Green Thumb Series:
Fertilizers & Plant Food
Fertilizers or “plant food” come in many forms. All the different fertilizer brands – and there are many – must offer products for both the very specific and very general needs of the people who buy them. Knowing the exhaustive list of mineral nutrients that make our plants thrive is usually the task of those with an academic background. However, even the beginner gardener can get very far with some of the highlights! The broad stroke information – things like NPK values, organic fertilizers, the functions and deficiency symptoms of different nutrients, and how to apply your own fertilizer – are what we’ll be covering in this article.
What is NPK?
If you’ve spent much time in the fertilizer section of your local garden center, you’ve probably seen the three values printed on all the different bags. In the industry, we refer to these as “NPK” values, and they always appear in that exact order. Simply put, these values represent a ratio of N, P, and K within a given bag of fertilizer. Before we get too in the weeds, what is N-P-K?
N stands for Nitrogen. Nitrogen is essential for leafy-green, vegetative growth. It helps plants synthesize proteins, enzymes, and chlorophyll. Without enough nitrogen in the soil, your plant’s beautiful green growth will begin to look pale and eventually yellow, usually starting at the base of the plant. This could mean more time spent dropping damaged leaves to produce new ones, or even stunted growth.
P stands for Phosphorus. Phosphorus is crucial for energy transfer within the plant. It promotes root development, flowering, and fruiting. Without enough Phosphorus, plants will start to exhibit dark-green or purplish leaves, spindly growth, and delayed maturity. Flower and fruit production will be lacking, if not completely unproductive.
K stands for Potassium (it’s weird, we know). Potassium aids in plant metabolism, water-uptake, enzyme activation, photosynthesis, protein synthesis, and disease / cold resistance. Plants with a Potassium deficiency exhibit slowed growth, scorched leaves (yellowing on the leaf’s edges and margins), and shriveled fruit.
(You might see “Potash” rather than Potassium on some fertilizers. Though technically different in the context of production, they are functionally the same for most garden applications. )
How To Read The NPK Ratio
As you can see, all three of these nutrients serve specific roles to ensure your plants remain happy and healthy! Some plants will benefit from a higher concentration of one or more nutrients over the others, which is why the industry standard NPK ratio was created. For example, an NPK ratio of 5-10-5 means the bag consists of 5% Nitrogen, 10% Phosphorus, and 5% Potassium. In this instance, you might see the bag branded as being for smaller flowering plants like bedding flowers. Remember, Phosphorus is great for stimulating early growth and flowering! This mix of branding and visible NPK ratios helps anyone from beginner gardeners to landscapers to botanists quickly identify the purpose behind the fertilizer. Another quick tip for larger bags: a 5-10-5 NPK ratio can also be used to get exact measurements of each nutrient. For a 20lb bag of fertilizer, 1lb (5%) would be Nitrogen, 2lbs (10%) would be Phosphorus, and 1lb (5%) would be Potassium. This brings the major mineral nutrient content to a total of 20% of the bag.
Other Important Mineral Nutrients
So, what is the other 80% of the bag? Like we mentioned before, there are a ton of other mineral nutrients that make a plant thrive. Some of that percentage is dedicated to that laundry list, but the other big three are Calcium, Magnesium, and Sulfur.
Calcium is usually found in its heaviest concentrations in leafy green edible fertilizers. Deficiencies can lead to dying plant tips and stubby roots. Celery blackheart, lettuce tip burn, and internal browning in Brussels sprouts are all signs of Calcium deficiency.
Magnesium deficiency can lead to light green or pale looking leaves. The leaf margins might begin to turn upwards.
Sulfur is found in plant structures in the form of amino acids, often functioning as enzymes. A deficiency can cause the areas between the veins of young leaves to turn a light green. You might also see spindly growth.
Between NPK and the nutrients just listed, you’ve got the macronutrients your plants need in the soil.
Micronutrients
The flip side of macronutrients is your micronutrients. Micronutrients are usually present in extremely small amounts, and include the following: Iron, Manganese, Copper, Zinc, Molybdenum, Boron, and Chlorine. Here’s a chart of the symptoms of their deficiencies on your plant:
Again, the amounts are usually extremely small for any of the micronutrients. However, different plants can benefit from outsized amounts. For example, citrus fruit-tree fertilizers will tend to have a lot of zinc because of the notable increase in yield. Branded and plant-specific fertilizers often tell you one or two benefits that their mix of micro and macro nutrients bring. You can learn a lot by just reading the marketing materials on the bags!
Always Read Your Product Labels
You can always check for the exhaustive ingredient breakdown on the bag of the bag / bottle of fertilizer. The majority of that missing 80% is usually mineral or organic filler – some of which will aid in the absorption or activation of the processes behind the macro and micronutrients. Don’t worry about all the particulars too much, as the biggest priority is matching the NPK ratio to the plants that utilize it. Remember that these bags tend to tell you what kind of plants they work best with, and some brands even offer plant-specific fertilizers. At Louisiana Nursery, our great partner Ferti-lome has Azalea and Hibiscus specific bags. This can give you an added layer of confidence when making your purchasing decisions and teach you something about the plant’s soil makeup preferences.
Fertilizer Categories Based on NPK Ratio
| Incomplete | Complete | Balanced |
|---|---|---|
|
Missing one or more of the major mineral nutrients (Nitrogen, Phosphorus, Potassium) |
Contains all three major mineral nutrients |
A Fertilizer where the NPK values are the exact same. 10-10-10 is a balanced fertilizer. |
The landscape and nursery world loves to categorize! Incomplete, complete, and balanced fertilizers all have their defenders and opponents depending on which specific product you’re talking about. We’ve covered some examples of complete fertilizers already, so lets get into incomplete and balanced next:
Incomplete Fertilizers
Bone Meal & Blood Meal are examples of incomplete fertilizer products. Their NPK ratios show a complete lack of either nitrogen, phosphorus, or potassium. While both of these examples are missing two major mineral nutrients, even missing just one would be categorized as an incomplete fertilizer.
Incomplete doesn’t mean worse! Many experienced gardeners will use information from a soil test to raise or lower the levels of certain mineral nutrients in their soil. We support anyone avoiding the overuse of products, or avoiding the use of unnecessary products.
Balanced Fertilizers
Balanced fertilizer products have NPK values that are exactly the same, with the ratio being one. You’ll usually find balanced fertilizers labelled as being “All Purpose” and “Houseplant Food”. Because different nutrients will be present at different levels in the soil, it may not always be the best product for any situation. This is because our landscapes naturally have varying amounts of N, P, and K, and a balanced proportion doesn’t necessarily mean better. That being said, plenty of gardeners have positive experiences with these products. I would personally only use it in container gardens and houseplants, a setting where you have more control over a smaller amount of soil.
Soil Additives & Amendments
Additive products can blur the definition of an incomplete fertilizer, and deserve an article all on their own. In the meantime, you can think of additives as having a distinct purpose in changing the properties of the soil itself. On the left, Aluminum Sulfate will help raise the acidity of your soil. On the right, Agricultural Limestone will lower the acidity of your soil.
I try to differentiate these products from incomplete fertilizers based on the purpose of the product, rather than its chemical makeup on the ingredient label. Both pictured products are fixing an issue to do with acidity. Gypsum by Hoffman, meant to loosen clay soil, could be described the same way.
Osmocote & Slow Release Fertilizers
Another fertilizer option that has grown in popularity is Osmocote, a Scotts product. If you’ve ever seen “slow-release” fertilizers – like Louisiana Nursery uses in their potting mix – it’s the same idea! A semi-permeable coating (the “cote” in osmocote) surrounds the typical granules of essential nutrients. This coating allows water in to dissolve the granules, and is eventually released through heat activation. The claim is an all-in-one solution that doesn’t burn – when applied properly – and only has to be re-applied every four-six months. We use it on our bedding house stock and have had great success. It may not be the absolute best way to get the showiest flowers or highest yield possible. However, it definitely benefits the plants more so than no fertilizer program at all, and is a general use product that less experienced and master gardeners alike can get a lot out of.
Organic Fertilizers
When searching for organic options, there’s no shortage of great brands and applications. Be sure to look for certifications such as OMRI, CDFA, or read the full label to make an informed decision. Organic fertilizers are unique in that their mineral nutrient contents are (usually) derived from natural sources.
Foxfarm Soil & Fertilizer Company, for example, markets many of their fertilizers for including “Active Soil Microbes”. When looking at their ingredient label, you see alfalfa meal, feather meal, fish bone meal, bat guano, and many more depending on the bag.
True Organic – another organic fertilizer brand – lists their mineral nutrients as being derived from seabird guano, shrimp & crab shell meal, poultry manure, and soybean meal. They also guarantee no sludge, fillers, or additives.
How To Apply Fertilizer
A few caveats: Testing your soil – especially the beds themselves – will help you make an informed decision. You may already have especially acidic (Nitrogen heavy) soil, or vice versa. Applying more fertilizer blindly could further damage your gardening success! We offer test kits at no charge that can be mailed directly to Louisiana State University for professional analysis of your soil. There are ways to adjust your soil’s properties, such as pH levels, which LSU’s Agricultural Department has written about extensively. Referring to soil additives, products like agricultural limestone claim neutralizing excess soil acidity as one of its benefits.
If you’re applying to already established plants, remember that over-application is just as damaging to your plants as extreme deficiencies. Again, soil test kits are helpful. If you happen to use fertilizers on a very regular basis, the hallmarks of deficiencies might be too much fertilizer rather than too little.
Regardless, when applying fertilizer, you’ll want to keep the granules or properly mixed liquid concentrate away from the base of the plant itself. The tips of the root system are what will be absorbing these nutrients, and root systems fan out to varying degrees away from the base of the plant. For trees and shrubs, this may mean as much as a foot (or a couple) from the base for application. For smaller flowers and edibles, you can apply closer. Many products will instruct you to mix fertilizers into the soil, usually 2-4 inches deep. This can help reduce waste and runoff.
Be Mindful With
Any Chemical Product
Overuse is a big issue in the horticulture industry. Whether we’re talking about pesticides, herbicides, fertilizers, etc., we tend to overdo in fear of not giving our plants everything they need. Even in watering – especially our beautiful houseplants – the most common cause of failure is the overabundance of water rather than neglectful watering. Worse than its potential impact on your individual plant is the impact on our forests, our waterways, and the environment at large. Plants have been thriving in all manner since the beginning of time, without man-made fertilizers and the like. A healthy respect for this fact will do more for your garden’s success than all the knowledge of chemistry in the world!
[Much of this information came from the excellent materials provided by the Louisiana Nursery & Landscape Association. Louisiana Nursery is proud to send staff members to their certification program every year. The benefits of horticulture education goes beyond the nursery business, creating more successful and responsible gardeners for life!]
Thanks for reading,
Written by Beau Mutrie
