“What fertilizers am I supposed use in my garden, and how do I know which ones are needed for each plant?”
Gosh, this is a fun one – and we get asked questions similar to this all the time. So, I’m ready to dive in to decoding fertilizers today.
The first thing I think you need to know about fertilizers is what the 3 numbers on the front of the container mean. These numbers indicate the amounts of the Macronutrients of Nitrogen, Phosphorus, and Potassium in the formula.
Often called the “N-P-K”, which stands for “Nitrogen-Phosphorus-Potassium,” this fertilizer indicator shows you two things: 1) The ratios of nutrients relative to each other; and 2) How many pounds of this product you would need to add 1 pound of that nutrient to the soil.
N-P-K Indicates Relative Ratios
The N-P-K numbers give you what is called “guaranteed analysis” that shows the amounts of the macro nutrients in each bag. It sets the standard for the contents of each bag.
For the ratios, a 10-10-10, or 3-3-3, will have the same exact amounts of all 3 nutrients throughout the entire bag. A 12-6-8, or 6-4-6, will have different amounts of nutrients throughout the entire bag. Higher numbers mean more of that nutrient was added to that formulation, in relation to the relative amounts of the other nutrients.
Since all these different formulations exist, it stands to reason that different ratios of nutrients are good for different plants. In fact, specialty ratios have been developed for certain types of plants. This is why you see specialty fertilizers such as Rose, Azalea, Palm, Citrus, Strawberry, Blueberry, and more.
These special formulations have been shown to increase the growth and production of these specific plants because of the ratios of the three main nutrients. It is based on how those plants assimilate nutrients, how they use them, and observational studies of which nutrients give the maximum amount of the desired result. Yep. it’s science.
Often, these specialty fertilizers will also have additional ingredients too, like micronutrients, humates, mycorrhyzae, etc – check out the back of the bag to see what goodies are in there for those special types of plants.
Alright, up next, it’s time for some math. Ugh, adulting is hard.
N-P-K Indicates Relative Weight
Now, speaking about the second characteristic of the N-P-K rating, the amount to apply to reach 1 pound of that nutrient in the soil, we need to do a little math.
Here’s a simple example: Shell’s 10-10-10 N-P-K fertilizer indicates that you will need 10 pounds of the fertilizer to add 1 pound of each of the nutrients into the soil. You do this math by taking 100 (indicating 100%) and dividing it by 10.
100 / 10 = 10 lbs of fertilizer is needed to yield 1 pound of nutrient applied to soil.
If the N-P-K numbers are all different, like our Shell’s 4-6-8 Citrus Fertilizer, then you’d take each of those numbers and divide them into 100. Your answers will tell you how many pounds of that fertilizer you’d need to put 1 pound of that nutrient into the soil, like this:
100 / 4 = 25 lbs; 100 / 6 = 16.7 lbs; 100 / 8 = 12.5 lbs
So basically the N-P-K numbers are “weights” in a ratio format. Basically the higher the number, the LESS fertilizer you need to achieve the same result.
I know, it’s a little confusing sometimes, especially when you’re dealing with the N-P-K’s with different values for each nutrient.
What Nitrogen Does for Plants
Nitrogen is essential for healthy plant growth. It is an essential component in the process of photosynthesis, and as such it is needed to grow leaves and the stems to support them.
Plants with not enough nitrogen will show yellowing leaves that fall off easily. Plants with too much nitrogen will have leaves that yellow then brown and become “crispy” as the whole plant shrivels and dies (nitrogen burn). So nitrogen has to be in the right balance to help plants.
What Phosphorus Does for Plants
Phosphorus helps plants convert nutrients taken in by the roots and converting them into building block for the plants in their growth process – for example, making proteins that keep stems strong. Phosphorus is essential for root development.
Plants with not enough Phosphorus will have a classic “failure to thrive” identity – they will be small, and will have difficulty flowering or fruiting. Their root systems will be under-developed. They may have either a very bright green or a purple hue to them – these are both signs of Phosphorus deficiency.
What Potassium Does for Plants
Even in modern science today, the specific interactions of Potassium with plants remains a mystery. That said, we know what happens with plants when they don’t have enough, so we can extrapolate from that.
Potassium increases the rate of growth of the whole plant. It makes them use water more efficiently and makes them more drought resistant. With the correct amount of potassium plants are able to fight off disease and pests. They also produce more flowers, and fruits.
Potassium deficiency is hard to spot, but sometimes in the older leaves there will be brown spots, yellow edges of the leaves, yellow or brown veins. Overall the plant will not perform well.
How Do I Know If My Soil Has What My Plants Need?
Well, that’s pretty simple, really. Soil can be tested!
There are soil test kits – we carry them. There is also your local UF Extension office – they can test your soil for you, and if you need further testing, they have resources where you can have a more full analysis of your soil done for you.
If you’re serious about finding out a full analysis of your soil with a full report of what you can do to make it ideal, there are private and/or commercial facilities that will take your soil and test it with full reports.
I found a small list of providers at Fine Gardening’s website, edited to remove links that no longer work.
13611 B Street, Omaha, NE
68144; 402-334-7770 www.midwestlabs.com
Woods End Research Laboratory
PO Box 297, Mt. Vernon, ME
04352; 207-293-2457 https://www.woodsend.com
I definitely suggest starting with the local Extension service. They’re there to help!
Diagram of Plants and How Macronutrients Help Them
Side by Side Comparison of Nutrient Deficiencies
What Do I Use For My Vegetable Garden?
Now that you know all of this great information, the choice should be easy.
A well balanced fertilizer is great for your vegetables and herbs to help promote growth of the whole plant. Formulas like Shell’s 6-6-6, 8-8-8, and 10-10-10 are great for gardens. If you love the idea of organic food growing, Shell’s 3-3-3 Organic Garden Mix Fertilizer is AMAZING for gardens, lawns, landscapes, ornamentals…really anything you want to grow.
For your trees, ornamentals and specialty plants, there are a wide variety of specialty fertilizers available for them. Ask us when you get to the store and we’ll point you in the right direction.
I will say that it is very beneficial to have your soil tested for the Macronutrients. Your soil may not need fertilizer, and adding more would contribute to leaching of these nutrients in the the ground water and our beautiful Gulf of Mexico. When we add too much nitrogen, our porous soil allows the nitrogen to leach out, adding lots of nitrogen to our water table and our ocean. Then algae get a growth boost, and we have what’s called “red tide.”
“It’s like the old adage – too much of a good thing can be bad.”
The fertilizer ban legislation happened because of humans using too much chemical fertilizer in the environment and not planting plants that are native to Florida which tolerate the “challenging” soil we have here. Our soil is just fine for Florida native and naturalized plants – we only find it challenging because we try to grow things that normally wouldn’t grow here, and chemically force them to survive.
Compost, clean dry leaf litter, pine bark or needles, straw/hay, old mulch, and wood chips are all great organic matter to add to your soil. As it decomposes over time, all of the NPK nutrients in that plant matter is eaten by the critters and microbes in the soil. The Nitrogens, Phosphorus, and Potassium in those materials are thus made available to the plants that are growing now.
It takes time, but amending with organic material is the best long term way to ensure that your soil has plenty of N-P-K and other nutrients for your plants. And…organic materials can be collected from your yard either for free, or it’s kitchen scraps and other waste that would have gone to the landfill…which you can collect and compost yourself.
I hope this fertilizer tutorial was helpful to you! Let me know if you have any questions.
P.S. Want to know more about WHEN to use fertilizer? Check out this article I wrote previously: