The lone voice of horticultural reason

Fertilizer Frenzy Part II

6-3-1999 — Walk into any large garden center, find the aisle with the fertilizer and let the confusion begin. Rose food, tomato food, bulb food, blood meal, fish meal, bone meal, super phosphate, super root & bloom, all-purpose flower food, vegetable food, aluminum sulfate, hydrated lime-there are more choices here than in the cereal aisle of the supermarket. What do they do? What do they mean? Most important, what do you use?

Deciphering the mystical-and in many ways, bogus-world of today’s garden fertilizers is best begun by learning a few basic facts. First comes the understanding of N-P-K, the three numbers found somewhere on the fertilizer box, bottle or bucket.
Take Miracle-Gro, a popular water-soluble fertilizer. Right on the front label are the numbers 15-30-15. The first 15 represents the nitrogen content (N). The 30 represents the phosphate, or phosphorus content (P). The last 15 represents the potash, or potassium content (K, the one I always got wrong in eighth grade science class). N-P-K, nitrogen, phosphorus and potassium, are the three basic, essential macronutrients all plants require in relatively large amounts in order to live.

Nitrogen (N) makes plants vigorous and green. Green is good. This is why lawn fertilizers have such a big first number, such as 22 or 30 or 33.
Phosphorus (P) promotes root development, which is why bulb food has a relatively large middle number.
Potassium (K) aids in blooming and fruiting, but since, as we learned in the previous column, this is something plants have a pretty good handle on, this third number is never huge and is rarely larger than the first two.

Blood meal is 12-0-0; all nitrogen. Super phosphate comes in a bag marked 0-18-0; pure root food. An all-purpose granular flower food might be 5-10-5; some nitrogen (5), a good dose of phosphorous (10) to develop roots, finished off with some potassium (5) to simply aid the blooming that is bound to happen because of the presence of the first two.

The size of the number indicates the amount by volume; mix up a gallon of Miracle-Gro and the gallon of water contains 15% nitrogen, 30% phosphorus and 15% potassium. Note that the ratio of these three numbers is the same as the all-purpose 5-10-5 garden food mentioned above; Miracle-Gro just packs a bigger wallop. If it’s a box of granular fertilizer marked 10-10-10, the box contains 10% each of N-P-K, and 70% filler. And yes, in terms of potency, or how much to use, two cups of 10-10-10 equals one cup of 20-20-20, water solvable or granular. There the simplicity ends.

A difficult concept for some to accept is the fact that nitrogen is nitrogen is nitrogen. Put differently, if it’s nitrogen, it’s nitrogen, no matter where it came from. This flies in the face of organic growers who claim purity of product by using only organic fertilizers, such as cow manure (which provides faint wisps of nitrogen), sea kelp, fish emulsion, barber shop sweepings or anything else not manufactured by evil white men in corporate chemical factories. There is a comforting factor infused by organic growers not using pesticides on their vegetables, certainly, but they confuse the issue by insisting that the forms through which they supply the nutrients to their crops are somehow superior. Fact of the matter is, your roses, your tomatoes and your beans don’t know and most importantly don’t care if the fertilizer you feed them was manufactured from fish certified to have died only from old age, or made from the tailings of a Russian nuclear power plant; plants cannot detect any difference. Nitrogen is nitrogen is nitrogen, phosphorus is phosphorus is … well, you get the idea.

In my next, and last column on the subject, we’ll look at how to know what fertilizer your plants need, how often, and in which form. Then I’ll try not to confuse the issue when explaining what soil pH has to do with it.

Don Engebretson
The Renegade Gardener