Fertilizer Bag: What Is NPK 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 npk compound fertilizer labels?


If you’re recently a transplanted apartment dweller or a well-read book gardener, you may fear using any type of “balanced fertilizer” like 10-10-10 fertilizer.

You read somewhere that the use of anything but compost or organic fertilizers 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.

Before you arrive to conclusions, buy mineral fertilizers and proceed with fertilizer application, consider a soil test for trace elements and identify the nutrients present in the soil.

Organic Fertilizer NPK Ratio: What Is NPK?

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

  • N – Nitrogen encourages leafy growth.
  • P – Phosphorus stimulates root growth, build strong tissues, helps with flower development.
  • K – Potassium – improves water use, disease resistance, fruit quality and size

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 content minerals such as:

  • Copper
  • Zinc
  • Boron
  • Manganese
  • Magnesium

At least one manufacturer includes these minor minerals, 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 minerals.

And nitrogen must be combined with other minerals 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 drops into water, it explodes violently.

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

Combining Combustible Chemical Fertilizers And Other Materials

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

The nitrogen may be carried by the chemical compound fertilizer, 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 or phosphate fertilizers is derived from superphosphate, which is about one-fifth phosphoric acid, the rest being oxygen, hydrogen, calcium and sulfur. (The last two minerals, 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 minerals. Learn more about potash fertilizer.

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 minerals are also supplied as compounds of “carrier” minerals 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

So, what are fertilizers and what is in 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 of the fertilizer products are conveyed in batches to a rotary mixer which can combine one or two ton pounds of granular fertilizer at a time. Part of the nitrogen in the mixture may be added in liquid fertilizer 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 lawns fertilizer ratios of 5-10-5, 6-10-4 and similar proportions stated on the outside of a plant food bag tell only part of the story.

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

JOIN Our FREE Plant Care Newsletter 

By entering your email address you agree to receive a daily email newsletter from Plant Care Today. We'll respect your privacy and unsubscribe at any time.