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Fertilizer terms can be confusing. What is the N-P-K? What are nitrogen, phosphorus, and potassium? Let us take the guesswork out of fertilizing. We’ll explain the N-P-K ratio and what those numbers on fertilizers mean.
It’s all about the soil. Most soil doesn’t have all the nutrients needed for optimal growth, which means you won’t get the harvest or flower bloom desired.
There are six primary macronutrients that plants require—carbon, hydrogen, oxygen, nitrogen, phosphorus, and potassium. The latter three nutrients come from the soil.
Every fertilizer label will give information relating to its “N-P-K” content, which is the amount of nitrogen, phosphorus, and potassium (standardized in that order) in the particular fertilizer blend. This is expressed as a series of three numbers that give the percentage (also called “ratio” in this sense of the word) of each nutrient by weight. On the label, you’ll see numbers like 5-10-10, 10-10-10, and 10-6-4. For example, a 100-pound bag of 10-10-10 contains ten pounds each of nitrogen, phosphorus, and potassium. The rest is filler, which gives it bulk and makes it easier to spread.
When it comes to fertilizers, much attention is paid to nitrogen, phosphorus, and potassium, but there are other key nutrients needed for overall plant health as well, though in smaller quantities.
Secondary macronutrients are:
There are also micronutrients, or trace minerals, that are needed in much smaller amounts for plant health. These include boron, chlorine, copper, iron, manganese, molybdenum, nickel, and zinc.
Soil test results sometimes come with recommendations to add these other minerals to the soil if they are found to be deficient.
Read more about organic soil amendments.
How do you know what your soil needs? Knowing your plants’ growing medium is key to knowing what kind of fertilizer(s) will benefit them. You want to add what they lack, not what they do not need.
The very first step in almost any gardening endeavor should be a soil test. For accurate and useful results, go through the folks at your County Extension Office. They will test the soil, explain the results, and provide recommendations for actions to take. Read more about soil testing for a healthier garden.
Because soil continually changes, you should have your soil tested every 2 to 3 years. Keeping records of test results, fertilizer applications, and any other soil amendments you make is always a good idea.
Fertilizer providing the N-P-K nutrients mentioned above can come in both organic and synthetic versions. What’s the difference?
In general, organic fertilizers need time to enrich the soil, so they’re best applied in the fall so the nutrients will be available in the spring. For the spring, some fertilizers combine the best of both worlds with an organically-based fertilizer mix that also contains small amounts of synthetic fertilizers to ensure the immediate availability of nutrients.
Note that the N-P-K ratio of organic fertilizers is typically lower than that of a synthetic fertilizer. By law, the ratio label can only list nutrients that are immediately available.
You may also notice that there are both granular and soluble formulations.
To build the long-term health and fertility of your soil, we recommend using granular organic fertilizers. Supplementing with an additional water-soluble fertilizer is a way to ensure that your plants have the nutrients they need when they need a boost (during active growth).
Shopping for fertilizer can be confusing because plants have individual nutrient requirements.
Before buying bags or bottles of fertilizers, consider how you might add nutrients to the soil by improving its biology. Try compost tea, a liquid produced by extracting beneficial microbes (bacteria, fungi, protozoa, and nematodes) from compost.
Applying compost tea to both soil and plant foliage adds those beneficial microorganisms to the growing medium, which boosts plant health and encourages growth. Compost tea is made using a brewing process similar to that used for making beer. Active compost, a brewing kit, and a little information can go a long way toward turning your landscape into a thriving ecosystem. See information on how to make compost tea.
Now that you understand more about fertilizers, see how to apply fertilizer to your garden!
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