part of the N-P-K system, so there is no number for it. For that, we again look to the analysis where we find it is the greatest part of Calcium Nitrate.

Total Nitrogen (N) – 15.5%

       Nitrate Nitrogen – 14.5%

       Ammoniacal Nitrogen – 1.1%

Soluble Calcium (Ca) – 19.0%

Both elements are derived from Calcium Nitrate, which is itself a soluble ionic compound.

A third part used with Masterblend is Magnesium Sulfate, commonly known as Epson Salt. As expected, it provides Magnesium and Sulphur. It is a soluble ionic salt.

The complete Masterblend set is typically used in two concentrations, a weaker one for lettuce and other greens and a stronger one for fruiting plants. The maker web site does not present any information for using it in hydroponics, but there are a number of online resources.

 

So, to get what we want in our nutrient reservoir with Masterblend, we have to use three different products, why not put them all into one bag?

The brief answer is that some nutrients that can coexist in reasonable peace diluted to the fairly low concentrations at which we use them will inter-react at higher concentrations to first break down and then reform into compounds we do not intend and that are not soluble. These compounds are called precipitants, because they form solids and precipitate to lie on the bottom of the reservoir. Any nutrient that is lost by combining to form a precipitant is lost to the plant, and a deficiency is likely. Some untoward combinations can even form crystals inside the plant, damaging it.

The solution to the problem is to formulate hydroponic fertilizers so that none of the ingredients in one part react among themselves. That requires compounds that can react with the ingredients in the first part to be kept in a separate part. If added to the large volumes of water in reservoirs in the proper amounts, they are likely to behave and not form precipitates. But if we are lazy and try to make a concentrate of all the parts so that we can use only that one concentrate to make up a nutrient solution, we will find that in that high concentration, nearly all the compounds that can react will react, and we will find a layer of solids at the bottom of the concentrate bottle. This can also happen if we make up the reservoir improperly, adding fertilizer parts in the wrong order or failing to stir well after we add parts to make sure the next part doesn’t land in a concentrate zone of the earlier part.

Not all hydroponic nutrient sets are three-part. There are two part products. There are also products touted as “one-part,” but the user must check carefully that it doesn’t mean a one-part “fertilizer,” and not a complete one-part nutrient set, which is unlikely. How many parts depends on the specific ingredients. Elemental nutrients can be derived from different compounds, and they all may have different other compounds they react to. It’s a little like the logic problem of having a wolf, a goat and a cabbage on one side of the river and trying to get them all across in a rowboat that carries only you and one of the other things, without leaving any of them alone with someone or something that will eat if not supervised. The wolf won’t eat the cabbage but will eat the goat. The goat will eat the cabbage. You can’t leave reactive nutrients together, but sometimes you can exchange the goat for a bobcat.

This isn’t a commercial for Masterblend. I merely use it for its simplicity. But how do we know it’s formulated more or less properly for hydroponic plants? With most products, we likely depend on the maker telling us it is. Although by now, we should be able to confirm that from the label. But how do we even know what a good nutrient set for hydroponics might be?

There’s quite a long history of intensively studied hydroponic nutrition. Hydroponics came to prominence in 1929 when William Gericke at the University of California demonstrated his methods of growing in water, displaying nine-foot tall tomato plants. It became a hot topic nationally. In the 1930’s, when Pan Am wanted to open a Pacific clipper route that required island hopping, they used hydroponics to make it possible to sustain a crew on tiny Wake Island. Dennis Hoagland and Daniel Arnon developed formulas for mineral nutrition, known as Hoagland solution and revised them again and again. Many scientists studied the subject to feed troops in WWII.

There is a vast body of experience in formulating hydroponic fertilizers. Various makers may have different philosophies, but all the formulas will have the same general affect.

A maker with a very different approach is General Hydroponics. One of their products is their Flora series, consisting of liquid three parts, FloraMicro™, FloraGro™ and FloraBloom™.

 

The maker publishes a feeding schedule covering eight weeks of growth. During each week, the amounts of the three parts change, reaching their highest concentrations in the late growth stages and into early blooming. The proportions of the three parts do not change radically. So, it is not unlike varying the concentrations of other fertilizers like Masterblend, and some growers do exactly that. The maker distributes the three parts as liquid concentrates, but the parts are formulated to avoid inter-reactions within each part. The end result it s complete hydroponic fertilizer program with a schedule of varying concentrations that one may follow. And the ingredients in the three parts are chosen to make the changes during the feeding schedule conform to what is known about nutrient demands in the different stages of growth.

General Hydroponics makes other fertilizers. One of interest to us will be the Maxi Series, MaxiGro™ and MaxiBloom™.

 

These are very different from the Flora Series. Only one product is used at a time, beginning with MaxiGrow and changing to MaxiBloom. The feedchart recognizes only two periods, Growth and Bloom, rather than individual weeks. Something for everyone’s tastes. This is not a base fertilizer, like Masterblend where common additional components make it complete. Each of the Maxi Series is considered complete.

And we have to remember that the business nature of these makers influences the formulations of their fertilizers. Some micro-nutrients take on significant importance only in commercial settings where crops are counted in the tens of thousands of plants, and small difference in production add up to measurable revenues. Home hydroponic growers can afford to wait to see if the lack of something makes enough difference to them to matter.

Fox Farm™ takas another different approach.

 

 

Their product, Grow Big™, is an early growth fertilizer, so it’s suitable for lettuce and greens for the life of the plant. It is a true one part liquid, but when we examine the “derived from” list, we find some organics, Norweigan Kepl and Earthworm Castings, as well as more conventional compounds. So they can avoid some of the otherwise inevitable reactions. Grow Big’s NPK is 3-2-6, roughly in proportion with Masterblend when all the components are used, just not so concentrated as provided.

Fox Farm makes two “bloom” products to use with Grow Big and an array of other products in an almost bewildering variety. Again, something for everyone. With Fox Farm, where they make products for soil and hydroponics, you must be alert that you are buying products compatible with hydroponics. Grow Big, for instance, it sold in soil and hydroponic versions, with almost identical labels but for the word HYDROPONIC on the front.

You cannot just blindly purchase a product and put it to work. You must read the manufacturers’ resources on its use. Recently, there was a question from a grower who was confused by the fact that he was using the recommended amounts of his fertilizer, but the solution E.C. was very low. He was questioning if he had a defective meter. Also, the plants appeared to be suffering. His fertilizers was billed as 11-11-40, “Plus Micro Nutrients.” While that sounds like it should be complete, a look at the analysis reveals that it contains no Calcium or Magnesium. And sure enough, on the maker web site, they say to use it with Cal Mag and gave recommended amounts that actually brought it up close to the analysis of the Masterblend set and would no doubt produce a credible E.C. value.

Cal-Mag is a popular supplement, but in the above example is used as a required part of the nutrient set, something it can be used for where appropriate. 

 

It provides its nutrients from Calcium Nitrate and Magnesium Nitrate. The most popular brand includes chelated iron. Cal-Mag is frequently used to boost Calcium to prevent tomato Blossom End Rot, a condition that afflicts some varieties of tomato when they need a bit more Calcium. Many growers use it before the condition is seen, because a fruit afflicted with blossom end rot can only be destroyed.

People also ask if they can use other common water-soluble fertilizers. They usually mean Miracle-Gro™, because it’s available everywhere. Let’s look at Miracle-Gro. Here’s the analysis section of the label. 

 

 

 

In the following, MG means Miracle-Gro, and MB means Masterblend.

Nitrogen: MG 24%   MB 4%  But we’re going to make the MB work by adding the rest of the nitrogen below.

Phosphate: MG 8%   MB 18%

Potash: MG 16%    MB 38%

Boron: MG 0.02%  MB 0.20%

Copper 0.07%  MB 0.05%

Iron: MG 0.15%   MB 0.40%

Manganese: MG 0.05%  MB  0.20%

Molybdenum: MG 0.0005%   MB 0.01%

Zinc:  MG 0.06%  MB  0.05%

Miracle-Gro is water soluble, so no worries there.

I said we would pick up some additional Nitrogen for the MB. It comes with the Calcium Nitrate that’ is used with MB, and it is 15.5% Nitrogen. So total Nitrogen is close to equal.

Also, Calcium and Sulphur are missing from Miracle-Gro. But the Calcium Nitrate above might add the Calcium, albeit with a lot of Nitrogen we neither need nor want. MB also is combined with Magnesium Sulfate which adds Sulphur.  We can use it with Miracle-Gro, also.

So we’re left with Miracle-Gro very short on a number of important nutrients, including the primaries, Phosphate and Potash. Miracle-Gro is a 24-8-16 in N-P-K. Heavy on the Nitrogen, but not wildly so, considering the combined Nitrogen in Masterblend as used, but it’s going to make it hard to provide the Calcium we want. (See below.) And Miracle-Gro is significantly short of Potash and very short on Phosphate. And those are so important that they are two-thirds of the N-P-K system. It is also short on others.

The reason is that MiraclepGro is intended for soil use. Soil naturally contains quantities of the same nutrients, so Miracle-Gro doesn’t need to provide all of them, nor should we try it and risk toxicity. I think it seeks to address average soil fertility without being uselessly too weak or dangerously powerful.

If we try to use it in a hydroponic system, we know we are going to be way light on some micro-nutrients that we should provide, even in the small quantities normally present in hydroponic fertilizers. At first glance we think about just using more Miracle-Gro, but that would involve too much Nitrogen. We could work out supplements to provide the short nutrients, but that would be additional cost and work. And we could make up the short Manganese and Sulphur with Epsom Salt as we do with the basic Masterblend core fertilizer. Again, more expense and effort.

And we could try to make up the missing Calcium, nut we can’t use Calcium Nitrate as we do with Masterblend. Miracle-Gro already has a full portion of Nitrogen. We would be inviting Nitrogen toxicity. And useable substitutes are hard to come by. Common calcium sources used in soil fertilizers are not soluble. And Chelated Calcium, which is soluble, comes with a hefty dose of Sodium that can accumulate and harm plants. And the EDTA chelating agent is only weakly bound to Calcium and can disassociate and attach to other things, like heavy metals in the medium, making them soluble. And the EDTA itself can cause precipitation in concentration with Calcium.

So we have significant problems, because in crafting a good general purpose soil fertilizer, Miracle-Gro has, without malice, painted us as hydroponic growers into a corner if we try to use it to provide a decent nutrient balance. Essentially, it can’t practically be done. It’s not their fault. They never claimed it was for hydroponics. In fact Miracle-Gro owns a hydroponic division, Aerogarden™.

And the fact that Miracle-Gro is a poor nutrient set for hydroponics is supported by any number of growers trying it and a number conducting side-by-side comparison grows with Miracle-Gro and legitimate hydro fertilizers in which the crop on Miracle-Gro fared poorly. The cost of Masterblend drops to around 15-cents per five gallons working solution when bought in the 12.5 pound set. Compromising nutrient balance to try to reduce that small cost makes no sense.

When selecting a nutrient set, investigate to be sure it is (1) made for hydroponic use and (2) is complete or can be made complete by adding other, readily available products. Once you believe it is for hydroponics and is complete, you can consider recommendations for use for different plant varieties from the maker and from other users.