%Nitrogen in soil test results

[Brian]

The Mississippi State soil lab has recently added OM and nitrogen testing, with Nitrogen listed as a percentage. For example, my test results had a hand written notation "N - 0.20%." All of the other entries (P, K, Mg, CA, etc.) are listed in lbs/ac of extractable nutrients.

I'm hoping that someone can help me understand what "%N" represents … or at least what 0.20% N tells me about my fertilization needs for my fall plots!

 

[X-farmerdan]

And what was your organic matter percentage?

 

[Brian]

1.7% and 2.2% OM in plots with 0.20% N. Another plot with sandy silt soil had only 1% OM but 0.29%N

 

[X-farmerdan]

I guess you should ask the lab. There are methods to estimate the amount of nitrogen that is or will be available in you soil based mostly on available organic matter. Those estimates have been notoriously unreliable because that nitrogen measure is changing in the sample every moment. There may have been some breakthrough that makes the test more reliable today than what I remember it to be. But, a 2% level of N is way beyond my interpretation and understanding.

 

[Brian]

Lemme back up here... that pesky decimal got me in trouble. The N readings were 0.20% and 0.29%.

 

[X-farmerdan]

Well, I don't know. I looked for a stock who's symbol was N and that had a rate of return of two-tenths-of-one-percent. I couldn't find that either.

Here's what your lab has to say:
http://extension.msstate.edu/agriculture/crops/soils/nitrogen-fertility
"Small quantities of soil N are provided by residue from plants that do not fix atmospheric N. Organic matter in Mississippi soils typically ranges from 0.5 to 2.0 percent by weight of the upper six inches. Typically organic matter is approximately 5 percent N, so total N in the topsoil ranges from 500 lb/acre to 2000 lb/acre. However, only a very small portion of the total N is available to plants within a growing season. Organic matter is replenished by returning crop residues to the soil or introducing other organic sources such as manures or animal bedding."

Let's work through that. For ease let's assume your OM content was 2%. For rough calculation, the top six inches of soil weighs 2-million pounds. If 2% of that is OM, then you have 40,000 lbs of it - organic matter. Relying on the information above, then, 5% of the 40,000 lbs is N or 2,000 pounds in a million. Divide 2,000 by one million and you get back to 0.20%. See how I conveniently worked back to the answer you wanted? Just lose a million pounds of soil. I don't get it.....

My guess is the 0.2% is the percentage of potential N derived from OM in the soil plow layer, not all of which is available. How that's helpful I do not know.

At the end of the day I would pretend I didn't see that N number.

 

[Brian]

I followed up with the soil lab and received this response:

Hi Brian,
The %N numbers represent the total nitrogen present in the soil at the time it was sampled. Nitrogen rates change throughout the growing season, so the number you have represents a snapshot of nitrogen amounts in your fields.

Because nitrogen amounts are so variable, we do not use %N to determine your recommended nitrogen rates. For example, for your samples we recommend 30 pounds of nitrogen per acre. That recommendation is based on research – we know how much nitrogen is required for a healthy stand of crops typically used in a food plot.

%N of samples are useful to farmers/researchers interested in analyzing trends from one year to the next. They are less useful to landowners just trying to achieve a great food plot stand.

Let me know if you have further questions.

Keri Jones │Lab Manager
MSU Soil Testing Lab

Bottom line, it looks like the soil lab agrees with x-farmerdan's recommendation that I pretend I didn't see that N number!

 

[Drycreek]

X has got it going on !