Level 12 Guiding Question:
How did today’s food system develop?
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Level 12 Investigation: Fertilzer Types
Calculating Fertilizer Problems
Whenever you see a fertilizer product, it will have three numbers prominently listed on the package, usually on the front. These numbers are very important and tell a great deal about what this fertilizer will do. Nitrogen, Phosphorus, and Potassium (N,P,K) are what the numbers refer to. They are always listed in this order. It is the percentage within that package of each component. For example, a common type of all-purpose fertilizer is referred to as 10-10-10. This is a balanced blend of equal portions of Nitrogen, Phosphorus, and Potassium. If you purchased a 50-pound bag, five pounds (or 10%) would be Nitrogen, five pounds would be phosphorus, and five pounds would be potassium. The remaining 70% is simply filler, or inert ingredients, which are there mostly to help disperse the chemicals. Thus, the amount of nutrient in a fertilizer will depend on the fraction of the material that is the nutrient. Also, P and K are often expressed as P2O5and K2O, respectively. For example, P X 2.29 = P2O5; or P2O5/2.29 = P (see problem solutions below). In converting P2O5 to P, one may either divide by 2.29 or multiply by 0.44 to get the same answer.
*% nutrient = (pounds of nutrient/pounds of fertilizer) X 100
*pounds of nutrient = (% nutrient/100) X pounds of fertilizer
*pounds of fertilizer = pounds of nutrient/(% nutrient/100).
Examples:
1. What is the fertilizer analysis of pure CH4N2O? (at wts: C=12 H=1, N=14,O=16) One must determine the percentage of N in this molecule. To do this, divide the weight of N in the molecule by the molecular weight of the compound. The molecular weight is:
C + N + H + O
1C (12) + 2N(14) + 4H(1) + 1O(16) = 60
Therefore, in 60 g of CH4N2O, one has 28 g of N
The percentage N is 28/60 (100) = 46%, or the analysis is (46-0-0)
2. If 160 lbs of starter (6-10-8) were applied per acre, how many lbs of N, P (P2O5) and K (K2O) are applied?
160 lbs fert. X 0.06 = 9.6 lbs N
160 lbs fert. X 0.10 = 16.0 lbs P2O5 1
60 lbs fert. X 0.08 = 12.8 lbs K2O
Level 12 Videos: How read labels and Calculate Fertilizer problems
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Level 12 Review: Fertilizer Problems
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