Calculating NPK and fertilizer concentration vs EC

[z0so1337]

Hi everyone,

I'm trying to balance out nutrients I have at home by weight and I put together a calculator to help me out.

However, since I started this process I don't know if the fertilizer NPK directly correlates to EC in water.
Let's say I have a 1-1-1 and a 20-20-20 NPK no other micronutrients added, if both of the EC are 1.0, are they essentially the same fertilizer?
Is a 20-20-20 of recommended dose by the manufacturer a 10-10-10 if I add 50% more water?

The way I'm planning to use this is to mix and match fertilizers to get the right NPK and then dilute it to my desired EC to keep things simple. No manufacturer recommendation included. I'm also trying to not dilute too much the ones that have micronutrients in it. And if I do, I can always add some kelp/microbes to the pot.

Another question is should I be mindful of sulfur excess when using Potassium Sulfate with Magnesium Sulfate? Would that add too much sulfur to the mix?
Should I add mono potassium phosphate to the arsenal for that matter?

Do you guys have any advice or good reads in the subject?
Appreciate any insight.

Cheers.

 

[GreenGalaxyFarm]

The way I'm planning to use this is to mix and match fertilizers to get the right NPK and then dilute it to my desired EC to keep things simple. No manufacturer recommendation included. I'm also trying to not dilute too much the ones that have micronutrients in it. And if I do, I can always add some kelp/microbes to the pot.

interesting, what did you find you want to get the final veg and flower npk

 

[Trixie]

Yes, the same fertilizer just different size prills. Potassium sulfate and magnesium sulfate do not react, but will just dissolve in water. No H+ ions will be released, so the pH stays the same.

 

[z0so1337]

interesting, what did you find you want to get the final veg and flower npk

My goal is just to grow successfully with stuff I have at home. I'd like to stay close to 3-1-1, 2-5-3, 1-3-4 respectively. I might have to find something that boosts nitrogen or maybe I'll stick with 1-1-1 for veg.

Yes, the same fertilizer just different size prills. Potassium sulfate and magnesium sulfate do not react, but will just dissolve in water. No H+ ions will be released, so the pH stays the same.

Ah, nice! I just worry about too much sulfur causing toxicity... But I'm not sure if it's relevant. The goal is just to feed potassium or magnesium. I'm using coco for everything. The magnesium sulfate would be used with calcium acetate after flipping to flower so I don't get that nitrogen from the other option I have which is calcium nitrate. The problem is that I'd get 2x sulfur boost by using potassium and magnesium sulfate.

 

[z0so1337]

Like this. I'm boosting my potassium with this mix, and adding CalMag. However, my sulfur and nitrogen levels would be similar. Normally sulfur is 2% or 3%. My guess is that's okay otherwise people wouldn't use these products.

 

[z0so1337]

I'm getting super confused thinking about how to put together this calculator, but the concern about the sulfur persists. lol

 

[Observationist]

EC just tells you how salty the water is.

 

[Peat_Phreak]

I'm getting super confused thinking about how to put together this calculator, but the concern about the sulfur persists. lol

What is the maximum sulfur ppm of your formula? I also put together a nute spread sheet. Very handy to have. You just have to be absolutely sure your spreadsheet is correct.

 

[z0so1337]

EC just tells you how salty the water is.

Right! But the saltier it is, the higher concentration of the solution/fertilizer. Isn't it? It got me confused because I tested the EC of 2 different fertilizers and the one that had higher NPK had a lower EC. I just measured just once, though. I might try again tomorrow. But I thought the higher NPK, the higher the conductivity of the water. I can't find anything to read about this relationship.

 

[Zill]

zO,

I found this that converts dissolved salts to EC and back.



Electrical Conductivity

Soil and water salinity is often measured by electrical conductivity (EC). The most commonly used EC units are deciSiemens per metre (dS/m) and millimho per centimetre (mmho/cm), numerically:



1 dS/m = 1 mmho/cm



Simple relationships are used to convert EC to TDS, or vice Versa:

TDS (mg/L or ppm) = EC (dS/m) x 640 (EC from 0.1 to 5 dS/m)

TDS (mg/L or ppm) = EC (dS/m) x 800 (EC > 5 dS/m)

---

The ratio of TDS to EC of various salt solutions ranges from 550 to 700 ppm per dS/m, depending on the compositions of the solutes in the water. For soil extracts in the EC range from 3 to 30 dS/m, TDS can be estimated using the formula below:The US Salinity Laboratory (1954) also used the following empirical relationship between EC and the total soluble salt concentration (TSS, mmol/L)

---

TSS (mmol/L) = EC (dS/m) x 10

---

Sodicity

Changes in the mole fractions of Na+ vs. Ca++ and Mg++ on the cation exchange sites of soils may be assessed by exchangeable sodium percentage (ESP) or by the sodium adsorption ratio (SAR). The ESP of a soil is calculated from:

ESP(%) = Exch. sodium (meq per 100 g soil) / CEC (meq. Per 100 g soil)

Since it is usually difficult to obtain reliable soil exchangeable cation data for calculating ESP, the SAR of soil solution, soil extract, or irrigation water is often used:

SAR = Na+/Sqrt [(Ca++ + Mg++)/2]

where Na+, Ca++, and Mg++ denote the concentrations of respective cations of the water (meq/L).

 

[GreenGalaxyFarm]

Right! But the saltier it is, the higher concentration of the solution/fertilizer. Isn't it? It got me confused because I tested the EC of 2 different fertilizers and the one that had higher NPK had a lower EC. I just measured just once, though. I might try again tomorrow. But I thought the higher NPK, the higher the conductivity of the water. I can't find anything to read about this relationship.

would think the same
i notice that some fert give npk ratio numbers ex 4-4-4
and other npk percentage numbers for gram per kg ex 53-7-17 aka (53g/kg-7g/kg-17g/kg)

 

[z0so1337]

What is the maximum sulfur ppm of your formula? I also put together a nute spread sheet. Very handy to have. You just have to be absolutely sure your spreadsheet is correct.

I'm pretty new to this, don't have one. And I'm not sure what you mean. I got confused when adding the NPK together. I thought averaging the NPK numbers was right, but apparently it isn't. Then now I'm just adding the NPK, not caring much about the numbers and next time I mix I hope the EC is in proper range. If not, I'll dilute it. I'm not sure if I can go just by EC and not NPK.

My editable spreadsheet
You just dial the parts in column A. Last iteration of this spreadsheet I'm just adding the NPK, which is obviously wrong. Mind sharing yours?

Thanks, all, for the replies!

 

[Peat_Phreak]

My spreadsheet is dialed in for specific nutes that I use. Jacks 5/12/26, calcium nitrate and a few others. Not compatible with your nutes.

You can download Hydrobuddy. Then you need to have a list of all the nute % in the nute mix. More than just NPK. Then enter data into Hydrobuddy. Which is not very easy to use, but gives accurate results.

 

[z0so1337]

zO,

I found this that converts dissolved salts to EC and back.



Electrical Conductivity

Soil and water salinity is often measured by electrical conductivity (EC). The most commonly used EC units are deciSiemens per metre (dS/m) and millimho per centimetre (mmho/cm), numerically:



1 dS/m = 1 mmho/cm



Simple relationships are used to convert EC to TDS, or vice Versa:

TDS (mg/L or ppm) = EC (dS/m) x 640 (EC from 0.1 to 5 dS/m)

TDS (mg/L or ppm) = EC (dS/m) x 800 (EC > 5 dS/m)

---

The ratio of TDS to EC of various salt solutions ranges from 550 to 700 ppm per dS/m, depending on the compositions of the solutes in the water. For soil extracts in the EC range from 3 to 30 dS/m, TDS can be estimated using the formula below:The US Salinity Laboratory (1954) also used the following empirical relationship between EC and the total soluble salt concentration (TSS, mmol/L)

---

TSS (mmol/L) = EC (dS/m) x 10

---

Sodicity

Changes in the mole fractions of Na+ vs. Ca++ and Mg++ on the cation exchange sites of soils may be assessed by exchangeable sodium percentage (ESP) or by the sodium adsorption ratio (SAR). The ESP of a soil is calculated from:

ESP(%) = Exch. sodium (meq per 100 g soil) / CEC (meq. Per 100 g soil)

Since it is usually difficult to obtain reliable soil exchangeable cation data for calculating ESP, the SAR of soil solution, soil extract, or irrigation water is often used:

SAR = Na+/Sqrt [(Ca++ + Mg++)/2]

where Na+, Ca++, and Mg++ denote the concentrations of respective cations of the water (meq/L).

Thanks! That's good info but unfortunately it doesn't relate back to NPK concentration

 

[z0so1337]

My spreadsheet is dialed in for specific nutes that I use. Jacks 5/12/26, calcium nitrate and a few others. Not compatible with your nutes.

You can download Hydrobuddy. Then you need to have a list of all the nute % in the nute mix. More than just NPK. Then enter data into Hydrobuddy. Which is not very easy to use, but gives accurate results.

Gotcha. I guess that's an app? I'll give it a look, but I'm more interested in the logic and math of such calculations.

 

[z0so1337]

I think I finally figured it out! EC is concentration. The more you dilute in water, the lower the NPK.
The manufacturer will give you a recommendation, and that will represent the NPK in the package.

Let's say someone is using nutrient X (1-3-3), as an example. That EC will be very low when used as recommended (my guess: 0.2). But if someone is increasing the EC on each feeding with a target in mind, like 2.0 for a mature plant (super high), they are essentially not using the NPK number stated on their bottle anymore.

With that in mind, I have fixed the calculator. It will give me an NPK number that will probably match the EC of other fertilizers. This will work to understand roughly what's going on in your solution and how much of micronutrients you're losing when mixing with others that don't have any.

Let's say if you're adding 0.4 parts of potassium sulfate to 1 part MGT18-18-21, you have essentially a 13-13-30 at higher EC, or a 6.5-6.5-15 when lower EC. The important part is the ratio though, since we're guiding ourselves by EC.

I also think EC/NPK should correspond to your light level/VPD. If you fertigate your plants in 20-20-20 and you're using 600 PAR, you're probably just washing nutrients down the drain and wasting money. But I guess you can fine tune to exactly what your plant needs with your current light and environment just by monitoring solution EC vs runoff EC.

These are all assumptions of mine, though. But it makes a lot of sense now. I will definitely know if this works or not in the upcoming months. Hopefully someone chimes in the meantime.

 

[GreenGalaxyFarm]

I think I finally figured it out! EC is concentration. The more you dilute in water, the lower the NPK.
The manufacturer will give you a recommendation, and that will represent the NPK in the package.

Let's say someone is using nutrient X (1-3-3), as an example. That EC will be very low when used as recommended (my guess: 0.2). But if someone is increasing the EC on each feeding with a target in mind, like 2.0 for a mature plant (super high), they are essentially not using the NPK number stated on their bottle anymore.

With that in mind, I have fixed the calculator. It will give me an NPK number that will probably match the EC of other fertilizers. This will work to understand roughly what's going on in your solution and how much of micronutrients you're losing when mixing with others that don't have any.

Let's say if you're adding 0.4 parts of potassium sulfate to 1 part MGT18-18-21, you have essentially a 13-13-30 at higher EC, or a 6.5-6.5-15 when lower EC. The important part is the ratio though, since we're guiding ourselves by EC.

I also think EC/NPK should correspond to your light level/VPD. If you fertigate your plants in 20-20-20 and you're using 600 PAR, you're probably just washing nutrients down the drain and wasting money. But I guess you can fine tune to exactly what your plant needs with your current light and environment just by monitoring solution EC vs runoff EC.

These are all assumptions of mine, though. But it makes a lot of sense now. I will definitely know if this works or not in the upcoming months. Hopefully someone chimes in the meantime.

View attachment 1322620

imo feel npk ratios (not percentage) should be adjusted after seedling, after flowering begins, after pistils start turning orange and finally the last 2 weeks. what are you finding

 

[Peat_Phreak]

Yes, Hydrobuddy is an app that can calculate nutes if you learn how to use it.

You are currently headed in the wrong direction with your spreadsheet. EC can not be calculated with the NPK numbers on the bag. Because there are substances that are not NPK in the nute mix that contribute to EC.

Yes, EC and PPM are related to nute concentration. However, it's not clear what you are trying to accomplish here. It would be easier if you just told us what nutes you want to use and asked for dose recommendations.

 

[z0so1337]

Yes, Hydrobuddy is an app that can calculate nutes if you learn how to use it.

You are currently headed in the wrong direction with your spreadsheet. EC can not be calculated with the NPK numbers on the bag. Because there are substances that are not NPK in the nute mix that contribute to EC.

Yes, EC and PPM are related to nute concentration. However, it's not clear what you are trying to accomplish here. It would be easier if you just told us what nutes you want to use and asked for dose recommendations.

Sadly Hydrobuddy didn't run on my phone. I don't need anything specifically, am just interested in NPK vs EC relationship. Most micronutrients that are added are so low percentage like 100:1 or 1000:1 that I don't think they hold significant weight in the EC.

imo feel npk ratios (not percentage) should be adjusted after seedling, after flowering begins, after pistils start turning orange and finally the last 2 weeks. what are you finding

From my huge experience of 1 failed grow 😎, I'd say no nutrients till a full grown set of true leaves, then gradually increase EC in a 2:1:1 ratio starting around 0.3 EC until 1.4EC at the end of veg, then start mixing in some 1:3:2 ratio solution gradually till 50%/50% (veg/bloom) in the last week of veg. Flip to flower and go full 1:3:2, then 1:2:4 for the last weeks. This is my plan for this grow. I jumped in cannabis growing using a coco and peat mix medium, switching from organic nutrients to synthetic nutrients back and forth. The plant is almost 8 months old now. I light burned it, nutrient burned it, overwatered it, underwatered it, re-vegged it and flipped to flower without a tent at the end of summer. It was a mess, couldn't find help anywhere. Learned a lot though! Thankfully, I successfully monster cropped 2 out of 4 clones after it re-vegged and I'm gonna feed them this solution I calculated. Going full synthetic for this next run.