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Adding Nitrogen to Finished Compost


January 30, 2016

I have a question about adding Nitrogen to finished compost. If I wanted to have a 5:1:1 (50) to a 100 pound bag of compost.  Meaning the bag contains 50% of added fertilizer and 50 percent of compost.. how do I calculate? How many  pounds of Nitrogen, P and K to add to the bag? I am going to use Urea 46 as the nitrogen component.


Answer by JH: This is a math question. 5:1:1 in pounds is 5 lb:1 lb:1 lb which equals 7 lb. Multiply it by 14 and you get 98 lb. But urea 46 is 46:0:0 and John had advised that finished compost averages 1:1:1. Again, you do the math to get to 50:1:1.

Be aware that urea 46 must be stored dry and changes chemically when added to soil. If not administered properly it can actually deplete the N in the soil. It is most effective as a fertilizer when watered into the soil or dissolved in water and used as a foliar spray. It also must be applied evenly particularly on seeds. So I'm wondering what is the intended use of this product you are making. Such a high nitrogen content calls to mind a green, bushy plant that doesn't flower. If that's not the type of plant you want, your question is best answered from a gardening point of view. But generally speaking since you're using urea 46, it seems you'd be better off amending the soil with compost first and then adding the desired amounts of N, P and K.

Does this address your issue?

Addendum by JH: So sorry, I am just re-reading my answer to you and realizing in the first paragraph that the 50:1:1 may read like a typo but is actually an error in my thinking and the reason for my comment about high N and a bushy plant with no flowers so just ignore that whole first paragraph. So much for responding before coffee. I apologize for this error.

Aside from that, knowing what plants you are growing would help. I  But I believe your question is more about how to achieve the 5:1:1 ratio with urea 46 and compost.

To be more clear, I would eliminate compost in the calculation. It is a soil amendment to add organic matter and should be worked into soil prior to planting or otherwise used as a top dressing or mulch. So I'm reading your question as how to take the urea 46, a 46:0:0 N source and add it to P and K to achieve a 5:1:1 fertilizer, and as previously stated, that's just doing the math.

But if you are amending the soil with sufficient compost, there really shouldn't be a need for any fertilizer. So I recommend being sure that the amount of compost is adequate to add 3-4" of it to the soil and gently turn it in to the top few inches. If you are planting in a planter or raised bed, you could even use 100% compost. In either case, I'd question the need for any fertilizer (again functioning blindly here as to what is being planted and what it's environment is).

The primary reason for soil amending with compost is to improve the soil biology - repopulating microorganisms, improving water holding capacity and soil tilth. Do keep in mind the earlier comments about problems with storing and applying urea 46 - that was not a typo or other error.

Is this at all helpful? Perhaps other of my colleagues will respond with more pointed information.

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