Fertilizing Basics: The ABCs and NPKs of Fertilizer

A couple of weeks ago I started a series of posts on dirt…err, soil.  The first was on the basics of what soil is…your plants’ home and its food.   Last week I gave you some basics on why and how to amend your soil.  Today’s post I’m going to explain the ABCs of fertilizing your plants and what those pesky #s and letters are on every bag of fertilizer.

Simply put…your plants need to be fed.  If you have good compost-rich soil they’ll get much of what they need from there.  However, most of us don’t have good soil…and like we need to have a good multi-vitamin to supplement what we get from the food we eat, your plants need a similar boost of nutrition…fertilizer.

Even if you start with great soil, over time your plants will absorb the essential nutrients they need to continue to grow big and strong…think tastier tomatoes, brighter flowers, and greener grass.  Plants need 6 essential nutrients…3 of which they get from “above”…the air and water – Oxygen, Carbon, and Hydrogen.  The other 3 they have to get from “below”…the soil they’re living in – Nitrogen, Phosphorus, and Potassium.  That’s where those pesky #s and letters come in.

On every bag of plant food…fertilizer, you’ll see 3 #s separated by dashes.  If you take the time to look at the finer print you’ll see that they represent the 3 nutrients your plants are looking to get from the soil…again, Nitrogen (N), Phosphorus (P), and Potassium (K)…in that order.

I could go all high school chemistry on you here by talking about what percentage of weight by element is in each bag of fertilizer and how the Phosphorus and Potassium are actually present in oxidized forms; however, all you need to know is that the bigger the number the more there is of that element…nutrient.  So in the bag of fertilizer to the right…it is made up of 11% Nitrogen, 7% Phosphorus, and 7% Potassium.  The remaining 75% are inert fillers.  And don’t feel jipped by the words inert and filler…these fillers (e.g., clay and limestone) actually serve a purpose…like appropriately distributing the Ns, Ps, and Ks and preventing the chemical burn to your plants that would take place without them.

Now that you understand what these #s and letters stand for, let me give you a quick and dirty explanation as to why each of these nutrients are important to the health of your plants:

• Nitrogen (N) aids in leaf and vegetation development.
• Phosphorus (P) provides your plants’ roots what they need to develop. It is also what is primarily responsible for flower and fruit production.
• Potassium (K) helps your plants’ roots function properly (e.g., water regulation and moving nutrients to plant cells), as well as making your plants more disease resistant.

You might be saying… “That’s great and all, but I’m still confused…why are there so many choices in fertilizers and combinations of numbers? Well, just like we need different vitamins and nutrients if we’re sick or depending on how old we are, plants need a different proportion of nutrients depending on what’s going on with them.  So…looking back at what value each nutrient provides:

• All-Purpose Fertilizers:  Like a good ole’ multi-vitamin, fertilizers with equal parts N-P-K (e.g., 10-10-10) are used to promote your plants’ overall health.
• Promote lush green growth: If you’re looking to green things up…look for a fertilizer with a higher first number.  Logically, you’ll see that grass fertilizers have a greater percentage of Nitrogen.
• Promote fruit and flower production: Since your plants only have so much energy to give, if you don’t supplement your plants with Phosphorus (essential for flower and fruit production) they will likely use what energy they do have to produce foliage.  That said, to promote fruit and flower production, look for a fertilizer that has a higher middle number.
• Promote root growth: You’ll also find that starter fertilizers will have a higher middle number…greater percentage of Phosphorus…which is needed to promote root growth.  Clearly this is an important function when a plant is first planted.
• Winterizing and disease prevention: Since during times of stress more phosphorus and potassium are needed to aid in plant cell development and to increase disease resistance, you should use a fertilizer with higher second and third numbers.

Lastly, I’d like to provide you with several pointers when using fertilizers:

• Labels: Make sure you read the label before using any fertilizer…for both reasons of safety and health for both you and your plants.
• New plants: With the exception of potentially using a root stimulater or starter fertilizer, you should resist the temptation to fertilize newly planted plants until after a full growing season.  Otherwise, you risk burning their roots.  But…if you read last week’s post and appropriately amended your soil, your plant should have all that it needs from the composted material to get it through its first year.
• More is not better: Generally speaking, when we get more vitamins than we need…well, we just “relieve” what we don’t use.  That’s not the case with plants. If you give your plants too much fertilizer…thinking more is better, odds are you will burn the plant’s roots and make the soil a bit toxic.  So less is more in this case.  If you still want to show your plants so extra love, feel free to add more composted material in and around their soil.
• Root Zone = Drip Line:  You should evenly distribute granular fertilizer at the plant’s root zone…which, generally speaking is also their drip line…where water would drop from its outer leaves.  This will get the fertilizer where it most needs it…at its roots.
• Let it soak in: If you are using a dry fertilizer make sure to water afterwards.  This will not only activate the fertilizer, but it will also minimize the risk of burning the surface of the roots.
• When to fertilize:
  • Early spring is a great time to fertilize your perennials and vegetables…just before or as they start sending out new shoots.
  • Your lawn, trees, and shrubs like a dose of fertilizer in early spring, as well as in early fall.
  • Annuals (i.e., growing annually in different seasons), like to be fed during their respective growing seasons.

So there you have it…the ABCs and NPKs of fertilizers.  I hope you found all this helpful.  Please let me know if you have any questions…I’d be more than happy to help where I can.  Next week I’ll conclude this series on dirt talking about soil samples and pH levels…again, trying not to get all high school chemistry on you.

Take care,