Fertilizer calculations, (if you're into that kinda thing)

[ChinaVoodoo]

I just realized I was wrong in my thinking.
From Wikipedia: The first number represents the percentage of nitrogen in the product; the second number, P2O5; the third, K2O. Fertilizers do not actually contain P2O5 or K2O, but the system is a conventional shorthand for the amount of the phosphorus (P) or potassium (K) in a fertilizer

So there's no need to calculate actual potassium and phosphate. It's actually the equivalent of phosphorus pentoxide and potassium oxide which npk represents.

 

[Yultanman]

And a liquid:

 

[Yultanman]

But when calculating how much is removed from the soil you do need to account for the actual element removal not how its expressed on packaging which was what i wanted to convey

 

[Yultanman]

But when calculating how much is removed from the soil you do need to account for the actual element removal not how its expressed on packaging which was what i wanted to convey

i think! As my current understanding is lol

 

[ChinaVoodoo]

So my box of K2SO4 is apparently equivalent to 40% potassium oxide.

 

[ChinaVoodoo]

But when calculating how much is removed from the soil you do need to account for the actual element removal not how its expressed on packaging which was what i wanted to convey

That's next level. I don't know how I could calculate removal. There's soil compactness, runoff, rain amount, temperature, growth of plants...

 

[Yultanman]

I think you have more science understanding than me lol....

These numbers you posted are called removal numbers. Ex tobacco removes 50-80lbs per acre

thos variables you mentioned are why it is a range 50-80

There are a lot of different fertilizer recommendations out there, and they are often dependent on regional soil composition and soil test results. The purpose of this thread is to explain how you take these recommendations and figure them into how much to actually apply.

I will be taking recommendations from Virginia Tech's 2019 report for flue cured tobacco. FYI, the nitrogen rates recommended for burley are significantly higher.

Their recommendations are as follows:
Nitrogen 50-80lbs/acre depending on soil depth
Phosphorous 40-230lbs/acre depending on soil test
Potassium 100-175lbs/acre depending on soil test

We will go with 80, 80, & 175 (respectively) for all of these example calculations.
This is represented as 80-80-175 lbs/acre

"but my garden is not that big!"
Yes. So let's switch to a combination of metric and imperial. This is how it makes sense to me: 1lb/acre = 1.04grams per 100 square feet.

This means that our recommendation for lbs per acre is basically the same as our recommendation for grams per 100square feet. So for every 100square feet of garden, you need 80g N, 80g P, & 175g K.

Calculations:

Let's say you have a pure N fertilizer, something like 60-0-0.

Only 60% of the weight of the fertilizer is N. Therefore if you need 80g of N, it makes sense that you would need more than 80g of the fertilizer.

The calculation is 80/0.60 = 133g.
So 133g of the 60-0-0 will give you 80g N and you will apply that amount to every 100square feet of your garden.

Let's say you have a fertilizer which is not purely one chemical.
Let's say it's 20-20-40.

So let's calculate how much fertilizer is required to get the amount of N we need.
80/0.20 = 400g per 100square feet

.

 

[ChinaVoodoo]

And a liquid:
View attachment 36072View attachment 36073

Well thanks for clarifying. I thought you were saying that the npk numbers were inaccurate in regards to how much to use. And you revealed to me that PK doesn't represent phosphorus and potassium, but phosphate and potash. I was mistaken.

 

[ChinaVoodoo]

I think you have more science understanding than me lol....

These numbers you posted are called removal numbers. Ex tobacco removes 50-80lbs per acre

thos variables you mentioned are why it is a range 50-80

Maybe it's semantics but I understood it as recommendation regarding how much to use depending on various conditions.

 

[Yultanman]

Well in this case this study represents the removal as k2o5 In which case your original math ahould be accurate but here is the removal reference :

“
Phosphorous Phosphorus is not very leachable, even in sandy soils, and a good tobacco crop only removes about 15 pounds per acre (as P205).

“

 

[ChinaVoodoo]

The recommendations are for phosphate and potash.

 

[Yultanman]

As an aside fyi I calculated the n removal for 100sq ft for manure and i takes about 1.8kg per 100sq ft to orovide the required N for tobacco

 

[ChinaVoodoo]

As an aside fyi I calculated the n removal for 100sq ft for manure and i takes about 1.8kg per 100sq ft to orovide the required N for tobacco

What percent nitrogen is this manure?

 

[Yultanman]

Something is weird with those numbers...

50-80 of N
Then
230 of P
And 150-175 of k

??? That sounds like a fruiting formula. I must be missing some context of that page

i see something like:

50-80N 15-20P 90K

 

[Yultanman]

What percent nitrogen is this manure?

1-1-1

Multi Fert. Rate Calculator

Fertilizer Rate Calculator-calculate the optimal amount of given fertilizer(s) for a given recommendation rate

www.rs.uky.edu

 

[ChinaVoodoo]

Something is weird with those numbers...

50-80 of N
Then
230 of P
And 150-175 of k

??? That sounds like a fruiting formula. I must be missing some context of that page

i see something like:

50-80N 15-20P 90K

230 is the charging rate if it's initially low. Subsequent years based on soil test, you should be between 40 and 100 P2O5 for flue cured tobacco grown in VA.

 

[ChinaVoodoo]

1-1-1

Multi Fert. Rate Calculator

Fertilizer Rate Calculator-calculate the optimal amount of given fertilizer(s) for a given recommendation rate

www.rs.uky.edu

Edit: mistakes were made - ignore this post. I left it up to show that calculations should be done with a calculator.
Well then we can do those calculations in our head. That equals around 170lbs of Nitrogen per acre which is about right if we are aiming for 100 and have deep soil and a high amount of rain.

 

[Yultanman]

Well then we can do those calculations in our head. That equals around 170lbs of Nitrogen per acre which is about right if we are aiming for 100 and have deep soil and a high amount of rain.

View attachment 36075

View attachment 36076

haha you can those in your head!!!
im in over my head!
this page you just posted was where i got alot of my info from as well

 

[ChinaVoodoo]

Nono no.

1.8kg is only 18lbs per acre. That's actually quite low.

I'm going to have a nap. I shouldn't be giving erroneous information.

 

[ChinaVoodoo]

So. According to the charts, we should have in medium rich soil that's two feet deep with little rain, 70-80, 60-100, & 100-150.

To get 80 N with 1-1-1, you would need 80/0.01= 8000lbs/acre = 8320g/100 square feet.