Moon's Graden

Can someone refresh my mind on fertilizer calculations?
I If I recall correctly, in NPK terms, N is reported as N, P is P2O5, and K
as K20. So using the gravimetric factors for the P & K, a 10-10-10 fert
gives me 10% by weight N, 4.4% P, and 8.3% K. Am I OK so far?

If it's a liquid, and is dosed at 1 teaspoon per gallon, that's an addition
of approximately 5 ml into 3785 ml of water.

If I assume a sp. gr. of 1 for the liquid fertilizer solution (probably not
correct, but OK for the calculation verification), my 5 ml weighs 5 g, of
which 0.5g is N, 0.44g is P and 0.83g is K.

If I have 0.5g N in 3790g of total solution, my nitrogen concentration is
0.5/3790 or about 132 ppm, right? And my TDS (assuming pure water) would be
(0.5+0.44+0.83)/3790 = 467ppm?

--

Ray Barkalow <> First Rays orchids
http://www.firstrays.com
Secure Online Ordering & Lots of Free Info!

Ray:

Your calculations are good as far as the concentration of the various
elements go, but incorrect for the total dissolved solids (TDS). Thats
because

1) The fertilizer contains other stuff than pure nitrogen, phosphorus and
potassium. For example nitrogen is usually present as either ammonium (NH4+)
or nitrate (NO3-). In the first case about 78% of the ammonium is nitrogen,
in the second only 23% of the nitrate is nitrogen.

2) The fertilizer usually contains other ions: micro-nutrients, pH buffers,
fillers such as sodium or chlorides.

3) Tap water contains usually anywhere between 200 and 1000 ppm of dissolved
solids and thus cannot be neglected in calculating TDS.

I hope this helps,

Marc

Marc,

Thanks.  I am aware of the minors and trace elements, not to mention those
minerals dissolved in the water.  I was looking for validation of my
factors, as much as anything, and for the calculation purposes was using
theoretically pure water and a fertilizer with only N, P, & K.

I use Dyna-Gro and ProTeKt in RO water, and while my 1/2 teaspoon per gallon
each would give me a calculated NPK of 120 ppm, my true TDS is 175 ppm

--

Ray Barkalow <> First Rays Orchids
http://www.firstrays.com
Secure Online Ordering & Lots of Free Info!