Fertilizer Magic What Do All These Numbers Mean

Plant nursery in Southern California with a variety of fertilizers, helping you understand the meaning behind the numbers.

Written by David S.

When you look at fertilizer products, there is a series of three numbers separated by a hyphen – 7-7-7- or 10-5-6. What do those numbers mean and how do they apply to your garden project. Keep reading, and we will answer that question so that you never have to wonder what all those numbers mean. 

Fertilizer and the Numbers 

The numbers follow some logic. They are always in the same order, and they represent three necessary chemicals or minerals needed at various stages of a plants development. They are:

Nitrogen – Phosphorus – Potassium. If you saw, 7-7-7 on a bag of fertilizer it would translate as 7% nitrogen – 7% Phosphorus, and 7% Potassium. The order never varies though the numbers do. For example, the code 10-5-6 means 10% nitrogen, 5% Phosphorus, and 6% potassium. The order is always N-P-K. N is the symbol for Nitrogen. P is the symbol of Phosphorus. K is the symbol for potassium. In short, the numbers on the fertilizer bag represent the percent of each chemical or mineral found in that product. So, a hundred-pound bag of 7-7-7 fertilizer would have seven pounds of each of the three chemicals. In a 100-pound bag marked 10-5-6 there would be ten pounds of Nitrogen, five pounds of Phosphorus, and six pounds of potassium. The order of these chemicals never change. 
What Do Nitrogen, Phosphorus, and Potassium do?

Each of these chemicals has a role. Nitrogen, generally, is all about leaf, bud, stem, and trunk production. Phosphorus helps plants to develop healthy and vigorous root systems. Potassium helps the entire plant maintain and improve health levels. We will go a bit deeper into the roles of these chemicals later in the blog. 

The Plant Cycle 

There are a few ways to look at the plant cycle. They are seedling, young plant, developed plant or growth stage, foliage stage, reproductive stage. In some plants, there is an overwintering stage. 

Seeds start growing in the spring, flowers occur in the spring and early summer, Fruit develops over the summer, and in the fall, we harvest many crops. What about Winter? Due to the decreasing amount of light from the sun after the fall solstice, not much plant growth occurs. The trees drop their leaves and go to sleep. Flowering plants stop growing, for the most part. Winter is a time of rejuvenation for the soil. The top layer of the Earth’s crust we call the Humus layer. It is where all of the fallen leaves sit. That layer is much like a compost pile, and the little soil organisms work hard to break down the humus layer into available plant foods. In the spring, what is the first thing we notice? We might see that everything is turning green. That greening is due to the increase in Nitrogen in the soil. Beneficial bacteria have spent the winter, converting free Nitrogen into usable Nitrogen so that when the plants wake up, and the seeds germinate, they have usable Nitrogen to grow. Well, a green plant is only green because it has healthy roots. Those same beneficial organisms have also spent the winter breaking down organic matter into other usable chemicals. Chiefly, potassium, and Phosphorus, which is why these elements appear on fertilizer bags. Other elements such as magnesium are available as trace elements in fertilizer products or in products that are made to amend the soil. 

That is the long way to discuss what chemicals do for plants. The shorter version is as follows. 

Seed and Seedlings – When you plant your garden, you want a fertilizer that is kind to seeds and seedlings. Generally, this means lower numbers 3-3-3-, 5-5-5, etc. What are seeds? Seeds are little energy packets from which a tap root forms. A seeding will have some energy from the seed casing, but it will need soil that is healthy and full of the right “stuff” to help it develop from a sprouted seed into a plant. If it has too much “stuff,” it will die. If it has to little “stuff,” it will struggle to become establish and be prone to pests and disease. If you are planting seeds or seedlings, reach for a fertilizer engineered with seeds and seedling care in mind. 

A good option for seedlings is Down To Earth brand Vegetable Garden 4-4-4, which is organic. The lower doses of N/P/K help seedlings without overpowering them. Note: when the chemical numbers are even, we consider that fertilizer to be a general fertilizer product. For more mature plants a general fertilizer with 7-7-7 would be a good option. 

Young Plants – Plants go through stages. The first is a growth stage, and this applies to seedlings and younger plants. The foliage stage is next, and the plant focuses on growing leaves, developing branches, and gaining height, etc. Most young plants do fine with a 4-4-4 fertilizer. The Down To Earth 4-4-4 is feeding the plant so that it grows above the ground, produces healthy roots, and is thriving. Nitrogen for the top, PPhosphorus for the roots, and potassium for the overall health of the plant. 

The Foliage Stage – The plant is ready to pack on leaves and develop branches. Nitrogen-rich fertilizer is helpful. Look for something higher in Nitrogen. For example, 10-7-7 has a nitrogen boost but is still promoting root growth and plant health. Sometimes the numbers can be high, such as 17-7-10, which is the ferti-lome brand of Hibiscus & Tropical plant food. You can look at the numbers and imagine what is going on here. A larger plant with a lot of foliage and lots of flowers or buds.

Another example is the fert-lome 16-0-8, which is a lawn fertilizer. Lots of green grass, not a lot of root support, and a little bit of plant health. 

At this stage, we have walked through the numbers and what they mean and what they do. So what happens when you have a plant problem, and you think fertilizer will help? The second or third-year potted plants are an excellent example of this scenario. You planted the contain a year or two ago. The first year the blooms were crazy beautiful, but this year, there is not as many, and they are not as large? Quite possibly, the container has lost its nutrient load. What kind of fertilizer would you use? If you said something with more Nitrogen than Phosphorus or Potassium, then you’d be correct. For that scenario, we’d go back to the 10-7-7 or something similar. What happens if you had a lovely plant that is not doing well? 7-10-10 or something similar would be a correct solution.  It breaks down like this: “7” to support leaf production, “10” phosphorus to promote healthy root development and “10” potassium to improve plant health. 

There is a Lot of Products Out There

Fertilizer can still be challenging. Be sure to stop into one of our nurseries and look over the fertilizer options. You can also ask one of our garden experts for help. Some plant problems are not about N/P/K at all. There may be a pest problem, or you may have a plant that needs specialized nutrients. The bottom line is that we are always here to help. 

If you are looking for a specific fertilizer, be sure to call ahead to make sure we have it in stock. Our products frequently change at all of our five locations. You can call ahead, shop online, or stop by. We are happy to answer your questions, help you diagnose fertilizer issues and plant disease, or give you recommendations that help your garden and landscaping projects to thrive. 


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