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Establishing ppm in Solution from Nutrient Labels

 

Excerpt from Integral Hydroponics Evolution by G.Low. Coming soon!

 

nutrient-label-Ca-Compliant

 

Above is an example of a nutrient label (“guaranteed analysis”) that would be compliant to California ‘specialty fertilizer’ regulations. I’ve used the California compliant label as an example because many multinational companies (those supplying their products worldwide) tend to adhere to U.S. (particularly Californian) standards where they aren’t in conflict with local fertilizer guaranteed analysis regulations in other regions. Therefore, in many locales, this is what a nutrient label will look like.

 

Firstly, the guaranteed analysis tells us how much of each element (NPK etc) is in the hydroponic nutrient concentrate at percentage weight by volume (%w/v). So, for example, when looking at a single part, full spectrum hydroponic nutrient the guaranteed analysis may look something like this.

 

Guaranteed Analysis:

 

  • Total Nitrogen (N): 2.0%, 0.09% Ammoniacal Nitrogen, 1.91% Nitrate Nitrogen
  • Available Phosphate (P2O5): 2.0%
  • Soluble Potash (K2O): 3.0%
  • Calcium (Ca): 2.5%
  • Magnesium (Mg): 0.5%, 0.5% Water Soluble Magnesium (Mg)
  • Sulfur (S): 1.14%, 1.14% Combined Sulfur (S)
  • Manganese (Mn): 0.05%, 0.05% Water Soluble Manganese (Mn)
  • Molybdenum (Mo): 0.0005%

 

 

This provides us with enough information to dissect that nutrient and establish a reasonably accurate ppm in solution.

 

By saying “reasonably accurate”, one important thing to highlight here is that analyzing the ppm in solution from fertilizer labels typically won’t provide 100% accurate ppm in solution numbers because what is listed on a label doesn’t necessarily always represent what is actually in the bottle. That is, due to labeling ‘compliance’ regulations and issues such as batch irregularities the nutrients that you are actually adding to solution can be somewhat different from what is listed on the label. For example, where labeling compliance regulations are concerned, fertilizers sold across the US and elsewhere are often only required to be listed accurately to within 0.4% at best to their listed NPK ratios. That is, they can only be below the labeled guarantee percent by this percentage. So if a nutrient has 1% in solution, manufacturers can often get away with listing 0.6% on the label.

 

When looking at Colorado fertilizer listing regulations, listing requirements are a sliding scale based on the guaranteed percent that is listed. For example, if the guaranteed percentage is 4% or less, a product needs to be listed within 0.49% for N, 0.67% for P, and 0.41% for K. As the guaranteed percentage rises, so does the range of error. This caps at that guaranteed percentage of 32% or more which allows for an error of 0.88% for N, 0.76% for P, and 1.44% for K. This also means manufacturers can get away with not listing the NPK ratio if the product is below 0.49% for N, 0.67% for P, and 0.41% for K. For calcium and sulfur, manufacturers don’t even have to list them on the guaranteed analysis if they are below 1% for calcium and below 0.5% for magnesium. For micro nutrients, the cutoff is different with each ion species having their own value assigned to them. All of this makes it difficult to know exactly what an off the shelf hydroponic nutrient or additive really contains.

 

To meet CDFA (Californian Department of Food and Agriculture) compliance, every nutrient is simply required to be labeled above a guaranteed minimum. Unless, according to § 2307, it is “labeled only for hydroponic, continuous liquid feed programs or ready-to-use foliar products.” In which case, “the minimum percentages acceptable for micronutrients stated in Section 2303, do not apply to guarantees for those water soluble nutrients or micronutrients”.

 

So, many of these liquid blends are labeled in an ambiguous way where manufacturers aren’t required to even specify that micronutrients are in the product. Essentially the legislation surrounding nutrient labeling allows the manufacturers to be deliberately vague about what a formula actually contains.

 

Given this, if the data on the labels isn’t 100 percent correct what you equate from this data isn’t strictly accurate where ppm in solution is concerned. However, in most cases with the macronutrients the labels guaranteed analysis listings are reasonably accurate and will provide close to actual ppm in the nutrient solution figures.

 

Besides this, from a practical perspective, without lab analyzing a nutrient (about $40 per solution in the U.S) for its macro and micronutrient guaranteed analysis, listings on labels is all we have to work with in establishing the ppm of each nutrient species in solution the product provides when used at a given ml/L usage rate.

 

In other words, analysing the ppm in solution from the guaranteed analysis is not a perfect science, but certainly when compared to EC measurements it provides us with a far more accurate picture of what is actually in solution with regards to nutrients (species and levels).

 

Interpreting the Qualities of a Nutrient from the ‘Guaranteed Analysis’

 

Fertilizer formulations are defined and listed by manufacturers in percentages, either as percent weight by volume (%w/v) or percent weight by weight (%w/w). For now, %w/v is the important measurement, so let’s focus on this.

 

Various regulatory bodies around the world require that NPK etc values to be presented in a somewhat ambiguous fashion. Therefore, listings for the same nutrient or additive may appear to vary on a country-by-country or, even, state-by-state basis. For example, when looking at our California compliant label ‘guaranteed analysis’ you will note that it states; 1) “Available Phosphoric Acid (P2O5) and 2) “Soluble Potash (K2O)”. This information becomes important when interpreting the guaranteed analysis.

 

That is, it is important to note that the P and K numbers found on the guaranteed analysis do not always reflect the actual amounts of elemental phosphorous and potassium by percentage. With our California compliant label example this is the case and P (phosphorus) is listed as P2O5 (phosphorous pentoxide) and K (potassium) is listed as K2O (potassium oxide) % w/v. When looking at nutrient’s label/s check them closely to see whether they list P as P2O5 and K as K2O because this becomes important in interpreting the data and using this data to establish any nutrients qualities.

 

When phosphorus is listed as P2O5 it is only 43% elemental P and when potassium is listed as K2O it is only 83% elemental K.

 

Therefore, when this system is in use, a 20-20-20 NPK ratio fertilizer truly reflects elemental NPK 20- 8.6- 16.6.

 

However, in some other countries (e.g. Australia) you may find that some labels list P and K as actual elemental P and K. This means that in some countries NPK ratios will reflect actual (elemental) NPK ratios while in other countries the P and K numbers will be much higher.

 

This situation becomes even more confusing in places such as Europe where P and K can be listed as elemental P and K or as P205 and K2O or both. Additionally, other nutrients such as calcium (Ca), magnesium (Mg), sodium (Na) and sulfur (S) can be listed in their oxide form (CaO, MgO, Na2O, SO3) or in elemental form, or both. It’s a crazy situation (no single universal standard) but one that you need to be aware of.

 

If a product is listed as 20% “P as P2O5” and 20% “K as K2O”, convert K2O to elemental K by multiplying by 0.83 and convert P2O5 to elemental P by multiplying by 0.43.

 

E.g. NPK (P as P2O5 and K as K2O) = 20-20-20

 

N = 20 (elemental value)

P as P2O5 = 20 x 0.43 = 8.6

K as K2O = 20 x 0.83 = 16.6.

 

= NPK (elemental) 20- 8.6- 16.6

 

To convert other nutrient listings that may appear on some labels use these equations.

 

Ca0 to Ca multiply by 0.714

MgO to Mg multiply by 0.6031

SO3 to S multiply by 0.4

 

Understanding Percentage Weight by Volume (%w/v) Listings

 

In chemistry the concentration of a chemical solution (e.g. a hydroponic nutrient concentrate) refers to the measure of the amount of solute that is dissolved in a solvent. We normally think of a solute as a solid that is added to a solvent (e.g. adding table salt to water), but the solute could just as easily exist in another phase. For example, if we add a small amount of ethanol to water, then the ethanol is the solute and the water is the solvent. If we add a smaller amount of water to a larger amount of ethanol, then the water could be the solute.

 

Rule 1. In chemistry, a solution is the mixture of two or more substances that are dissolved and mixed until homogenous. The mixture is made up of a solute dissolved in a solvent. E.g. nutrient salt (solute) is dissolved in water (solvent).

 

Rule 2. In chemistry, concentration is used to express the amount of a given substance mixed with another substance. This applies to any sort of chemical mixture of solids, liquids or gases, but more frequently is used to measure active ingredients in homogenous solutions (the amount of solute in the solvent).

 

The way we measure how much solute is in a given solution is typically via %w/v (weight by volume) or %w/w (weight by weight). The concentration can also be measured in lots of other units like moles, but for now you need to know about %w/v.

 

Percentage Weight by Volume (%w/v)

 

A simple way of understanding how to convert a %w/v listing found on the guaranteed analysis into grams per litre is by understanding that 1ml of demineralized water weighs 1gram. Therefore 1000ml (1L) of demineralized water weighs 1000 grams.

 

Percentage weight by volume (%w/v) refers to the total weight of elements contained within a finished concentrate of a given total volume. For example, 10%w/v equals 10% of an element incorporated/integrated within a total volume of 1000ml (i.e. water plus element equals 1000ml).

 

So, in the case of a 1 litre nutrient concentrate that lists total N as 10%w/v this would mean that there is 100grams of N in 1Ltr of nutrient concentrate.

 

A handy sum for equating %w/v is:

 

1000 (ml nutrient concentrate) times 10% (%w/v listing) = 100 (grams N)

 

By the way, when using this sum on a calculator be sure to use the % button to arrive at the final calculation.

 

More Examples of %w/v

 

0.5%w/v equals 5 grams per litre

1.0%w/v equals 10 grams per litre

2.5%w/v equals 25 grams per Litre

5.0%w/v equals 50 grams per litre

 

Establishing how many parts per million of a nutrient is added to the nutrient working solution by using the guaranteed analysis of a given nutrient concentrate at a given ml/L usage rate

 

This is perhaps more chemistry than the average novice grower needs right now (enter at your own risk). However, I will cover it in depth for the more advanced grower. Knowing these equations will enable the reader (you) to dissect hydroponic nutrient formulas and establish their suitability for use with various crops. This said, for the hydroponic newbie and advanced grower alike, you’ll find a calculator on the Manic Botanix website that does these calculations for you (here). Simply use this calculator by entering how many ml/L of nutrient you are using; then enter the data from the guaranteed analysis (%w/v or %w/w with SG) and push “calculate”. The calculator will then tell you how many ppm of each element is in the nutrient working solution. It’s a handy little calculator we devised and programmed for hydroponic growers. Other than telling you the ppm in solution it converts ml/L to ml per U.S. gallon. Additionally, there are calculators on the same page that convert K2O to elemental K and P205 to elemental P, and ppm to %w/v and vice versa. This said, for those of you who want to understand the chemistry let’s run the sums. You may find it helpful to grab a pen and paper and calculator and work through things as we go.

 

Converting %w/v to ppm and ppm to %w/v

 

To establish ppm from %w/v you simply need to multiply by 10,000. I.e. 3 (%w/v) x 10,000 = 30,000 (ppm)

 

To establish %w/v from ppm you simply need to divide by 10,000. I.e. 30,000 (ppm) ÷ 10,000 = 3 (%w/v).

 

Nutrient element concentration in the working nutrient solution yielded by a specific dose working from guaranteed analysis at %w/v

 

To establish the concentration of individual elements in the ‘working nutrient solution’ (the solution that is being fed to the plants), the guaranteed analysis %w/v specification should first be converted into ppm, then multiplied by the usage rate (per litre), then divided by 1000. For example, if a nutrient lists 2.5% nitrogen (N), when it is used at 4ml per litre it will yield 100ppm N in the nutrient working solution.

 

That is:

 

Step 1. N = 2.5 (%w/v) x 10,000 = 25,000 (ppm)

 

Step 2. 4ml per litre yields 100ppm. I.e: 25,000 (ppm) x 4 (ml/L) ÷ 1,000 (ml) = 100 (ppm).

 

Keep in mind that where elements such as phosphorus (P) are listed as P2O5 and potassium (K) is listed as K2O you first need to convert P205 to elemental P and K2O to elemental K before using these equations.

 

So if our guaranteed analysis listed 2%w/v available phosphoric acid as P205 we first convert P2O5 to elemental P with the sum 2 (% P205) x 0.43 = 0.86 (% elemental P).

 

Then we would establish the ppm using the sum 0.86 x 10,000 = 8,600 (ppm of elemental P).

 

Then at 4ml/L we use the sum 4 x 8,600 ÷ 1000 = 34.4 (ppm elemental P in the working solution).

 

i.e.

 

Step 1. P as P2O5 = 2%w/v = 2 x 0.43 = 0.86 (elemental P)

 

Step 2. 0.86 (%w/v) x 10,000 = 8,600 ppm (elemental P in concentrate)

 

Step 3. 4 (ml/L) x 8,600 ÷ 1000 = 34.4 (ppm elemental P in working solution)

 

Comparison of Two Single Part Nutrients – Flairform GreenDream Bloom versus Botanicare CNS17 2-2-3 Bloom

 

Okay, so let’s now put the ppm in solution theory into practice to demonstrate how we can establish what we are feeding to the plant/s from the guaranteed analysis. We’ll begin by comparing two single part bloom formulas against one another and then lining them both up side-by-side against the optimum nutrient ppm in solution required by the tomato plant (mid to late bloom).

 

A single part nutrient is a full spectrum hydroponic nutrient in a single bottle and this is reflected on the labels guaranteed analysis. So, for instance, when looking at Botanicare CNS Bloom Formula NPK 2-2-3 the guaranteed analysis reads:

 

Guaranteed Analysis:

 

  • Total Nitrogen (N): 2.0%, 0.09% Ammoniacal Nitrogen, 1.91% Nitrate Nitrogen
  • Available Phosphate (P2O5): 2.0%
  • Soluble Potash (K2O): 3.0%
  • Calcium (Ca): 2.5%
  • Magnesium (Mg): 0.5%, 0.5% Water Soluble Magnesium (Mg)
  • Sulfur (S): 1.14%, 1.14% Combined Sulfur (S)
  • Manganese (Mn): 0.05%, 0.05% Water Soluble Manganese (Mn)
  • Molybdenum (Mo): 0.0005%

 

 

While the guaranteed analysis of Flairform GreenDream single part bloom reads:

 

Total Nitrogen (N)          2%

0.1% Ammoniacal Nitrogen

1.9% Nitrate Nitrogen

Available Phosphate (P2O5)      2%

Soluble Potash (K2O)                6%

Calcium (Ca)                             1.4%

Magnesium (Mg)                       0.6%

0.6% Water Soluble Magnesium (Mg)

Sulfur (S)                                  1.2%

 

 

Flairform Green Dream has an NPK ratio of 2- 2- 6 and Botanicare CNS17 Bloom has an NPK of 2- 2- 3.

 

Keep in mind that by North American standards P is listed as P2O5 and K is listed as K2O. This means that if the products were listed in elemental P and K the would be 2- 0.86- 4.98 (Green Dream) and 2- 0.86- 2.49 (CNS17).

 

When comparing these two products macronutrients side-by-side it looks like this in elemental form (i.e. P2O5 as elemental P and K2O as elemental K). See the following table.

 

 

Nutrient Elemental Values %w/v Botanicare CNS17 2-2-3 Flairform GreenDream Bloom
Total N 2.0% 2%
Ammoniacal Nitrogen 0.09% 0.1%
Nitrate Nitrogen 1.91% 1.9%
Elemental Phosphorous (P) 0.86% 0.86%
Elemental Potassium (K) 2.49% 4.98%
Calcium 2.5% 1.4%
Magnesium 0.5% 0.6%
Sulfur 1.14% 1.2%
Total Nutrients %w/v 9.49% 11.04%

 

Let’s now break this down into ppm in solution when each product is used at 5.5ml/L. I’ll also compare the ppm in solution from the CNS17 and Flairform Green Dream to optimum ppm in solution for tomato plants in mid to late bloom. See the following table.

 

Element CNS17 @ 5.5ml/L in working solution (ppm) Flairform @ 5.5ml/L ppm in working solution Optimum Tomato Nutrition mid/late bloom
Total N 110ppm 110ppm 150-200ppm
Ammoniacal Nitrogen 4.95ppm 5.5ppm ———-
Nitrate Nitrogen 105.05ppm 104.5ppm ———-
Elemental Phosphorous (P) 47.3ppm 47.3ppm 30-50ppm
Elemental Potassium (K) 136.95ppm 273.9ppm 200ppm
Calcium 137.5ppm 77ppm 150ppm
Magnesium 27.5ppm 33ppm 40-50ppm
Sulfur 62.7ppm 66ppm 60ppm
Total Nutrients ppm 521.95ppm 607.2ppm 710ppm
EC mS/cm @ 500ppm = 1EC 1.04 1.21 1.42

 

Interpreting these Results

 

What we are looking at here is two single part bloom nutrients made by different manufacturers. In this example, the Flairform product (made in Australia and sold internationally) is 16% more concentrated than the Botanicare product (made in North America) so if you were paying the equivalent price for 1L of each product you’d be getting 16% more bang for your buck if purchasing the Flairform product. However, there are more things to consider than price. For instance, if one product yields a few percent more produce than another product which is marginally cheaper, saving a few dollars (price of nutrient) to lose even more dollars (based on yield) is simply bad mathematics. This said, it is a math that far too many indoor growers apply and thus lose yield and profit as a result.

 

Besides the concentration differences, we are looking at quite different formulations. For instance, the calcium (Ca) to magnesium (Mg) ratio in the Botanicare CNS17 is 5:1 while the Ca to Mg ratio in the Flairform Bloom is 2.3:1 (rounded down = 2:1). CNS17 is quite unusual on this front. To touch on the Ca to Mg ratio of hydroponic nutrients, many plants have a Ca to Mg ratio of about 2:1 in their tissues. Because of this, a nutrient formulated for these plants should reflect this and contain about a 2:1 Ca to Mg ratio. Most single part nutrients typically have a 1.5:1 to 2.5:1 Ca to Mg ratio, so the 5:1 Ca to Mg ratio in CNS17 presents as higher than is normally found in many other single part hydroponic store sold nutrients.

 

Additionally, the N to K ratios are significantly different between the two products. In the case of the Flairform Bloom, the N to K ratio is 0.4:1 while the CNS17 has an N to K ratio of 0.8:1 (double the N to K ratio of the Flairform bloom).

 

Because the concentration of the two products is 16% different let’s compensate for this by using 16% more of the CNS17 (6.4ml/L or 24.22ml per U.S. gallon) and have a look at what this equates to as ppm in the working solution when compared to Flairform Green Dream used at 5.5ml/L . See the following table.

 

Element CNS17 @ 6.4ml/L in working solution (ppm) Flairform @ 5.5ml/L ppm in working solution Optimum Tomato Nutrition mid/late bloom
Total N 128.16 110ppm 150-200ppm
Ammoniacal Nitrogen 5.76 5.5ppm ———-
Nitrate Nitrogen 122.24 104.5ppm ———-
Elemental Phosphorous (P) 55.04 47.3ppm 30-50ppm
Elemental Potassium (K) 159.36 273.9ppm 200ppm
Calcium 160 77ppm 150ppm
Magnesium 32 33ppm 40-50ppm
Sulfur 72.96 66ppm 60ppm
Total Nutrients ppm 607.52 607.2ppm 710ppm
EC mS/cm @ 500ppm = 1EC 1.21 1.21 1.42

 

 

Keep in mind that we are establishing ppm in solution using demineralised water (e.g. RO water) and when working with mains water, nutrients from the water supply would also need to be factored in. i.e. using an example of just one mains water supply, see the following tables.

 

Mains Water Analysis

 

Phosphate 1.69
Nitrate 0.15
Ammonium 0.30
Potassium 7.4
Calcium 43
Magnesium 14.4
Sulfate 12.3
Iron 0.07
Sodium 55
Chloride 97
Total ppm 231.31

 

 

Element CNS17 @ 6.4ml/L in working solution (ppm) Flairform @ 5.5ml/L ppm in working solution Optimum Tomato Nutrition mid/late bloom
Total N 128.45 110.15ppm 150-200ppm
Ammoniacal Nitrogen 6.06 5.5ppm ———-
Nitrate Nitrogen 122.39 104.65ppm ———-
Elemental Phosphorous (P) 56.73 48.99 30-50ppm
Elemental Potassium (K) 166.76 281.6ppm 200ppm
Calcium 203 118.9ppm 150ppm
Magnesium 46.4 47.4ppm 40-50ppm
Sulfur 85.26 78.3ppm 60ppm
Total Nutrients ppm 686.6 685.34ppm 710ppm
EC mS/cm @ 500ppm = 1EC 1.37 1.37 1.42

 

It’s worth noting that I haven’t equated the ppm of the iron, chloride and sodium found in the mains water because I’m demonstrating the ppm of particular nutrients and not the total values of all known elements combined. If I did factor in the sodium, chloride and iron this would give us a total of 838.83ppm (EC 1.67) in the CNS17 working solution when used with this particular mains water supply. Keep this in mind when calculating ppm in solution; ideally you want to factor in all known elements to establish the exact ppm of each element that is being provided to the plant via the nutrient working solution.

 

Comparing Two Part Formulas

 

The guaranteed analysis for two part nutrients are usually based upon the %w/v or %w/w of nutrients/elements in the individual bottles. As such, to establish a total weight by volume of each nutrient’s value that will ultimately be in the working solution (as ppm) when part A and B are used at equal proportions it necessary to add up all the guaranteed analysis numbers to establish e.g. A + B = N total, A+B = K total, A+B = Mg total etc. Let’s take an example of a U.S. based multinationals bloom product to demonstrate. I’ll only list the macronutrients in this example to keep it simple. However, when adding part A and B nutrients be sure to equate all of the nutrient elements in the bottles (i.e. calculate all of the macronutrients and micronutrients from each bottle). See the following table.

 

Part A %w/v Part B %w/v Part A + B totals
Total N 7.55 2.3 9.85
P as elemental P Nil 2.3 2.3
K as elemental K 3.9 5.5 9.4
Ca 4.9 Nil 4.9
Mg Nil 0.97 0.97
S Nil 0.56 0.56

 

Using the totals in the right column we can then begin calculating %w/v to ppm in the working solution.

 

Three Part Nutrients

 

Three part nutrients, unlike two part formulas, are typically not used at equal ratios in solution. E.g. the manufacturer may recommend 4ml/L part A, 3 ml/L part B and 5 ml/L part C etc. Therefore, to calculate the ppm in solution from a three-part formula you will need to equate the recommended dilution rates of each bottle separately and then use these to establish totals.

 

Percentage Weight by Weight (% w/w) v. Percentage Weight by Volume (% w/v)

 

In some instances nutrient labels may list the nutrients in the product as percentage weight by weight (% w/w). Therefore, we had better cover the difference between %w/v and %w/w and discuss how to convert %w/w into ppm in solution.

 

At this point you already understand the concept of %w/v. That is, if we have a solution that contains 10g of a solute (e.g. calcium nitrate) in a 1L product you have 1%w/v. If we have 100g of solute in a 1L solution we have 10%w/v and so on.

 

So when discussing %w/v to mix a 1L solution which has 100g of calcium nitrate in it we would start with, for this example, 700mL of demineralized water. We would then add 100g of calcium nitrate, mix/circulate until the calcium nitrate dissolves completely into solution and then top up with demineralized water to 1L. Final volume of solution equals 1L; therefore the calcium nitrate in weight is expressed as a percentage of the total volume (1000mL). 10%w/v of calcium nitrate (100grams per 1000mL).

 

On the other hand, when comparing %w/v to %w/w let’s say that we make the 1L solution above with 100g of calcium nitrate. It takes 965mL of water to dissolve the calcium nitrate and make up the final volume to 1L. That would mean that we have a 1L solution with a total weight of 100g + 965g = 1065g. We know we have a solution that is 10%w/v but what if we want to convert it to %w/w? To do this we would simply express the100g of calcium nitrate as a percentage of 1065g. This gives a concentration of 9.389%w/w. I.e. 100 (grams calcium nitrate) ÷1065 (grams) = 0.09389 x 100 = 9.389 (%w/w). So, the concentration of the calcium nitrate solution is 10%w/v or 9.389%w/w.

 

Now, let’s say that we take 1L of water and dissolve into it 200g of calcium nitrate, what will we have now? Well, firstly, we’d have more than 1L (1000mL) of final solution. I.e. 1000mL (solvent) + 200g (solute) = ? (ml). Secondly, we would have a final solution that weighed 1,200g (i.e. 1000mL demineralized water = 1000g + 200g calcium nitrate = 1,200grams). We know the weight of the final solution and the weight of the calcium nitrate. This gives us a figure of 16.66%w/w. I.e. 200 ÷ 1,200 = 0.16666 x 100 = 16.66 %w/w.

 

To convert this to %w/v we will need to know one of two things, the final volume or the specific gravity of the solution. For now let’s say that we measure the final volume and use this in our calculations. We will talk about specific gravity a bit later. Therefore, we measure the volume and we have 1.07L or 1070mL. We can then calculate the %w/v and we will have 18.69%w/v. I.e. 200 (g calcium nitrate/solute) ÷ 1070 (ml) = 0.1869158 x 100 (%) = 18.69 (%w/v).

 

Therefore, 16.66%w/w equals 18.69%w/v in the equivalent volume of solution.

 

Based on this information %w/v gives the impression of a more concentrated solution than %w/w as the same concentration is expressed as a higher percentage (i.e. 16.66%w/w versus 18.69%w/v in 1L). However, this is deceptive because the equivalent %w/w and %w/v listing would tell us that the %w/w product is more concentrated than the %w/v product. In other words a 1L product that lists NPK percentage as 5%w/v N, 0.8%w/v P, 4%w/v K, is less concentrated (i.e. less solutes in solution) than a 1L product listed with 5%w/w N, 0.8%w/w P and 4%w/w K.

 

This is generally true except for some cases where the solute actually weighs less than water, usually a gas that has been dissolved in solution. In this situation the weight of the solution decreases as the concentration rises. Of course, this information isn’t relevant to hydroponic solutions as the solutes used in formulation are always heavier than water.

 

Nutrient element concentration in the working nutrient solution yielded by a specific dose working from guaranteed analysis at %w/w

 

In order to calculate how many ppm of any nutrient will be contributed to the working solution from a nutrient concentrate that has a guaranteed analysis listed in %w/w it is necessary to know the specific gravity (SG) of the product. Where a product has a guaranteed analysis listed in %w/w the SG should also be listed. However, if the SG is not listed it is easy to establish the SG by taking a precise liquid volume of the solution and weighing it. So, for example, if you take 500mL of a solution and weigh it and it weighs 650g you divide the volume by the weight and get an SG of 1.3. I.e. 500ml of liquid weighs 650grams. Therefore 650 ÷ 500 = 1.3 (SG)

 


 

About Specific Gravity: SG will be listed as either s.g or SG. SG stands for Specific Gravity. Some use the term density but density can also refer to solids. Specific Gravity refers to the weight/volume of liquids in relation to water, usually measured in g/mL or grams per millilitre. So one ml of pure water will weigh 1.0g and has an SG of 1.0. 1ml of Phosphoric Acid 75% weighs 1.58g so phosphoric acid at 75% is 1.58 times the weight of water and has an SG of 1.58. Therefore 1,000L will weigh 1,580kg (1000 x 1.58 = 1580). SG is usually measured at a standard tem­perature of 25 degrees Celsius (77F).

 

You are able to measure the SG yourself by taking a precise liquid volume of the solution and weighing it. So, for example, if you take 500mL of a solution and weigh it and it weighs 650g you divide the volume by the weight and get an SG of 1.3.

 

I.e. 500ml of liquid weighs 650grams. Therefore 650 ÷ 500 = 1.3 (SG)

 

You can also use an instrument like a hydrometer to measure the SG of a liquid. A hydrometer is a sealed glass vial with a bulb on the bottom. It is placed in the liquid and floats. The tall thin part at the top has measurements inside that you read against the meniscus of the liquid i.e. it will have gradations like 1.11, 1.12, 1.13.

 


 

 

Okay, so let’s now hypothetically say that we have N listed at 2.5%w/w and an SG of 1.2.

 

To equate how many ppm of N this would contribute to the nutrient working solution when used at 4ml per litre we would run the same sums as when we were working with %w/v but then multiply the final figure by 1.2 (the SG).

 

I.e.

 

  • 5 (%w/w N) x 10,000 = 25,000 (ppm)
  • 25,000 (ppm) x 4 (ml/L) = 100,000 ÷ 1000 = 100 (ppm)
  • 100 (ppm) x 1.2 (SG) = 120ppm (N in working solution)

 

 

Anyway, let’s for a moment take a break from the chem quagmire of %w/v and %w/w to ppm conversion and ppm in solution material and mix things up a bit with some less intense theory on how and why this cluster of E equals MC squared (E equals what?) type material is important to understand. Take a breather, relax and let’s talk about off-the-shelf hydroponic nutrients and mains water supplies.

 

We’ll be tossing around a load more numbers in this material. The good news is that the numbers aren’t so important as the message so you can give your brain some much needed rest.

 

Keep in mind that we have a calculator on manicbotanix.com that does all these equations for you. view calculator now