Chem. Orchid fertilizer recipe wanted

[DavidCampen]

One thing I don't like about the formulation given above is that the ratio of ammonium to nitrate nitrogen is very low. In the next batch I will probably decrease the amounts of calcium and magnesiun nitrates and replace those amounts with ammonium aspartate to increase the percentage of nitrogen from ammonium.

[DavidCampen]

Here is the formula of my latest fortnightly batch of nutrient concentrate. I reduced the amount of magnesium nitrate to increase the Ca/Mg ratio and I replaced the potassium nitrate with ammonium aspartate and potassium aspartate to increase the ratio of ammonium to nitrate.

60g Calcium Ammonium Nitrate
30g Magnesium Nitrate
24g Potassium Aspartate
21g Ammonium Aspartate
10g Potassium Dihydrogen Phosphate
5g Ammonium Sulfate
2g Citric acid

I increased the amount of each minor element by 50%.
Fe 0.18g
NaCl 0.10g
Cu 0.09g
Mn 0.09g
Zn 0.09g
B 36mg
Co 2.4mg
Mo 1.8mg

This gives an N-P2O5-K20-Ca-Mg-S ratio of
15.7-5.2-10-11.4-2.9-1.3

The nitrate to ammonia ratio is: 12.6 - 3.1
There is also nitrogen from the aspartate equal to about twice the nitrogen from ammonia.

This formula is diluted to 2 liters as stock solution for my chemical metering pump and from there it is diluted 1 - 150 to produce the ready to use mixture.
The stock solution has a pH of 5 - 5.5 and the RTU solution a pH of about 6.

[Intruder]

Interesting David. Why do you add acetic acid or citric to your salts mixture? I prepare also myself the orchids fertilyser I use. Also, I agree that pure nitrate as nitrogen source is not optimal. I have calculated a fertlyser having 1/6 of its nitrogen which is in the ammonium form. I see also that your second preparation has a ratio N/P/K near 3-1-2 what is considered as optimal for green plants.

[DavidCampen]

I added acetic or citric acids to lower the pH. Sulfates and phosphates have better solubility in acidic solutions. I am trying for a pH of about 5.5.

In the next iteration of the formula I will replace some of the potassium and ammonium aspartate with potassium and ammonium citrate (probably dipotassium citrate and diammonium citrate and then adjust the final pH with citric acid) to get more buffering capacity.

[Intruder]

@David. Thank you for your response. Keep us informed about the results reaches using your homemade fertilyser.

[Shiffdaddy]

This is a great thread.. especially for someone taking his 3rd year of college chemistry. This is great info and i think i may try this or maybe change it up a little and do the calculations for fun lol.

[DavidCampen]

I had been sterilizing my tank concentrate mixtures by filtering through a water purification filter.
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But with my last batch I got lazy and did not sterilize, that was a mistake, within a few days I had copious amounts of white fungus/mold growing in the concentrate tank. I had to resterilize my tank and make a new batch of sterile nutrient concentrate solution.

I had started sterilizing back when I was using DynaGro to prevent algae from growing in the nutrient tank. I had never noticed anything other than algae. The aspartic and citric acids in my formulations must provide a good carbon source for fungus/mold.

[DavidCampen]

Originally Posted by DavidCampen

Here is the formula of my latest fortnightly batch of nutrient concentrate. ...

60g Calcium Ammonium Nitrate
30g Magnesium Nitrate
24g Potassium Aspartate
21g Ammonium Aspartate
10g Potassium Dihydrogen Phosphate
5g Ammonium Sulfate
2g Citric acid
...
This formula is diluted to 2 liters as stock solution for my chemical metering pump and from there it is diluted 1 - 150 to produce the ready to use mixture.
The stock solution has a pH of 5 - 5.5 and the RTU solution a pH of about 6.

The above formulation deposited a small amount of white precipitate, probably a phosphate, it was soluble in 10% nitric acid but not in dilute citric acid.

So, my next batch, that I am using now, has the formulation:
45 g Calcium Ammonium Nitrate
20 g Magnesium Nitrate (Magnisal)
15 g Potassium Nitrate
45 g monoAmmonium Aspartate
5 g Citric acid

It has the N-P-K-Ca-Mg-S composition of:
15.985-5.2-10.3-8.55-1.92-0.6 (16-5-10-9-2-0.6))
It has not deposited any precipitate.

I was going to attach a spreadsheet that I use but I see that that file type is not allowed. I will try to attach it as a zip file.

Attached is a zip file.

Oh, about the spreadsheet, I need to mention that I don't work with percentages but rather with ratios so that the numbers for the N-P-K-Ca-Mg-S values are in weight not percentage.

[DavidCampen]

For my next batch I am going to try to increase the amount of phosphate to get a 15-10-10 N-P-K ratio.

I amended the spreadsheet to include citric acid. I ignore it in my N-P-K-Ca-Mg-S calculations since it doesn't contribute any of those elements. But it is important for maintaining an acidic pH so that various phosphates and sulfates don't precipitate.

If the quantities in this and my other spreadsheets were taken to be in grams then this would be my batch sheet for preparing 2 liters of concentrate to be delivered at a dilution of 1:100 to my watering wand (thus producing 200 liters of ready-to-use solution). Also in these spreadsheets the numbers for the N-P-K-Ca-Mg-S components are in units of weight (grams if you are working in grams). So for the formulation in this spreadsheet, if the components are given in grams then there is 15.11 grams of Nitrogen in a batch; thus diluted to its ready-to-use volume of 200 liters it will supply 75 ppm Nitrogen. (15 grams in 1000 liters would be 15 ppm so 15 grams in 200 liters is 75 ppm).

These formulas are intended for use with RO water so it is also important to include trace elements. To a 2 liter batch of concentrate I add:

180 mg Iron, from Ferric Ammonium Citrate
90 mg Copper, from Cupric nitrate
90 mg Manganese from Manganese sulfate
90 mg Zinc, from Zinc nitrate
12 mg Cobalt, from Cobalt sulfate
36 mg Boron, from Boric acid
100 mg Sodium chloride
6 mg Molybdenum from Sodium molybdate
(mg is milligram)
I copied these trace element amounts from the Dyna-Gro formulations. The proportion of copper strikes me as being a bit high (even thought at the final dilution of 200 liters the copper will only be 0.45 ppm).

When I prepare a batch of concentrate I make 2 solutions, one containing the monoPotassium Phosphate, ammonium sulfate, boric acid and sodium molybdate plus half the citric acid, the other solution contains everything else. Each solution is diluted to a little less than 1 liter then they are rapidly combined and diluted to the final volume of 2 liters.

In the spreadsheet I use abbreviations for many of the components:
CAN = Calcium Ammonium Nitrate
Mag. Nitrate = Magnesium Nitrate (hydrated)= Magnisal
KNO3 = Potassium Nitrate
KH2PO4 = monoPotassium Phosphate
NH4NO3 = Ammonium Nitrate (not used)
NH4AspH = monAmmonium Aspartate
Amm. Sulfate = Ammonium Sulfate

Magnisal is the brand name of a greenhouse grade magnesium nitrate. Its N-P-K-Mg composition is typical for any greenhouse grade magnesium nitrate.

Calcium Ammonium Nitrate (CAN) is the form that greenhouse grade calcium nitrate is supplied.

It is important to get "greenhouse grade" calcium nitrate and magnesium nitrate. Other fertilizer grades of these materials may be only partially water soluble. Even with the greenhouse grade materials, they are not highly purified laboratory reagents - the CAN that I have dissolves leaving a gray scum and the Magnisal dissolves into a solution with suspended small black particles so I filter these when I prepare my stock solutions.

When I prepare a batch of nutrient solution I do not weigh out the components. Instead I have each component already dissolved in a stock solution so that I can measure the required amounts by volume using gradutated cylinders.

CAN and Magnisal produce stock solutions containing 400 grams per liter. Ammonium Aspartate 200 grams per liter, monPotassium Phosphate 100 grams per liter. Citric acid and Ammonium Sulfate, 250 grams per liter.

Ammonium Aspartate is the one component I use that is not typically used in nutrient formulations. I use it for 2 purposes. It supplies Ammonium nitrogen and it also acts as a chelating agent to help prevent the cationic trace elements and also the calcium and magnesium from precipitating as phosphates and sulfates. In many commercial nutrient formulations the trace elements are chelated with EDTA but I believe that EDTA is too strong a chelating agent for this purpose so I prefer aspartic acid. Aspartic acid is an amino acid that can be purchased as the pure bulk material from retail stores for use as a human nutrient. To make Ammonium aspartate I add the requisite amount of aspartic acid to a solution of ammonium bicarbonate. Ammonium bicarbonate also called ammonium carbonate and Baker's Ammonia can be purchased in 1 pound quantities for use as a leavening agent in cooking.