by Keith Baldwin, CFSA’s Farm Services Coordinator

If you use the NCDA&CS Soil Test as a guide for nutrient application rates, it’s a good thing to know that the lab does not test your soil sample for N content. Instead, the report sent to you makes a “standard” pre-plant N application rate recommendation for the crop you’re going to grow. Usually, the amount that the report recommends is 50 lbs/acre.  Or, if you’re a small grower, it will recommend (the roughly equivalent) rate of one pound of N per 1,000 square feet.  Importantly, you have to do the math yourself to figure how much of fertilizer will supply the rate that the report recommends (or whether to even use their recommended rate).

 

 

Here’s How to Do the Math:

 

EXAMPLE ONE: Fish Emulsion

 

Let’s say you’ve done your research and determined that fish emulsion is the way to go for your crop.  Say you’re putting the stuff out as a side-dressing to a growing crop.  You’ve found Alaska Fish Emulsion 5-1-1 fertilizer from an online vendor for $20/gallon.

 

All the “books” say: 6 weeks after planting, apply 20 pounds (lbs) of nitrogen (N) per acre.  Quick, what’s the right application rate of the fish emulsion?

 

First question: Which one of the numbers is N?  Answer: The first number (5) is the percent N in the mix.

 

Next question: How much do you need to get to 20 lbs of Nitrogen per acre?

 

As a rule of thumb, one gallon of liquid fertilizer weighs 10 pounds. Five percent of 10 lbs (0.05 times 10) is 0.5 pounds of N.   Hmm, let’s see.  At 0.5 pounds N per jug, to get 20 pounds N out per acre, you’re going to need 40 jugs (20 / 0.5) ! Wow! At $20 per gallon that’s $800!

 

On the other hand, the product description recommends two tablespoons (in one gallon of water) for every 25 square feet of soil. At that rate, how much N are you providing to the crop?

 

Well, two tablespoons is one ounce. So, 16 ounces (one pound) covers 16 X 25 square feet (400 square feet).   Then, 10 lbs (one gallon) will cover 4,000 (10 X 400) square feet, about 0.1 acre.

 

Answer: a whole acre would take 10 gallons.

 

That’s certainly going to cost you less: 10 jugs instead of 40.   But, the crop only gets a quarter of the N, or about 5 lbs of N per acre as opposed to 20. Your cost would be $200 at the lower rate.

 

The long and short of it is: No matter what the rate, can you afford to pay $40 a pound for N?

 

EXAMPLE TWO: Dry Fertilizer
We’ve already seen that a recommended rate of 20 lbs per acre calls for 40 jugs of fish emulsion.   Fifty lbs/acre would add up to 100 jugs of fish per acre ($2,000)!  No one is going to do that. Most farmers are probably going to use some kind of dry fertilizer material to put out 50 pounds of N.   But how much dry fertilizer do you need to supply 50 lbs of N?

Let’s use two different fertilizers and a very simple (and handy dandy) calculating tool. Feather meal is 13-0-0. Harmony fertilizer is 5-2-2. They have 13% and 5% N, respectively. The calculator works like this:

Take the recommended application rate.   Multiply it by 100.   Divide by the % N.

For example, 50 lbs/acre (recommended) X 100 = 5,000.

ü  5000 divided by 13% (feather meal) = 385 lbs (about eight 50-pound bags) of feather meal.

ü  5000 divided by 5% (Harmony) = 1,000 lbs (twenty 50-pound bags) of Harmony.

The same calculation can be used for any other dry N fertilizer, as well as for calculating P and K application rates (for P and K, you divide by the percent P or percent K, the second and third numbers on the bag, respectively).

By the way, at those rates the feather meal ($30 for a 50 lb bag) would cost you $240 (Feed and Seed in Pittsboro) and the Harmony ($18 for a 50-lb bag) would cost you about $360 (on-line).