Section 6: Reading and Understanding Fertilizer Labels
Fertilizer labels contain a great deal of valuable information. In order to prepare fertilizer solutions, properly apply a fertilizer, or to effectively use a fertilizer, the information on the label needs to be understood.
A fertilizer label lists the % by weight of N, P2O5 and K2O (not N-P-K). Therefore, a 20-20-20 is 20% N, 20% P2O5 and 20% K2O. A 100 lb bag would contain 20 lbs N, 20 lbs P2O5 and 20 lbs K2O. However, this does not tell us how much actual P and K are in 100 lbs of 20-20-20. To determine this we need to know that P2O5 is 43% P and K2O is 83% K (determined from molecular weights). Therefore, a 20-20-20 contains:
0.20 N x 100 lbs = 20 lbs N
(0.20 P2O5 x 0.43) x 100 lbs = 8.6 lbs P
(0.20 K2O x 0.83) x 100 lbs = 16.6 lbs K
Therefore, a 20-20-20 is actually 20% N - 8.6% P - 16.6% K. All of these calculations are based on molecular/atomic weights, so if you know the atomic weights you can determine the actual amount of any element from any fertilizer or fertilizer compound. For example, 100 lbs of Epsom salts (MgSO4 - 7 H2O) will contain: 24 + 32 + 4(16) +7(18) = 246 and (24/246) x 100 = 9.8 lbs of Mg.
This is not the only important information on a fertilizer label. Fertilizer labels provide information regarding the actual components used to formulate the fertilizer, the ratio of NH4+ to NO3-, whether the fertilizer is acidic or basic, the electrical conductivity of different concentrations of the fertilizer, and application recommendations. The links below connect to sample fertilizer labels.
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