PO 32. Calculate fertilizer application rates from fertilizer analysis information.

Before using any fertilizers, it is important to understand how to read a fertilizer label.  All fertilizers are labeled as %N - % P₂O₅ - %K₂O.

• For example, a fertilizer labeled as a 20-5-10 means that the product contains, by weight: 20 percent N, 5 percent P₂O₅, and 10 percent K₂O.

The nature of the fertilizer (liquid or dry) must also be taken into account.  Generally there is no measurable difference in crop response, as long as they supply the same amount of soluble nutrient.  From a plant's perspective, a pound of N from urea is equal to a pound of N from UAN.  The agronomic differences among N fertilizers are the risk for N losses due to volatilization, denitrification and leaching.  Ultimately, the choice between wet and dry P fertilizers is based on economics and farmer preference.  Note that for liquid fertilizers where the rate is given in gallons per acre, the density of the fertilizer must be known to determine the amount of nutrients applied per acre.  For example:

Material                         Weight (lbs/gal)

10-34-0                                     11.7

9-18-9                                      11.7

28-0-0                                      10.7