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Today I received the results for my garden soil test, which show that I have only 5 ppm of nitrogen.

So my research (here, and elsewhere) has suggested that the best way to (organically) raise the nitrogen level is to add some blood meal. But how much blood meal do I need? The "Fertilizer recommendations" that came with my test results says:

You should apply your nitrogen at a rate of ¼ lb N per 100 sq. ft. You can divide this amount in half and apply half at planting and half a month later during the summer.

Then later it says:

To Calculate the Amount of Fertilizer to Use:
( % of nutrient in fertilizer ÷ 100) = Pounds fertilizer per 100 sq. ft.

But using that formula, if I had a theoretical fertilizer with 100% nitrogen, I would apply 1 lb per 100 sq. ft., or 4x the recommended amount above.

What is an accurate formula to use to determine how much (organic) nitrogen fertilizer I need to apply?

EDIT I live in Central Kansas, and will be growing a variety of vegetables.

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it depends what you are growing! Corn requires a lot of nitrogen, leaf vegetables not so much. Do you expect to take one crop per year or multiple crops? Where are you located and what type of soil do you have? – kevinsky Feb 19 '12 at 16:23
@kevinsky: I live in central Kansas, and will be growing a variety of vegetables. What do you mean by soil type? Clay vs. sand, etc? The soil is more clayey than sandy. I'm not sure how to quantify that. – Flimzy Feb 19 '12 at 16:55
@kevinsky, leaf vegetables need more nitrogen than most others. – J. Musser Feb 20 '12 at 2:06
up vote 6 down vote accepted

The NPK numbers on the fertilizer represents the percent, by weight, of Nitrogen, P2O5 and K2O, respectively. So if your blood meal is labeled 12-0-0, then a 100 pound bag would contain 12 pounds of nitrogen.

If you have a 400 sq ft garden, you'd want to apply a pound of nitrogen, or 100/12 = 8.3 lbs of 12-0-0 fertilizer.

Note that the calculation for phosphorus and potassium is a little trickier, since the weights in the NPK number are the amount of phosphate and potash and not the pure element. See this answer for details.

@kevinsky's comment about varying requirements for different vegetables is a good point. You probably shouldn't make a blanket application over your entire garden. Check a good gardening reference for the relative amounts of nitrogen required by various plants and add more for plants that want it and less for plants that don't. Squash and corn, in particular, do well with extra. But if you give extra nitrogen to your tomatoes then you'll end up with a lot of foliage and not much fruit.

Also keep in mind that you can reduce your long-term need for nitrogen amendments somewhat by increasing the organic matter content in your soil. The organic matter acts like a sponge, absorbing nitrogen and releasing it for plants gradually over time.

Lastly, be aware the nitrogen is volatile -- if you add the full amount at the beginning of the planting season, you may lose it into the air or it may wash away into the soil. Soil type is an important factor because sandy soils will tend to lose nutrients faster than clay soils. It's definitely a better idea to add half the amount at the start and then side-dress a month or two later.

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Thanks for the info, but how do I know that I want 1 pound of nitrogen for that area? Is there a formula to tell me that if my current nitrogen level is A, and my target level is B, that I need to add X pounds? Or is it just a rule to always add 1 pound per 400 sq ft? – Flimzy Feb 19 '12 at 22:05
@Flimzy: You're looking for a formula to get from 5ppm to X pounds of nitrogen? I don't know of a formula; I'd recommend a table like the one about halfway down this page. In your case, assuming >2% organic matter, the 5ppm level will give you 3.3#/1000 sq ft, or 0.33#/100 sq ft, which is not far off from the 1/4# recommendation you got in your soil test. I'm not a soils expert, but I think different methods give different measurements so it's best to follow the advice given in the test results. – bstpierre Feb 19 '12 at 22:45
+1 for @bstpierre for a complete answer! – kevinsky Feb 20 '12 at 1:56
@bstpierre: Thanks for the link. The problem with following the advice I got with my soil test is that it provides conflicting advice. It says 1/4 lb in one place, and 1 lb (by way of expanding their formula) in another. – Flimzy Feb 20 '12 at 4:42
@Flimzy: I'd trust the 1/4 lb. The formula doesn't make sense: I don't see how the "per 100 sq ft" part belongs at the end. – bstpierre Feb 20 '12 at 12:25

Why not start with some beans or other legumes? These will help put some nitrogen into your soil, and you'll get a crop as well.

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If you can get over the "ick factor," you have an easily-applied source of organic nitrogen fertilizer close at hand. It costs nothing, and you should be able to get several litres of it per day with little inconvenience -- your urine!

Unlike humanure, urine should be sterile, so you don't need to go through long composting nor testing for pathogens. It has a nominal rating of 11-1-1 or so, so together with the other answers posted, you can easily calculate how much you need.

I recommend diluting 10:1 or so, since urine is so "hot" with nitrogen that it will "burn" some plants if applied directly. Put it in a pump sprayer, and fertigate away!

A simple plastic container next to your toilet will suffice. A well-fitting lid is essential. I recommend the square plastic one-litre containers that organic plain yoghurt comes in. When you fill it, put it in your ten-litre pump sprayer, top off with water, and you're set.

Using urine for fertigation helps close the "nutrient cycle" that our "modern" sewage systems have broken, neatly carving one natural solution into two problems.

The only disadvantage (besides the mentioned "ick factor") is the odour, which is considerably attenuated when you dilute it. But besides, have you smelled other organic fertilizers? They all stink! So tell yourself you're saving the planet, and just get over the smell and "ick factor."

In general, urine fertigation does great for starts, leafy greens, and other things that produce leaves. Root crops are going to want more potassium than you can get from urine, and seed/flower crops are going to want more phosphorous.

For leafy greens, apply at the base of the plant, rather than foliar, to further reduce the "ick factor." (You do wash your greens before eating, right?)

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+1 for unconventional, natural, and free solutions. How long have you been doing this? :) (Although this doesn't actually answer my question of how much) – Flimzy Jun 12 '12 at 1:13
Hi Flimzy. I've been doing it informally for decades. This year, we've done some semi-formal greenhouse trials, and the results were impressive enough that we've built a 90 gallon fertigation system that can be valved into the greenhouse watering system. (Kids, don't try this at home unless you know what a "check valve" is, or you may end up with urine in your drinking water -- double ick.) – Jan Steinman Jun 13 '12 at 0:47
As for "not answering your question," I apologize -- I didn't want to repeat what others had written. You say you need "¼ pound per 100 sqft." In round numbers, urine weighs 8 pounds per gallon and has about 10% nitrogen, so a quart would have nearly ¼ pound of nitrogen. Dilute 9:1 and use ten quarts of the diluted solution. – Jan Steinman Jun 13 '12 at 0:51

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