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Fertilizer Ratios - What is NPK?


What is Fertilizer and Why Do I Need It?

Plants need nutrients to grow. The major nutrients they need are carbon, hydrogen, oxygen, nitrogen, phosphorous and potassium. They also need magnesium, sulfur and calcium. Without all of these nutrients, plants may grow slowly or fail to grow at all.

Plants absorb the nutrients they need from the soil. Over time, the soil will have less nutrients than before. Gardeners add fertilizer to the soil to replenish the soil with the nutrients plants need. A soil test will show you what nutrients your soil needs.

What Is NPK and Why Is It Important?

Most fertilizers or plant foods list the NPK fertilizer ratio on the bag. NPK stands for nitrogen, phosphorous and potassium, the three major macronutrients plants need. Nitrogen is mostly responsible for plant leaf growth; phosphorous is needed for the growth of plant roots, fruits and flowers; potassium is responsible for the plant's overall functioning. At different times in the plant's growth cycle, it may need more of one macronutrient than another.

When looking at NPK fertilizer ratios, it may be listed as 6-3-3. This means that it has six parts nitrogen, three parts phosphorous and three parts potassium. Often the NPK ratio for vegetables like corn and lawns have higher percentages of nitrogen. Other NPK fertilizer ratios may be listed as 10-10-10. This mean the fertilizer has equal parts nitrogen, phosphorous and potassium. These are sometimes called balanced fertilizers.

What Is the Best NPK Fertilizer Ratio for My Plants?

To determine the best NPK fertilizer ratio for your plants, start with a soil test. This will help you determine what macronutrients are in your soil and what nutrients should be added. For instance, many lawns and gardens have plenty of phosphorous, but may need more nitrogen. If that is the case with your garden, look for fertilizer ratios where the first number, nitrogen, is higher than the middle number, phosphorous. When adding fertilizer to your soil, you should add nutrients that are lacking in the soil. Remember, plants take the nutrients they need from the soil.

Expert Advice

Q. Twice this year friends have come to me and said, "My tomatoes are all vines and no fruit." I asked what they were fertilizing with (thinking they were applying too much nitrogen), and to my surprise both replied: "10-10-10". I said, "That'll do it." What I don't know is why anyone would think 10-10-10 is fine for their garden—or how to help them fix the problem they've caused. Maybe by next year the crappy chemical fertilizer will be flushed out by rain and winter?,

    ---Michael in Albemarle, NC

A. I have mixed feelings about you, Michael. On one hand, you're my new favorite listener for knowing how bogus so-called "balanced" fertilizers like 10-10-10 are. On the other hand, if more people DID know things like that I'd have to go out and find a real job.

Seriously, you're absolutely correct: 10% nitrogen is only appropriate for non-flowering plants like sweet corn and lawns; it's way too much 'N' for plants that flower, like tomatoes,squash, beans,peppers, melons, eggplant, and—oh yeah, flowers! Bogus 'even number' fertilizers like 10-10-10 and 20-20-20 are always composed of concentrated chemical salts; and the super-fast growth they cause makes plants extremely attractive to pests and diseases. And those salts—originally designed to be used as high explosives—ruin the soil, and kill the soil life that keeps plants naturally healthy. And finally, despite their arithmetic rhythm, fertilizer ratios like 10-10-10 are also unbalanced. No plant uses those three nutrients in equal amounts.

A little background: The three numbers (commonly referred to as "N-P-K") that appear on the label of every packaged fertilizer represent the three main plant nutrients: Nitrogen, Phosphorus and Potassium (which is sometimes called 'Potash'). Studies have found that the ideal NPK fertilizer ratio of those nutrients for flowering plants is 3-1-2. (That's 3% Nitrogen, 1% phosphorus & 2% potassium.) So look for that ratio on the label of packaged fertilizers; anything close to a 3-1-2, a 6-2-4 or a 9-3-6 should be ideal. (Beware higher numbers—that's the realm of chemical salts.)

My advice to folks like Michael's friends is to water on the heavy side to wash those salts out of the soil as quickly as possible, feed with compost alone for the rest of the season, and then move to organic fertilizers and/or compost in the future and sin no more!

Q. I've been growing heirloom tomatoes for a few years with reasonable success. But I would like to know how to build on that success. I understand that some fertilizers promote root growth, some promote leaves, and some encourage flowering. I have fish emulsion and a seaweed solution. Which should I use and when to maximize my tomato yield this year?

    ---Nina in Central NJ

I'm curious about the difference between bone meal and blood meal. Would I use either for new plantings?

    ---Ray from Front Royal, VA

A.Nitrogen—the first number of an NPK rating—grows big plants with lots of leaves. But too much nitrogen, especially combined with a lack of other nutrients, will inhibit flowering and fruiting. The plants that thrive with this nutrient are the non-flowering grasses and grains (i. e. lawns and sweet corn). Blood meal is a high nitrogen fertilizer (it rates a 12-2-0; a very high number for a natural product), as are fish meal (and fish emulsion), horse and poultry manure and corn gluten meal (which also prevents seed germination, making it the only natural springtime weed and feed for lawns).

Phosphorus—the middle number—is best known as the nutrient that produces more flowers and fruits, but it's also essential to strong root growth early in the season. Bone meal (1-11-0) is the organic source that becomes available the fastest. Many growers prefer rock phosphate or colloidal rock phosphate, which release the nutrient slowly, and for a long time after application—three to five years. But that slowness means you should try and apply rock phosphates the season BEFORE you want your blooms boosted, to give it time to get ready to work.

Potassium—the third number—helps plants process all nutrients more efficiently, improves the quality of fruits, and helps plants resist stress. The best single-ingredient source is green sand. Also known as glauconite, this mined mineral formed in prehistoric oceans also contains lots of important trace elements and minerals. It releases its nutrients the slowest of all—over the course of a decade; so, like the rock phosphates, always try to spread green sand in the fall in preparation for the following season.

But it's important to remember that all fertilizers—chemical and organic —rely on soil life to make their nutrients available to plants, and that high levels of organic matter in your soil are vital to the healthy growth of all plants. So all fertilization plans should begin with an inch of high-quality compost applied to the soil (preferably on top as opposed to tilled in). Then as the season progresses, you can give your plants a little boost by adding more compost or using a well-balanced organic fertilizer (remember, the NPK ratio you want to come close to achieving is 3-1-2).

I like to use a liquid fish and seaweed mix for that boost, as these products provide a nice balance of the basic nutrients, and lots of essential trace elements from the seaweed component. Just pour some into a watering can, dilute it as directed and water it into the soil around the root zone. If you prefer granular fertilizers, spread the material evenly over the soil beginning about six inches away from the plant stem and then cover it with some soil or compost to help it get to the plants faste

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