NPK of compost: comparison with chemical fertilizer

[Dchall_San_Antonio]

Someone just wrote to me off the list with the following question. I thought it would be helpful to share my reply with the lawn forum readers who might also have the same question.

Quote: I see the chemical fertilizer bags at Home Depot. They have numbers for the nitrogen, phosphorus, and potassium like 12-3-12 ect.... How can you compare that to the chemical tests of my compost?
Nitrogen .403
Phosphorus .230
Potassium .310

And I replied/ranted...
The numbers on the fertilizer bags are expressed percentages of the different chemicals in the bag. So for a 12-3-12 fertilizer, it would contain 12% nitrogen, 3% phosphorus, and 12% potassium. That adds up to 27%, so who knows what else they are using as a filler in the bag to get up to 100%? Assuming your chemisty test expresses the results in %, then your compost is a 0.40-0.23-0.31 fertilizer.

That means if you apply 100 pounds of compost to a spot, you will get 0.40 pounds of nitrogen, etc. over that area. In order to get 1 pound of N, you will need 250 pounds of compost. So if you wanted to apply 1 pound of nitrogen per 1,000 square feet (which I find to be a common application rate in the chemical world), you would need to apply 250 pounds of compost.

When I recommend using compost on turf, I always recommend using 1 cubic yard per 1,000 square feet. One cubic yard of finished compost weighs on the order of 500 pounds or so. So applying 1 cubic yard of compost would add up to two times as much N as a turfgrass manager might apply if he were applying a chemical fertilizer. Fortunately finished compost cannot burn turfgrass, so the "extra" nitrogen is not harmful. But does this tell the whole story on the differences between compost and fertilizers? Not hardly.

Compost provides valuable beneficial microbes to the soil underneath it. Here's a list of things an organic program can do that no chemical can do. The beneficial microbes from compost can do all the following:

1. Decompose plant residues and manure to humus.
2. Retain nutrients in humus.
3. Combine nitrogen and carbon to prevent nutrient loss.
4. Suppress disease.
5. Produce plant growth regulators.
6. Develop soil structure, tilth, and water penetration/retention.
7. Clean up chemical residues.
8. Shift soil pH to neutral and keep it there.
9. Search out and retrieve nutrients in distant parts of the soil.
10. Decompose thatch and keep it from returning.
11. Control nitrogen supply to the plants according to need.
12. Pull minerals out of inorganic soil components for plants.
13. Provide the exact chemical nutrients to the plant that the plant has evolved with rather than man's cheapest chemical approximation.
14. Provide exactly the required quantity of nutrients that the plant needs.
15. Provide the nutrients at exactly the right time that the plant needs them.

No chemical can do any of that. To be fair, no single microbe can do all of that either. In fact, it could be that it takes 10 different species, one working right after the other, to do any one item in the above list - sort of like a microbiological assembly line. But at least it's real easy to get all the right microbes. The biology of the soil is very complicated.
At the same time, many chemicals inhibit the microbe's natural abilities to do these things. Herbicides, fungicides, and pesticides are all designed to kill various biological life. As a byproduct, they often kill off the beneficial microbes that are doing 1 through 15 above. Any break in the assembly line can interrupt the process, damage the mini ecosystem, and lessen the benefit of the organic methods.

Sorry for ranting. I got on a roll there and couldn't stop :roll:

[Gar]

Dave,
Thanks alot, very informative. I never looked at it as a 'microbiological assembly line'. But you are right. That is what it is.

[Billusa99]

And this is what I simply say to my "chemical friends" about their fertilizers which are breaking down into all those varied chemical salts in their soil.

I'll buy you a goldfish and a bowl. Everyday I'm going to come over and mix 1 tsp of table salt in the fish's water. How long do you think the fish will live? How long do you think the beneficial critters in your lawn/soil will live??

[CaptainCompostAL]

I was just reading a soil book, borrowed from a college library published by the "Acres USA" magazine company. It says that the cell wall membranes in the beneficial soil aerobic bacteria and fungi, have a NPK rating equivalent to a 10-5-2 synthetic fertilizer in the soil interfacing with the plants' roots!

That is why don't be fooled by the seeming low NPK ratings of natural fertilizers/soil amendments like homemade compost (1-1-1 to 4-4-4), fish emulsion (5-1-1), seaweed tea (0-0-1), etc. It's all in the microbes that really makes the real difference in how our organic plants get fertilized.

Happy Gardening!

[Dchall_San_Antonio]

Captain,
Thanks a TON for that tidbit :D

THUS, it has been said, the organic matter that matters is the living microbes and not the dead leaves that we think of as organic matter. If you feed the microbes with protein and sugar (from the plants' roots), then, when the microbes die, they fertilize the soil, too. And their offspring fertilize the soil, and their offspring...