How can I change a fertiliser formulation?

I have recently gotten into hydroponics and came across your blog containing much detailed information. I am trying to move away from buying expensive pre-mixed solutions to making my own. I have a query about DIY soluble fertilisers and hope you could help.

I might buy the following fertiliser: N = 6%; P2O5 = 12%; K2O = 36%; MgO = 2.5%; SO3 = 10%; Ca = 0%. It also includes trace elements B, Cu, Fe, Mo, Mn, Zn.

I want to upgrade the N content to 20 and P to 18. What do I need to buy to do that? Also I have seen all commercial soluble fertilisers do not have Calcium, so how do I add this to my mix and in what dosage? Will adding Calcium affect the NPK?   Answer by RICK DONNAN

Fertiliser labelling convention
From years ago, the fertiliser industry continues to use a strange NPK convention for labelling fertilisers. This quotes most nutrient elements as oxides and dates from early last century when it was the way of analysing and reporting nutrient analysis.

The figures you quote use this convention, that is, phosphorus is quoted as P2O5, rather than P. There is actually no P2O5 in the fertiliser, or any of the other oxides, so a conversion factor is required to convert the oxide figures to the actual nutrient content. Comments on forums indicate many growers are unaware of this. Surely it would be more useful and sensible to quote the actual figures.

That said, in England it is now compulsory to include the elemental percentage in brackets alongside the oxide figure. Also, thankfully, most commercial greenhouse grade fertilisers, as used for hydroponics, list the nutrient elemental percentages.

What are the conversion factors? This requires the use of atomic weights (AWt). For example, P2O5: AWt of P (phosphorus) is 31.0, and of O (oxygen) is 16.0.

P2O5 is 2 x 31.0 + 5 x 16.0 = 142.0. Proportion of P in P2O55 = 2 x 31.0/142.0 = 0.436

That is, your fertiliser contains 12% P2O5, equivalent to (x 0.436) = 5.23% P, which is the figure we need for calculations.

Commercial fertiliser analysis
Table 1 lists the converted figures for the percentage nutrients in your fertiliser.

Table 1. (click to enlarge)

Table 1. (click to enlarge)


Adding calcium
Putting aside increasing the P, calcium is added as greenhouse grade calcium nitrate, which typically contains 18.8% Ca and 15.5% N (14.4% as NO3—nitrate, and 1.1% as NH4—ammonium). This is also what you would add to raise your nitrogen level. If you plan to dissolve the fertilisers to make concentrates for convenience, the calcium nitrate would be part A and your other fertiliser would be part B, similar to the liquid fertilisers you buy. Don’t dissolve both of them as a single strong solution, or it will precipitate calcium sulphate and calcium phosphate. Combined at working strength is OK.

To raise the N level to equivalent to 20% you need to add 14% N. This would bring with it 14 x 18.8 / 15.5 = 17.0% Ca.

How much do you need: 100 gram (g) of single fertiliser gives 6 g of N. To get 14 g of N from calcium nitrate requires 14 x 100 /15.5 = 90 g calcium nitrate. So, whatever units you use, for every 100 units of your solid part B fertiliser, you need 90 units calcium nitrate.

The combination shown in Table 2 is very close to a standard vegetative lettuce formulation, just slightly lower in N, K, Mg and slightly higher in S and Ca.

Table 2.

Table 2. (click to enlarge)

Increasing phosphorus
Adding the calcium nitrate changed the ‘NPK’ as you wanted.
Any other changes that you make involve adding fertilisers. These always contain at least two different nutrients, and increasing these will automatically reduce the relative proportions of all other nutrients in the total fertiliser.

To increase the P content from P2O5 12% to 18% (P content 5.23% to 7.85%) requires the addition of either 13 g MKP (mono potassium phosphate) also increasing K. Alternatively, add 10 g MAP (mono ammonium phosphate) also increasing N. In both cases, the relative concentrations of all the other nutrients will fall in proportion as shown in Table 2.  Ω

PH&G July 2015 / Issue 157