Managing Soil To Keep It Productive
Module 2
Lesson 2 - Reading The
Fertilizer Bag And Fertilizer Application Rates, Activity
Acknowledgement: Taken from "Living on
the Land 2001"
Now that you have a rough idea of what your soil actually needs, it is time to fertilize.
Remember that the numbers on the fertilizer bag indicate the percentage of elements in a specific order: N - P - K (Nitrogen, Phosphorous, Potassium). If there is a fourth number, it indicates the amount of sulfur. Let’s make sure we understand by answering the next few questions.
Which bag has the highest percentage of phosphorous?
__ Bag A with 16-16-16 or __ Bag B with 16-20-0-15?
Which bag has the highest percentage of nitrogen?
__ Bag A with 16-16-16 or __ Bag B with 16-20-0-15?
Which bag has the highest percentage of potassium?
__ Bag A with 16-16-16 or __ Bag B 16-20-0-15?
Which bag contains sulfur?
__ Bag A 16-16-16 or __ Bag B 16-20-0-15?
Now, the tough question – how much fertilizer do we need to add? The soil test results will recommend a certain number of pounds of a certain element to add to your soil, generally on a per acre basis. But there are some things to remember about the calculation. The soil test will tell you how many pounds of a certain element to add to your soil. It is up to you to calculate the actual pounds of fertilizer to apply to achieve that concentration of a specific element. Remember that a 100-pound bag of 16-16-16 fertilizer actually contains only 16 pounds of nitrogen, 16 pounds of phosphorous and 16 pounds of potassium; the rest is inert ingredients. If your soil test suggests adding forty (40) pounds of nitrogen per acre, you will actually have to apply 250 lbs of the 16-16-16 fertilizer per acre to get to the desired level of nitrogen. You must also remember to calculate your actual fertilizer needs based on your acreage.
Let’s do some examples:
Example 1: Your irrigated pasture has been tested and it is suggested that you fertilize at a rate of 140 pound of nitrogen per acre. You are using Ammonium Sulfate, a 16-20-0-15 fertilizer. A 100-pound bag of this fertilizer will contain 16 pounds of nitrogen. How many pounds of this type of fertilizer should you apply, per acre?
140 pounds divided by 16 = 8.75 bags. Each bag is 100 pounds, so the grand total is 875 pounds of 16-20-0-15 fertilizer per acre to actually apply 140 pounds of nitrogen per acre.
What does this cost? If Ammonium Sulfate sells for about $12 per 100 pound bag, the fertilizer cost per acre would be: 8.75 bags X $12 per 100-pound bag = $105.00 per acre
Now the question is, do you need the phosphorous and sulfur? Is there a cheaper fertilizer? These questions will take some research on your part to see what is available in your area.
Example 2: Your dry land pasture has been soil tested and the recommendation from the testing lab is to add 40 pounds of nitrogen per acre. You would like to do this as cheaply as possible. You have access to two (2) different fertilizers:
Ammonium Sulfate 16-20-0-15 cost = $12.00 per 100 pounds
Ammonium Nitrate 28-14-0 cost = $17.50 per 100 pounds
Which is cheaper to use?
Ammonium Sulfate: 40 pounds nitrogen required divided by 16 pounds of nitrogen per 100-pound bag = 2.5 bags per acre or 250 pounds of Ammonium Sulfate per acre. 2.5 bags at $12.00 per bag = $30 per acre
Ammonium Nitrate: 40 pounds of nitrogen required divided by 28 pounds of nitrogen per 100 pound bag = 1.4 bags per acre or 140 pounds per acre. 1.4 bags at $17.50 per bag = $24.50 per acre
If there is no need to add sulfur to your soil, the Ammonium Nitrate would be a cheaper fertilizer.
PLEASE remember that cost is not the only factor to consider in deciding on a fertilizer. Generally, the less expensive fertilizers are the quickest to dissolve, some of which will flush away with water movement through your soil. Slow release fertilizers are exactly what the name implies: they release the nutrients slowly due to a chemical encapsulation that inhibits rapid dissolution. But, of course, you pay more for these types of products because they cost more to produce. They are applied less frequently and reduce pollution by reducing nutrient runoff. It is a matter of weighing all the factors involved and deciding on a fertilizer that best meets all your needs, goals and objectives.
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