# percentage for dry fertilizers

#### Fitos

##### Seedling
I want to buy some dry ferts but see on the internet different percentage of the elements.

KH2PO4
KNO3
MGSO4

in what percentage should be these elements for better results?
also for micro does anyone can tell what to look for??

#### dw1305

##### Expert
UKAPS Team
Hi all,
I want to buy some dry ferts but see on the internet different percentage of the elements.
KH2PO4
KNO3
in what percentage should be these elements for better results?
It may be to do with the way that phosphorus and potassium content are displayed for agricultural fertilsers.

In terrestrial fertilisers, for N: P: K, nitrogen (N) is quoted in its elemental form, the phosphorus (P) content as the pentoxide (P2O5) and potassium (K) as the oxide (K2O). To convert from one to the other you need to know the RAM of the element.

So N =14 and O =16, the RMM of NO3 is 14 + (3*16) = 62 and 62/14 = 4.43 multiply the quoted value of N by 4.43 and that is the amount of nitrate NO3.

PPM is the same unit as mg/L (there are 10^6 milligrams in 1 litre (1000 g) of water).

To convert N to NO3:
Nitrate-NO3 (mg/L) = 4.43 * N (mg/L), so 10 mg/L N is 44.3 ppm NO3
and to go from NO3 to N:
N (mg/L) = 0.2259 x Nitrate-NO3 (mg/L)

To convert P2O5 to P you multiply by 0.436.

If you want to know how much PO4--- that is, it is bit more complicated because you need to know the percentage of P in P2O5 (43.7) and the percentage of P in PO4 (32.6), the divide 43.7/32.6 = 1.34, so 10 mg/L P2O5 = 13.4 mg/L PO4, to go the other way around PO4 to P2O5 its 32.6/43.7 to give you multiply by 0.747.

K from potassium oxide (K2O) is fairly straight forward, just multiply by 0.83 to give the value.
This one is slightly different, you have to take into account the <"water of crystallization">, the maths is in the link but basically it is 10% Mg.
micro does anyone can tell what to look for??
If you have <"really hard water"> (which is likely on Cyprus) you need a mix with FeDTPA or FeEDDHA as the iron (Fe) source.

You may be able to buy a micromix aimed (designed for Hydroponics) locally? Yara <"Tenso Cocktail"> perhaps?

cheers Darrel

• ian_m

#### Fitos

##### Seedling
I really lost in maths. what I want to know when I looking on the internet or order a fertilizer from a company what I must ask??
For example are the Kalium and phosphate the same amount in the KH2PO4?
Is the PO4 double percentage than the K??
In KNO3 how much is NO3 and how much K is correct?
Or it doesn't matter.

#### dw1305

##### Expert
UKAPS Team
Hi all,
I really lost in maths.
Use the dosing calculator at <"Rotala Butterfly">.
In KNO3 how much is NO3 and how much K is correct?
KNO3

That is an easy one to estimate, because you can just round the RMM to 100.

In solution you have one K+ ion for every NO3- ion. The RMM of KNO3 is 39 (K) + 14 (N) + 3*16 (O3) = 101, so you have 39% K and 61% NO3.

To simplify the maths, if you add 10g of KNO3 to a 100 litre tank, you have added 61ppm (61 mg/L) of NO3- and 39 ppm of K+.

If you wanted to add 10ppm NO3 (to the same 100 litre tank) it is 1.63g KNO3, and that would also have added 6.31ppm K.

KH2PO4
In this case you have one K+ for every PO4--- ion. The RMM is 136 and you have 28.5% K+ and 69.5% PO4---

Dry salts
To simplify the maths, if you add 10g of KH2PO4 to a 100 litre tank, you have added 69.5 ppm PO4--- and 28.5 ppm K+.

Stock solution
If you don't want to dose with dry salts you can make up a stock solution. In this case I've dissolved 100g of KH2PO4 in 1000 mL (1 litre) of water as my stock solution and then I've dosed 10 mL of this stock solution into my 100 litre tank, which has added ~7ppm PO4--- and ~3ppm K+.

If I wanted 5ppm PO4--- it is 7.16mL of my stock solution, or easier to do 71.6g of KH2PO4 in one litre as the stock solution and a 10cm3 dose. In both cases you would add 2ppm K+ as well.

cheers Darrel

• ian_m