How to Calculate Fertilizer Amendments Needed

  1. calculate the molecular weight (mw) of the molecule:
    • Example fertilizer: Nitrate (NO3)
      • 1 Nitrogen = 1 x 14 = 14
      • 3 Oxygen = 3 x 16 = 48
      • 14 + 48 = 62
  2. calculate the % element in the molecule:
    • Example: NO3 = 62
      • N / NO3 = 14 /62 = 22.5%
      • => 22.5% N in NO3
  3. calculate weight of fertilizer needed:
    • Example: Nitrogen target of 100 lb/acre
      • 100 lb/acre = 0.25 lbs per 100 sq ft
      • veggie bed = 150 sq ft
      • => N target = 0.25 x (100 / 150) = 0.375 lbs N
      • => 0.375 / 22.5% = 1.67 lbs NO3

=> 1.67 lbs NO3 needed for amending 150 square foot bed to reach 100 lb/acre target

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