Getting What You Paid For
The example in Table 7.3 is based on the fertilizer label in Table 7.2.
Table 7.3. Calculating Pounds of Nitrogen (N), Phosphate (P2O5), Potash (K2O)
Our bag contains 60 pounds of fertilizer. Since we know the percent of nitrogen, phosphate and potash, we can calculate how many pounds of each element using the following formula:
(% nutrient ÷ 100) x total pounds of fertilizer = pounds of nutrient in the bag
In our example the calculations would be as follows:
Nitrogen: (30 ÷ 100) x 60 lbs = 18 lbs nitrogen
Phosphate: (4 ÷ 100) x 60 lbs = 2.4 lbs phosphate
Potash: (4 ÷ 100) x 60 lbs = 2.4 lbs potash
In the cases of P and K, this calculation only determines the percentage oxide of these two elements and not the actual percentage of each nutrient. For example, the element P only makes up 44 percent of the P2O5 or phosphate molecule and the element K only makes up 83 percent of the K2O molecule. However, unless a soil test directs you otherwise, the amount of product to apply will be given in terms of the phosphate and potash designations. When occasions arise where the precise amount of P or K need to be determined, multiplying the phosphate percent by 0.44 and the potash percent by 0.83 will give the exact amount of P and K respectively. Table 7.4 illustrates how to calculate actual P and K.
Table 7.4. Calculating Actual P and K
Calculating actual P and K amounts using the example above:
Phosphate into phosphorus: 2.4 lbs x 0.44 = 1.1 lbs
Potash into potassium: 2.4 lbs x 0.83 = 2.0 lbs
Proceed to Types and Forms of Nutrient Source - Nitrogen.