The Home Food Garden

Dear Farmers’ Almanac – NPK

John

GUARANTEED ANALYSIS
TOTAL NITROGEN (N)……………………………14%
     8.20% Ammoniacal Nitrogen
     5.80% Nitrate Nitrogen
AVAILABLE PHOSPHATE (P2O5)…………….14%
SOLUBLE POTASH (K2O)………………………..14%

Dear Farmers Almanac:

On page 12 of the 2010 edition of the Farmers Almanac you have an article titled “Understanding Fertilizer.”  The article states “The three numbers on a fertilizer bag refer to the percentages by weight of Nitrogen, Phosphorus, Potassium, always in that order.”  Unfortunately, that is incorrect and perpetuates a common misunderstanding.

The three numbers, example 14-14-14, actually stand for the percentages of Nitrogen, Phosphate, and Potash.  According to Knott’s Handbook for Vegetable Growers, Third Edition, the 14-14-14 can be easily converted to the actual weights of Nitrogen, Phosphorus, and Potassium.

Multiply the first number, N, by 1.  Multiply the second number, P, by 0.437, which is the percentage of Phosphorus in Phosphate (P₂O₅) by weight.  Multiply the third number, K, by 0.830, which is the percentage of Potassium in Potash (K₂O) by weight.

Thus in a 100 pound bag of 14 – 14 – 14 fertilizer, you have 14 pounds of Nitrogen, 6.1 pounds of Phosphorus, and 11.6 pounds of Potassium.   

Save the handy chart:

Fertilizer N – P – K

N = Nitrogen, multiply by 1 = N

P₂O₅ = Phosphate, multiply by 0.437 = P

K₂O = Potash, multiply by 0.830 = K

Sincerely,

John E. Perkins