What Is NPK In Fertilizer and What Else Is In The Bag?

When the home gardener opens a bag of fertilizer for their lawn or landscaping plants he may see pink, blue, yellow, white or gray-colored, crumbly material.

Ever wondered what makes up all those little prills listed on the fertilizer label?


If you’re recently a transplanted apartment dweller or a well-read book gardener, you may fear using any type of “balanced fertilizer.”

You read somewhere that the use of anything but compost or organic fertilizer would poison the soil and plants or burn them up.

Plant foods used in caring for the lawn or the home garden are no more “poisonous” to plants and soil micro-organisms than ham and eggs are to the average human being.

Certainly, the farmers of the United States do not buy “poison” when they use millions of tons of plant fertilizer on their farms every growing season.

The NPK Fertilizer Ratio Explained

A typical home garden plant fertilizer mix includes an analysis with the “fertilizer numbers” printed on the bag which guarantees the percent or N-P-K ratio of nitrogen phosphorus and potassium in the mixture.

Thus, a 5-10-5 mixture contains 5% nitrogen, 10% phosphoric acid and 5% potassium. This adds up to 20%.

“But why not 100% plant food?”, the gardener may wonder. ” What is the other 80%?”

Part of the 80% may be made up of other essential trace element fertilizer ingredients – nutrient elements such as:

  • Copper
  • Zinc
  • Boron
  • Manganese
  • Magnesium

At least one manufacturer includes these minor elements, though the trace element quantities used are not specified on the bag label.

Most of this 80% in question, however, is present because no gardener on earth would want to touch a 100% plant food with a 10 foot pole – let alone put it on their flowers!

The fact is that a simple nitrogen-phosphorus-potassium combination is a practical impossibility. Pure nitrogen is a gas (78% of the air is nitrogen) which does not combine readily with other elements.

And nitrogen must be combined with other elements before any plants, except the legumes (clovers, alfalfa, peas, etc.) can use it.

Pure phosphorus is just the opposite of nitrogen insofar as its chemical activity goes. In fact, it must be kept under water at all times, because this dangerous stuff bursts into flame upon contact with the air.

Nearly as bad – pure potassium. It requires submerging in oil to prevent its combination with moisture in the air. If a piece of potassium dropsinto water, it explodes violently.

Other pure elements are equally useless to plants for other reasons. The metallic elements such as zinc, copper, manganese, magnesium and boron would be of little use to plants if supplied as bits of metal.

Combining Combustible Chemical Fertilizer Materials

Instead of marketing a dubious mixture of useless gas, a couple of fire hazards and assorted scrap metals, the fertilizer industry manufacturers offers a safe, clean, sometimes odorless mixture of chemical compounds of the nutrient elements which plant roots can take up from the soil.

The nitrogen may be carried by the chemical compound, ammonium sulfate, a by-product of industry. This is a white, water soluble material made from nitrogen, hydrogen, sulfur and oxygen and it contains about 20% nitrogen.

Probably the phosphoric acid in the plant food is derived from superphosphate, which is about one-fifth phosphoric acid, the rest being oxygen, hydrogen, calcium and sulfur. (The last two elements, incidentally, are also important plant nutrients.)

Superphosphate is made by treating phosphate rock which has been mined with sulfuric acid. This process makes the phosphate much more available to plants.

Potassium needs are met through the inclusion of muriate of potash, sulfate of potash or manure salts, which contain potash, the remainder being carrier elements.

Mined in New Mexico, or obtained from dry lakes in California, the various potash salts are harmless crystalline materials, similar to table salt, which easily dissolve in water.

The minor elements are also supplied as compounds of “carrier” elements instead of as the pure metals. Common sources are commercial borax for boron, and the sulfate salts of copper, zinc, manganese and magnesium.

The Mixing Of Bag Of Fertilizer

All of these raw materials and several others are assembled in separate large storage bays in the fertilizer mixing plant.

Carefully weighed portions of each material are conveyed in batches to a rotary mixer which can mix one or two tons of plant food at a time. Part of the nitrogen in the mixture may be added in liquid form – as ammonia – during the mixing process.

After leaving the mixer the fertilizer (plant food) is conveyed by cranes or endless belts to storage piles where it “cures” for several weeks. The curing process allows interactions to be completed between the raw materials and insures the consumer a granular, easy flowing product.

At the end of the curing period the plant food is re-mixed, screened and bagged, ready for shipment to hardware, garden centers and big box retailers.

So the lawn fertilizer numbers 5-10-5, 6-10-4 and similar proportions stated on the outside of a plant food package tells only part of the story.

The rest of it we have told here. With the calcium and sulfur “carrier” elements, plus the minor elements, there is a lot more to plant food fertilizers than the figures show – but it’s in the bag!

Filed Under: Fertilizer Numbers, LG, z-002

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