Example Fertilizer Calculation N, P2o5, K20
bove is an example of a nutrient labels guaranteed analysis.
It tells us how much of each element is in the bag at percentage weight by volume (%w/v).
This provides us with enough information to establish a reasonably accurate ppm.
Note that analyzing the ppm from fertilizer labels won’t provide 100% accurate ppms.
Fertilizers sold worldwide are often only required to be listed accurately to within 0.4%.
Regulations around the world require that NPK.. values be presented somewhat ambiguously.
Therefore, listings for the same nutrient may appear to vary on a country-by-country basis.
For example, when looking at our labels guaranteed analysis you will find note that it states;
Available Phosphate (P2O5)……….15.0%
Available Potash (K2O)…………..14.0%
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 %.
With our label, this is the case and P is listed as P2O5 (phosphorous pentoxide) and K is listed as K2O (potassium oxide) percentage.
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 5-15-14 NPK ratio truly reflects elemental NPK 5-6.45-11.62.
N = 5
P = 15 * 0.43 = 6.45
K = 14 * 0.83 = 11.62
Additionally, other nutrients such as calcium (Ca), magnesium (Mg) and sulfur (S) can be listed in their oxide form (CaO, MgO, SO3) or in elemental form, or both.
To convert other nutrient listings that may appear on some labels use these equations.
CaO to Ca multiply by 0.714
MgO to Mg multiply by 0.6031
SO3 to S multiply by 0.4
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 RO water weighs 1gram.
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, 5% of nitrogen added to 1 litre(1000ml) of RO water would mean that there is 50grams of N in the water.
1000 (ml) * 0.05 (5% nitrogen) = 50 (grams of N)
Converting %w/v to ppm and ppm to %w/v
To establish ppm from %w/v you simply need to multiply by 10000.
5% (nitrogen) * 10000 = 50000 (ppm)
To establish %w/v from ppm you simply need to divide by 10000.
50000 (ppm) / 10000 = 5% (nitrogen)
To establish the concentration of individual elements in the water, the guaranteed analysis (%w/v) should first be converted into ppm, then multiplied by the usage rate (per litre), then divided by 1000 (ml).
For example, if a nutrient lists 5% nitrogen, when it is used at 5grams per 4 litres it will yield 62.5 ppm of nitrogen per litre.
Step 1 : 5% (nitrogen) * 10000 = 50000 (ppm)
Step 2 : 5grams / 4L = 1.25g/litre
Step 3 : (50000 (ppm) * 1.25g/litre) / 1000ml (1 litre) = 62.5ppm of nitrogen per litre (1000ml)
Doing the math
Using what we’ve learned, we’re finally ready to find the ppm of our fertilizer.
5 * 10000 = 50k nitrogen ppm
6.45 * 10000 = 64.5k phosphorus ppm
11.62 * 10000 = 116.2k potassium
5 * 10000 = 50k calcium ppm
3.5 * 10000 = 35k magnesium ppm
4 * 10000 = 40k sulfur ppm
0.1 * 10000 = 1k iron ppm
50k + 64.5k + 116.2k + 50k + 35k + 40k + 1k = 356,7k or 356700 ppm.
(356700ppm * 1.25g/litre) / 1000ml = 445ppm(~0.9EC) per litre.
How Much Phosphorus and Potassium are Really in Your Fertilizer?
Fertilizer Calculations for Greenhouse Crops
Understanding phosphorus fertilizers
Common Fertilizers Table
Table 1: Percentages of water-soluble and available phosphate in several common fertilizer source
| P2O5 source | N | Total | AvailableP2O5 | Water soluble* P2O5 |
|---|---|---|---|---|
| Superphosphate (OSP) | 0% | 21% | 20% | 85% |
| Concentrated Superphosphate (CSP) | 0% | 45% | 45% | 85% |
| Monoammonium Phosphate (MAP) | 11% | 49% | 48% | 82% |
| Diammonium Phosphate (DAP) | 18% | 47% | 46% | 90% |
| Ammonium Polyphosphate (APP) | 10% | 34% | 34% | 100% |
| Rock Phosphate | 0% | 34% | 38% | 0% |
| *Water-soluble data are a percent of the total P2O5. | ||||
| Source: Ohio Cooperative Extension Service. |
More Fertilizer Calculation Examples with Videos
| Fertilizer calculation one | |||
| N | P | K | |
| % weight | 15 | 5 | 15 |
| desired ppm = mg/L or mg/kg | hoal | ||
| ( 1 liter oof water weighs 1 kg) | 200 | mg/L | |
| 0.15 mg of N per mg og Fertilizer | |||
| 200 / 0.15 = | |||
| 1333.333333 | |||
| mg N / L / mg N/ mg F = | |||
| mg F / L ) for desired 200 ppm) | |||
| divide by 1000 to get grans | |||
| 1.333333333 | g per L | ||
| for 2000 L solution | 2000 | ||
| 2666.666667 | grams F in 2000 L | 1000 | |
| 2.666666667 | kh of F in 2000 L | ||
| Fertilizer calculation two | ||
| 1 gallon = 3.7854 L | ||
| 5000 | gallon holding tank | |
| * 3.79 | ||
| 3.79 | ||
| 18,950.00 | Liters storage tank | |
| Calcium Nitrate | 0.155 | |
| 15.5 % N | mh N per Mg Ca Bitrate | |
| 645.1612903 | =100/0.155 | |
| desired ppm 100 )mg/L) | 0.64516129 | grams |
| 12,225.81 | times storage tank | |
| 12.23 | kg |
Videos
Other Nutrients
Potash
| Common name | Chemical name (Formula) | |
| Potash fertilizer | c.1942 potassium carbonate (K2CO3); c.1950 any one or more of potassium chloride (KCl), potassium sulfate (K2SO4) or potassium nitrate (KNO3).[9][10] Does not contain potassium oxide (K2O), which plants do not take up.[11] However, the amount of potassium is often reported as K2O equivalent (that is, how much it would be if in K2O form), to allow apples-to-apples comparison between different fertilizers using different types of potash. | |
| Nitrate of potash or saltpeter | potassium nitrate (KNO3) | |
| Sulfate of potash (SOP) | potassium sulfate (K2SO4) | |
| Permanganate of potash | potassium permanganate (KMnO4) |
Potassium oxide (K2O) is an ionic compound of potassium and oxygen. The chemical formula K2O (or simply ‘K’) is used in several industrial contexts: the N-P-K numbers for fertilizers,
P2o5
Phosphorus pentoxide is a chemical compound with molecular formula P4O10 (with its common name derived from its empirical formula, P2O5).
The phosphate or orthophosphate ion [PO 4]3− is derived from phosphoric acid by the removal of three protons H+
Lime
Agricultural lime, also called aglime, agricultural limestone, garden lime or liming, is a soil additive made from pulverized limestone or chalk. The primary active component is calcium carbonate.. Calcium oxide (CaO), is commonly known as quicklim.e
Calcareous (/kælˈkɛəriəs/) is an adjective meaning “mostly or partly composed of calcium carbonate“, in other words, containing lime or being chalky.
Calcium carbonate shares the typical properties of other carbonates. Notably it
- reacts with acids, releasing carbon dioxide (technically speaking, carbonic acid, but that disintegrates quickly to CO2 and H2O):
CaCO3(s) + 2 H+(aq) → Ca2+(aq) + CO2(g) + H2O(l)
Calcium carbonate reacts with water that is saturated with carbon dioxide to form the soluble calcium bicarbonate.
CaCO3(s) + CO2(g) + H2O(l) → Ca(HCO3)2(aq)
Agriculture and aquaculture
Agricultural lime, powdered chalk or limestone, is used as a cheap method for neutralising acidic soil, making it suitable for planting, also used in aquaculture industry for pH regulation of pond soil before initiating culture.[54]
Nitrogen
Ammonia is a compound of nitrogen and hydrogen with the formula NH3.
The ammonium cation is a positively charged polyatomic ion with the chemical formula NH+ 4. It is formed by the protonation of ammonia (NH3).
Urea, also known as carbamide, is an organic compound with chemical formula CO(NH2)2.
