Fertilizer Science & Math

Doctor Dave — Fri, 05 Mar 2010 16:55:00 GMT

Fertilizer Science & Math

Today let’s talk numbers, more specifically about fertilizer numbers. Most of us use a ppm meter (parts per million), an EC meter (electrical conductivity), or a TDS meter (Total Dissolved Solids.) All three of these measurements are different ways of expressing the same information. Basically it is the amount of fertilizer or solute that is dissolved in our fertilizer and water mixture. Knowing this value allows us to push our plants to grow to their maximum genetic potential!

First understand that PURE water actually conducts no electricity. That being said the more fertilizer dissolved in water the more conductive of electricity it becomes. So measuring the amount of electricity that is transferred from one electrode and received by a second electrode gives us an idea of how much fertilizer is present in a solution. This is the basic way all of these meters work. Suppose though you want to know more than just the total amount of fertilizers combined in a solution. What if you want to know how much of that PPM value Potassium is… then read on!

The N-P-K ratios are the 3 numbers present on most if not all fertilizer packages. But aside from figuring out which one is for vegetative growth (a high Nitrogen number,) or what might be better for fruiting or flowering (a higher 2^nd and 3^rd number,) what do they really tell us? If you want to really understand the N-P-K ratio on your fertilizer then the information below will be very helpful.

TDS or Total dissolved solids is a measure of everything that is dissolved in the water

PPM or parts per million is a proportional measurement of solute (dissolved stuff) in the water. IT IS A WEIGHT / WEIGHT PROPORTION… 1 part per weight of a solute in 1 million parts per weight water = 1 PPM
1 gallon of water (H20) has a volume / weight of 3.785 liters or 3,785,000 mg

(It can have both a volume and weight because at 3.98 degrees Celsius 1 liter of water weighs 1,000 grams & only changes by 4% from 0-100 degrees Celsius)

If we are looking to determine how much actual Nitrogen, Phosphorus, or Potassium is actually in a solution of XXX ppm then follow the steps below:
For this example lets use a fertilizer that has an N-P-K of 15-7-11. Our fertilizer after weighing 10 tsp has an average weight of 1.74 grams per tsp or 1,740 mg.
- First let’s find how many PPM there are in 1 tsp of fertilizer added to 1 gallon of water. Take the weight of fertilizer in grams and divide it by the weight/volume of 1 gallon of water.
  - 1,740 mg fert. / 3.785 Liters of water = 459.7 PPM
  - We will round up and call it 460 PPM
Now of that 460 PPM how much actual Nitrogen, Phosphorus, or Potassium actually is there.
- Nitrogen is measured as Elemental Nitrogen so it is the easiest to determine… Just multiply 460 PPM by the amount of Nitrogen in our fertilizer’s NPK…15% or .15 so there is (460 PPM x .15 = 69 parts per million Nitrogen in our solution
- Phosphorus is more complicated, the NPK uses P₂O₅ which is the oxide form of Phosphorus so we need to multiply by .437 to get the elemental amount of Phosphorus. So we have [460 x (.07 x .437)] = 14.07 Parts per million Phosphorus in our solution.
- Similarly Potassium is complicated; the NPK uses K₂0 which is the oxide form of Potassium so we need to multiply by .82 to get the elemental amount of Potassium. So we have [460 x (.11 x .82)] = 41.49 Parts per million Potassium in our solution.
So 1 Gallon of water with 1 tsp of 15 – 7 – 11 fertilizer has a total of 460 PPM, of which 69 parts are Nitrogen, 14 parts are Phosphorus, and 41 parts are Potassium.

So if you thought that simply 7% or 32ppm of the 460 parts per million in this water was phosphorus then you now see the value of the above calculations!

If math isn’t your favorite subject then feel free to ask our knowledgeable staff for their recommendations regarding what fertilizers or supplements might best complete or complement your feeding regiment.

Atlantis Hydroponics Blog RSS Feed

Fertilizer Science & Math