What the Heck is N-P-K and How do I Get Enough?


Ah, fall. Turning leaves, turning temperatures, turning garden soil. This time of year the gardener's mind turns to plowing everything under after the harvest. It is the perfect time of year to add amendments to your soil. Before you do this, however, you should have the soil tested so that you are enriching your soil properly. Any county cooperative extension service can tell you where to get a soil test performed and most offer the service themselves at nominal fees.

So, let's say you did the wise thing months ago and sent off a soil sample for testing. Your soil results indicate you need to add 30 lbs of phosphorus (P).

The numbers on a fertilizer bag are the percentages of nitrogen (N), phosphorus (P), and potassium (K) in the bag. The bigger the number, the higher the percentage of that nutrient. A 100-lb bag of 10-10-10 contains 10lbs each of nitrogen, phosphorus, and potassium. The other 70 lbs is filler used to stabilize the nutrients and make application easier.

Going back to your soil test, you will need 300 lbs of 10-10-10 to get 30 lbs of actual P. You will also get 30 lbs each of N & K you don't need and 210 lbs of filler. OHHHH, my aching back. Why not buy a fertilizer high in the requisite nutrient?

To calculate the amount of fertilizer needed, divide 100 by the percentage of nutrient on the bag, and then multiply the result by the amount of nutrient you want. For example, how much superphosphate (0-20-0) do you need to get 30 lbs of phosphorus? Divide 100 by 20, getting 5. Then multiply 5 times 30 lbs, for a total of 150 lbs of superphosphate, a much more manageable poundage to lug around and apply.

A bag of 0-20-0 may cost twice as much as 10-10-10 but you need only half as much to get the required 30 lbs of phosphate. You won't be adding unnecessary nutrients and filler to the soil, and you will be maintaining your garden's health.

Although the amounts of nutrients in organic fertilizers are less uniform, the calculations are done the same way. For example chicken manure has a 3-4-2 rating for N-P-K, while cow manure is 2-0-3, according to the Master Gardener's Handbook put out by the University of New Hampshire Cooperative Extension. But the amounts can vary by as much as 2.5%, depending on the source. Labels on all packaged fertilizers should include a nutrient analysis.

The copyright of the article What the Heck is N-P-K and How do I Get Enough? in New England Gardens is owned by Diana Morgan. Permission to republish What the Heck is N-P-K and How do I Get Enough? in print or online must be granted by the author in writing.

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