NPK Ratio

[tug]

I am not sure if ratios are absolutely necessary or relevant to what we are doing. IMHO, it is more important that we provide non-limiting nutrients for our plants by looking at typical nutrient uptake rates, CO2 deficiency and lighting intensities. I do agree, if you dose KNO3 and KH2PO4 at non-limiting levels then you are also adding enough K, but we can add a little K2SO4 for some plants without harming fish.

From Tom Barr's research, some typical uptake rates with non-limiting CO2, per day (24hours) are:

• NO3 1-4ppm
• NH4 0.1-0.6ppm (do not dose NH4! It will cause algae)
• PO4 0.2-0.6ppm
• Trace (CSM+B) - Fe is a proxy for all trace nutrient levels and is often dosed from 0.2-0.5ppm or higher, (virtually no hobbyist ever measures trace metals. So everyone is guessing about the traces as it is, even the most ardent proponent of testing for dosing!)

These rates do not assume that you will show deficiencies if you dose less than this, but providing more than these rates will not help further your plants health. Basically, it is extremely unlikely your plants will ever need more than these rates even at high light intensities. Adding enough nutrients to prevent anything from becoming deficient is the goal, not precise uptake and growth requirements.

These typical uptake rates with high light and CO2 enrichment offer some fairly good predictions/correlations of uptake rates for non CO2 planted tanks as well. The rate of uptake is reduced due to less light and less CO2. Basically the non CO2 tank grows 6-10x slower than a CO2 enriched tank.

If you want to calculate the amount of fertilizer you need to dose, then "wet" has a new all inclusive nutrient calculator that is worth a look, http://rota.la

[krisw]

I agree that precision is less important, but ultimately all the ratios are is a guideline to achieve the ppm doses that you outline from Tom. From personal experience, I know that if I dose about 1 part phosphate for 3 parts nitrate, the concentrations work out to what I want. I dose the minimum I need because I don't like overdosing and ultimately dumping unnecessarily nutrient rich water down the drain into the Bay's system during water changes. I realize my individual impact is small, but it's still important to me.

[John G]

... I don't like overdosing and ultimately dumping unnecessarily nutrient rich water down the drain into the Bay's system during water changes...

When I do a water change I use the old water to water some of our indoor plants, or if not needed there, to water various shrubs outside. So the excess nutrients are put to a good use.

[krisw]

I do for my non edible plants, but given Seachem warnings about not for use on plants for human consumption, I don't use much, as the majority of my outdoor plants are veggies/herbs.

[tug]

Kris, I defer to your experience. We are moving away from the idea that PO4 causes algae in our planted tanks. I see that as a good thing and recommend increasing phosphate doses in most cases, under favorable growing conditions.

The ratio of NO3 to PO4 dosed might go up or down depending on the situation. Personally, my diy solutions for NO3 and PO4 are dosed 5:1 on average. In time, I might increase the amount of PO4 (when my tanks have pressurized CO2) but for low light tanks without CO2 supplementation a 10:1 dose can be just as effective at providing non-limiting nutrients. The key starting out, is providing non-limiting nutrients within that ratio.

I do for my non edible plants, but given Seachem warnings about not for use on plants for human consumption, I don't use much, as the majority of my outdoor plants are veggies/herbs.

Really? What could possibly be different about Seachem that their products should not be used on food crops? A little nitrate and phosphate from our tank water should be fine for crops. It's not organic and won't help the soil retain moisture by increasing microporosity, or encourage the formation of good soil structure but Seachem warnings about not for use on plants for human consumption is a bit over the top.

[krisw]

Kris, I defer to your experience. We are moving away from the idea that PO4 causes algae in our planted tanks. I see that as a good thing and recommend increasing phosphate doses in most cases, under favorable growing conditions.

The ratio of NO3 to PO4 dosed might go up or down depending on the situation. Personally, my diy solutions for NO3 and PO4 are dosed 5:1 on average. In time, I might increase the amount of PO4 (when my tanks have pressurized CO2) but for low light tanks without CO2 supplementation a 10:1 dose can be just as effective at providing non-limiting nutrients. The key starting out, is providing non-limiting nutrients within that ratio.

I do for my non edible plants, but given Seachem warnings about not for use on plants for human consumption, I don't use much, as the majority of my outdoor plants are veggies/herbs.

Really? What could possibly be different about Seachem that their products should not be used on food crops? A little nitrate and phosphate from our tank water should be fine for crops. It's not organic and won't help the soil retain moisture by increasing microporosity, or encourage the formation of good soil structure but Seachem warnings about not for use on plants for human consumption is a bit over the top.

I think we're in agreement. Does in a ratio that's non limiting, and don't be afraid of adding phosphate.

Regarding Seachem in the garden, I'm primarily concerned with Excel. Even the Seachem rep at the AGA noted that it's the only bottle with a child proof cap.

[Ghazanfar Ghori]

Updated - 10 years later:

I've been using a new mix that seems to work very well for me. My tap water is pretty soft, so there is some good Ca and Mg in this mix that helps.

Stock Solution A (500ml)
Dissolve the following in 300 ml distilled water.
40 grams of Calcium Nitrate - Ca(NO3)2.4H20
24 grams of Magnesium Nitrate - Mg(NO3)2.6H20
20 grams of Potassium Nitrate - KNO3
Bring total solution volume up to 500ml

Dose 1 ml per gallon of tank water 3 x weekly (e.g. Dose 50ml of stock solution in a 50 Gallon tank, 3 times weekly)


Stock Solution B (500ml)
Dissolve the following in 300 ml distilled water.
13 grams of Potassium Phosphate - Kh2PO4
Bring total solution volume up to 500ml

Dose 1 ml per gallon of tank water 3 x weekly (e.g. 50ml of stock solution in a 50 Gallon tank, 3 times weekly)

Dose a commercial trace/iron mix on the other days


Quick note: You can safely double the concentration of the stock solution if you'd like.
Since I dose several tanks, I kept the concentration of the stock solution at a level where it was easy to remember to dosage (1 ml per gallon).

Pro Tip: I buy 2 x 1 gallon bottles of distilled water from the grocery store. That's close to 4 liters.
I can make close to 8x the recipe above which I pour back into the gallon bottles.

I use that to refill my Twin Neck dosing bottles.
Saves me from measuring and mixing over and over again and the Twin Neck bottles are frikkin awesome!