Basic Fertilizer Math

I have been asked to recommend a good fertilizer for late fall (for example) in New England . I suggested a product 32-3-6 with 30%CRN 2%FE . I was told “I don’t want to use that high of a number in the Fall”

What he was referring to is the 32 . The nitrogen. I then realized how many people don’t understand some basic fertilizer math . I know it because I use it daily, but others may not. Some may be basic, so bear with me if you know this stuff. So, here goes….

32-3-6 what’s that? Those 3 numbers on a fertilizer bag represent N-P-K.
Nitrogen , Phosphorous, and Potassium. N-P-K …

Each number represents the percentage of the contents of the bag.

N = 32% of the contents
P = 3% of the contents
K= 6% of the content

The rest is micro nutrients and fillers. From here on out I will use N,P & K to represent each nutrient instead of typing the whole word

In this instance it will be a 50 lb bag of 32-3-6 straight fertilizer containing:
.32 x50 =16 # N
.03 x50 = 1.5 # P
.06 x50 = 3 # K

Most commercial fertilizers and retail products have a setting for your spreader on the bag. Some will break it down for different #’s of N, but most are derived from 1# N per 1000 sq ft. That’s what most everyone is trying to achieve .

In this instance at 1#N per 1000 sq ft and 16 #s of total N in the entire bag, you will get about 16,000 sq ft from this product at the recommended rate which is usually defaulted to 1 # N per 1000 sq ft.

Most cool season grasses require about 4 lbs of N annually per 1000 sq ft. Never put it down at once!! That’s why you fertilize 4-5 times a year to replace the N that the plant uses in abundance. At about 1 lb of N per 1000 sq ft each time you fertilize, with 4 -5 fertilizer applications ,you get your annual requirement of 4lbs.

Here is the key to the whole story. This customer walked out with 20-1-5 instead of the 32-3-6. Nothing wrong with that at all, because the 20-1-5 is a fine product.

He asked how far it goes . I said 10,000 sq ft . He bought it.

The part that most people don’t realize is the recommended setting on the bag of the 20-1-5 fertilizer for his spreader was derived from 1lb N per 1000 sq ft ,so he gets 10,000 sq ft from that 50# bag.

The 32-3-6 goes down at the same 1lb N per 1000 (the exact same rate) , but he will get 16,000 sq ft from the same 50# bag as opposed to 10,000.
He didn’t want a “high number”, but is getting the same amount of Nitrogen on the lawn. As a matter of fact , the 32-3-6 was probably a better deal because what he was getting was 16,000 for a certain price, or 10,000 sq ft for a little lower price.

Combination products are a bit different. These are weed & feeds, crabgrass preventers with fertilizer, etc. Those recommended settings or rates are based on the amount of whatever active ingredient is on the fertilizer, like weed control or insecticides. In this instance , delivering the right amount of active ingredient per 1000 sq ft is more important than lbs of N per K.
Most combo products usually put down less than a lb of N per 1000 sq ft because of this. Usually 3/4 -1 lb per 1000 . This is where they get a recommended setting for these products in stead of with a straight fertilizer.

Retail products are a bit different from what I see. A bag will have 26-3-9 on it, and the bag says “covers 15,000 sq ft” . Any manufacturer can label whatever they want on a bag for coverage, but in this case if this fert is in a 50 lb bag , you probably should get about 13,000 sq ft instead of the 15,000 it claims.

One last nifty tip. As long as you are looking a 50 lb bag of fertilizer (most professional fertilizers are sold this way), you can simply take the N content (we will use 32 here ) and divide it in half for 16 . You get 16,000 sq ft at 1# N per 1000. It must be a 50lb , and it must be straight fertilizer .

If you can grasp these basic concepts, you understand more than most, even some lawncare folks


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