What is the N.P.K. of each ingredient?

NPK analysis given for fertilizer is given in percentages by weight of product.

So, 10-15-10 means that in a 100 pound bag of fertilizer, there's 10 pounds of nitrogen (N), 15 pounds of phosphorus (P), and 10 pounds of potassium (K). The remaining 65 pounds is something other than available N, P, or K.

So, to determine the analysis of your custom mix, you have to first figure out two things: 1) what is the analysis of the individual ingredients (cottenseed meal, bone meal, and kelp meal in your case) and 2) how much each ingredient represents in the mix by weight ... not by volume.

Hypothetically, let's assume the following analyses (although to be correct, you're gonna have to read the real analyses on each label of the ingredients you actually have)...

Cottenseed meal: 7-3-2
Bone meal: 4-12-0
Kelp meal: 0.6-0-1.5

Then let's assume your mix is by weight (pounds) and not by volume (cups or quarts, etc.), then you have:

64 ounces (4 pounds) 7-3-2

8 ounces (1/2 pound) 4-12-0

4 ounces (1/4 pound) 0.6-0-1.5

So:

Nitrogen (N) = 64(0.07) + 8(.04) + 4(0.006) = 4.48 + 0.32 + 0.024 = "4.24"

Phosphorus (P) = 64(0.03) + 8(0.12) + 0 = 1.92 + 0.96 + 0 = "2.88"

Potassium (K) = 64(0.02) + 0 + 4(0.015) = 1.28 + 0 + 0.06 = "1.34"

Therefore (assuming my math and assumptions are correct ... which is a big assumption!) I think you'd have something like:

4-3-1 or 4.25 - 2.9 - 1.34 or there abouts, depending on the true analyses of the original products and where you round off your decimals.

Now, someone surely is gonna do this differently and come up with a different answer.

Again, you gotta know what the analysis is for each of the 3 products you start with, and you gotta measure by weight and not volume.

See ... I already checked my own math and found one mistake right off the bat:

4.48 + 0.32 + 0.024 = 4.804 (N)

So let's say roughly 5-3-1 or in that neighborhood.

Bill

"The remaining 65 pounds is something other than available N, P, or K."

Most of the time, it's just inert filler.

In growing tomatoes, the NPK are important however the number that follows these 3 is important as well.

NPK analysis given for fertilizer is given in percentages by weight of product.It is really not that simple for the N component in OM products. For instance, simple urea (O=C-(NH2)2) has an N weight of ~50%. But it is not useable for most plants in that form.

If you interpret N to mean Nitrate-N (like in the blue stuff), then that is a rather unequivocal argument.

Reg

The urea I've been buying is supposedly 46-0-0-0