Fertilizer Calculations

Typically, commercial fertilizers come with a nutrient rating of three numbers, for example 10-13-10.  The respective nutrients are nitrogen(N),phosphorous (P) and potassium (K) and the numbers imply the amount of that element per unit of fertilizer.  In other words, the percentage of pound or weight or volume that each unit provides for that element is represented by a number.  For example a 10 lb bag of 10-15-12 fertilizer will provide 1 pound of nitrogen, 1.5 lbs of phosphorous and 1.2 pounds of potassium. Once soil tests have been taken, recommendations are made and fertilizer is applied to the area. To apply the proper amount of fertilizer, simply use the following formula: (lbs of element required/acre) / (lbs of element/lb fertilizer)= lbs of fertilizer/ac. For example: The results from your soil test recommend that the spot where you sampled requires 200 lbs of N.  If you are planning on buying 10-10-10' fertilizer how many pounds of this 10-10-10 fertilizer will you need to meet the 200 lbs of N requirement. (200 lbs of N/acre) / (0.10 lbs of N/lb fertilizer) = 2000 lbs of fertilizer per acre you will always want to use fertilizer that provides for the most deficient element(also known as the limiting nutrient), if your soil test requires 100 lbs of N, 500 lbs of P and 100 lbs of K you will want a fertilizer highest in Phosphorous for example 10-15-10.  The idea is to try and tailor the fertilizer to the requirements.  Ideally, if you could, for the requirements of 100 lbs of N, 300 lbs of P and 200 lbs of K you would want something like 10-30-20, but rarely will you find fertilizers tailored to your specific need.  Just do your best matching in order to save money.