You want 500 ppm nitrogen in the stock tank; injector is set at 1:100 and fertilizer is 20-20-20. How many ounces per gallon are needed in the stock tank?

• A. 16.67 oz
• B. 5 oz
• C. 66.66 oz
• D. 33.33 oz

Answer: (D)

The essential idea is that the injector ratio determines how much stock fertilizer ends up in the final delivered solution. With an injection set at 1:100, the final solution is 1 part stock concentrate to 100 parts water, so the stock must be about 101 times more concentrated in nitrogen than the target final concentration. To get 500 ppm nitrogen in the final mix, the stock tank needs 500 × 101 ≈ 50,500 ppm N.

Convert that to a per-gallon requirement. 50,500 mg of N per liter times 3.785 liters per gallon equals about 191,200 mg of N per gallon. Since the fertilizer is 20% nitrogen, the mass of fertilizer needed per gallon is 191,200 mg / 0.20 ≈ 956,000 mg, or about 0.956 kg. That converts to roughly 33.3–33.7 ounces of 20-20-20 fertilizer per gallon of stock solution.

So, about 33.3 ounces of 20-20-20 per gallon of stock tank is needed to achieve 500 ppm N in the final irrigation when the injector is set to 1:100.