How much fertilizer do I need for my vegetable garden?

Question

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.

Answer 1

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.

Answer 2

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.