4

I have gone through couple of similar questions in this forum but did not quite understand on how to interpret and calculate the required amount of fertilizer and type of fertilizer to choose from.

So, here is the screenshot of the fertilizer recommendation per 1000 sq.ft. soil test recommendation

I went through this post in order to understand and calculate the quantity that I would require but it is given based on different NPK number.

So my questions are as follows:

  1. How do I identify which NPK number to choose from different combinations available? I mean how do I determine whether should I choose 13-5-8 or 11-11-11 or some number?
  2. How will I calculate how much quantity do I require once a fertilizer based on the NPK number is selected for question #1?

I would appreciate very much for any guidance on this.

Thanks very much.

1
  • 1
    What on earth is "surplus pH" supposed to mean?
    – alephzero
    Aug 14, 2021 at 3:05

2 Answers 2

2

First, the recommendation is specific to target crops, so confirm that you're after a lawn (perennial ryegrass) and not veggies or shrubs. Some soil tests will give multiple recommendations depending on the target plant.

Second, you look at the proportion. There's a range for nitrogen, so to make it easier, I'll go with a middle number. You're looking for something with an NPK ratio of 4-3-0.75, or 8-6-1.5, or 16-12-3. Basically, something with the last number smallest, and the middle number about 4 times bigger, and the first number a little bigger than the middle number or about 5 times bigger than the last.

You're probably not going to find anything exactly matching. But that's ok. The easy way is to mix fertilizers that have only one number with the rest at 0...but you shouldn't mix chemicals you're not familiar with...so what you can do is apply at different times.

For this example, I'll use a 10-0-2 (typical of lawn fertilizers, and first number 5 times the last) and super phosphate 0-18-0.

Third, check how much space you have relative to the recommendation and cut down accordingly. This is based on 1000 square feet. For my example, I'll say I have a 270 square feet yard (30ft x 9ft). That means my yard is (270/1000 = 27% = 0.27x) the given area.

We take the npk and multiply by the smaller size. This means the 4-3-0.75 recommendation is 1.08-0.81-0.20. I'll say 1-0.8-0.2 to make the math easier. For my lawn, the recommendation is to add 1 pound of N, 0.8 pounds of P and 0.2 pounds of K.

Starting with the 10-0-2 fertilizer. We'll need 10 pounds. Because 10% of 10 pounds is 1 pound N, and then 2% of 10 pounds is 0.2 pounds. So this gives exactly what we need for N and K.

Then with the 0-18-0. Here's the algebra: I want 0.8 pounds of P, and have a fertilizer that's 18% (0.18) P by weight, with X being how many pounds I need of my fertilizer:

0.18X = 0.8

X = 0.8/0.18

X = 4.444...

So I need 4.44 pounds of the 0-18-0 super phosphate.

That phosphorus number is higher than I would expect for a lawn. I wouldn't reapply that again until another year and another test.

I hope the math makes sense.

PS. The recommendation also recommended application of sulfur (probably to decrease excess pH). It doesn't specify an amount. However, the phosphate application will also decrease pH so if you do apply sulfur, do so sparingly.

4
  • 1
    Are you reading the "APP - S" as "apply sulphur"? It might just as well mean "solid" (as opposed to "liquid") fertilizer.
    – alephzero
    Aug 14, 2021 at 13:16
  • Pretty sure it's about sulfur--though it's odd they didn't state how much. They're recommending zero liming and sulfur would be inverse of that. These soil test recommendations usually don't care whether you apply it in liquid or solid form. The extra boxes typically include the secondaries: Ca/S/Mg, and sometimes trace at extra cost.
    – Josh
    Aug 14, 2021 at 15:11
  • Thanks @Josh for your answer. You mentioned to "..apply at different times". I believe it means that I have to apply NK at one time and P at another time. How much gap should there be between these two applications? Also, how many times do I need to repeat it for NK?
    – Uresh K
    Aug 16, 2021 at 14:26
  • 1
    Yes, superphosphate should actually be compatible with most of the standard formulations, but if you want to be safe, give it a bit of extra time in between. A couple weeks should be plenty. As for how often to repeat, it'll be based on the timeline of the recommendation. It might say it in the extra details. Usually they're for a growing season, so you might be ok just doing that one application for the year. If you have extra of the NK, save it for the next year and apply it based on the dosage of the fertilizer label.
    – Josh
    Aug 16, 2021 at 21:21
5

Presumably the numbers in "pounds per 1000 sq ft" are intended for "single nutrient" or "straight" fertilizers which contain only one of N, P, or K. These are commonly used commercially because they are more concentrated than the "NPK" mixes sold to non professional growers, and can be blended in any combination as required.

On the other hand straight fertilizers may be hazardous to handle (for example some straight nitrogen fertilizers are explosives) and applying them at the wrong rate may cause severe damage to plants.

(This is what happens when you don't treat ammonium nitrate fertilizer with the respect it deserves ... https://youtu.be/tN69l78_Crc)

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.