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

The basic formula for calculating how much fertilizer to apply to a given area for a specific amount of nutrient is:

Fertilizer "rate" often refers to two things:

1. The desired lbs of actual nutrient per acre
• e.g. a crop nutrient goal of 100 lbs N/acre, 25 lbs P₂O₅/acre, and 50 lbs K₂O /acre
2. The desired lbs of fertilizer material per acre necessary to achieve the lbs of nutrient per acre goal
• e.g. given a 20-10-0 fertilizer, how many lbs of fertilizer material do we need to apply 25 lb N/acre?  How much P₂O₅ and K₂O tag along?

Example 1

• Needed: 45 lb N, 45 lb P₂O₅, 45 lb K₂O per acre
• Determine ratio of N : P₂O₅: K₂O: (45:45:45 = 1:1:1)
• Apply: for example, 300 lb of 15-15-15 (because 45 lb needed / 15% in fertilizer = 300 lb fertilizer)

Example 2

• Needed: 150 lb N, 30 lb P₂O₅, 120 lb K₂O per acre for corn
• Apply: for example

K preplant (broadcast)    150 lb/ac 0-0-61 =          0          0          91

Starter                          200 lb/ac 15-15-15 =      30         30        30

Sidedress UAN              37 gal/ac 30-0-0 =         120        0           0

Total:                           150      30        121

• In this example, UAN 30-0-0 weighs 10.85 lb/gal; therefore, 37 gal/ac = 37 x 10.85 = 401 lb of UAN/acre.  At 30% N, this equals 401 x 0.30 = 120 lb N/acre.