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What Does This Number Mean?

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What do the numbers on this soil test report mean? It is a question I hear often, so dont feel alone if you have asked it. I have previously written an article on what the individual nutrient results mean (Soil Test Results: What Do They Mean?, December 2000 issue of Ag News and Views). However, the nutrient values mean nothing if you dont know how to apply the fertilizer. I hope this article will answer the application portion of this question. Figure 1 shows a sample soil test report. In the box in the lower left hand corner are the fertilizer recommendations in pounds per acre of actual nutrient. You will notice it says N, P2O5 and K2O. This is the same way nutrients are expressed on a fertilizer tag. So if the number under the N in the fertilizer recommendation box is 60, it means 60 pounds per acre of actual nitrogen, not 60 pounds of nitrogen fertilizer. It is the same for P and K. A 40 under P2O5 in the fertilizer recommendation box is 40 pounds of actual phosphate not 40 pounds of phosphorus fertilizer.

To convert pounds of actual nutrient to pounds of product requires a little math. For example, lets say the recommendation is for 60-60-0. First we need to figure out the P because it typically comes with some nitrogen as 18-46-0 (diammonium phosphate). Different fertilizer materials contain different amounts of N-P-K by weight. The numbers 18, 46 and 0 are percentage concentrations of N, P2O5 and K2O, respectively. So every pound of 18-46-0 is 46 percent phosphate and 18 percent nitrogen. We want 60 pounds of phosphate, so we divide 60 pounds needed by 46 percent or 0.46 available and get 130 pounds of 18-46-0 needed to have 60 pounds of P2O5 available.

In that 130 pounds of 18-46-0, there is 18 percent nitrogen or 130 multiplied by 0.18, which is 23 pounds of actual N available. We need to credit ourselves for the N supplied in the 18-46-0, so we subtract this 23 pounds of N from the 60 pounds recommended. We still need 37 pounds of N. If we use 34-0-0 (ammonium nitrate), we again divide the 37 pounds needed by 34 percent or 0.34 available and get 109 pounds of 34-0-0 needed to have 37 pounds of N available.

So to get 60-60-0 per acre we need 130 pounds of 18-46-0 and 109 pounds of 34-0-0 blended together. Then this blend is spread at 239 pounds of material per acre.

Lets do another example for a recommendation of 100-40-30. We divide 40 pounds of phosphate needed by 0.46 from 18-46-0 and get 87 pounds of 18-46-0 needed to have 40 pounds of phosphate available. To credit ourselves for the nitrogen, we multiply 87 pounds of product by 0.18 from the 18-46-0 and get 16 pounds of nitrogen credit to subtract from the 100 pounds of nitrogen recommended.

Now we still need the other 84 pounds of N. This time we will use 46-0-0 (urea). Take 84 pounds of N needed divided by 0.46 (from the 46-0-0), and it will require 182 pounds of 46-0-0 to have 84 pounds of N available.

Repeat the process for potassium. Divide 30 pounds of K2O needed by 0.60 from 0-0-60 (muriate of potash) and get 50 pounds of 0-0-60 to have 30 pounds of K2O available. When we add this up, we need to apply a total of 319 pounds per acre of this blend.

I know many of you get to the fertilizer dealer, and he hits you with this 319 pound number, and it sounds like a lot to apply per acre. As one of my cooperators says, You can see it on the ground! when you spread it. If the pounds of product per acre seem like too much or you cannot afford it, please contact the person who made the recommendation or a Noble Research Institute soil and crop specialist. Some ways of cutting back are better than others, and we want to help you reach your goals as economically as possible.