Plant Fertilizer 101

The order I had placed recently for those tiny succulents came with instructions to fertilize the plants occasionally with a “5-10-10” fertilizer.  That got me thinking that it might be good to do a little bit of sharing of some of the information that I have been reading up on recently regarding plant fertilizers and what you can do with those numbers.

Very few of the common fertilizers that I purchase at the supermarket or home improvement stores come with explicit labels about their numbers (Although if you’re familiar with them, you might be able to look at the labels for the active ingredients and figure it out yourself.)  Those numbers are the “NPK” numbers – the ratio of how much nitrogen, phosphorus, and potassium.  A 5-10-10 fertilizer, for instance, will be composed of 5% nitrogen, 10% phosphorus, and 10% potassium when used in its instructed preparation.

Different plants are going to have different needs for their fertilization, so it will help to understand a little bit what each of those does for the plant. 

Nitrogen is one of the most common elements in the soil, and helps plants with the growth of greenery and foliage.  It’s good for usage on lawns, or large leafy plants.

Phosphorus is largely beneficial for root formation and flowering.  It’s mostly beneficial for flowering and ornamental plants.

Potassium helps overally plant function; it will help strengthen against diseases and pests, and assists in the formation of fruits and vegetables.

Higher numbers will signify different strengths of the content of the fertilizer, but really the most important thing to pay attention to is the ratio.  The only real difference between a 20-20-20 fertilizer and a 10-10-10 fertilizer is the strength.  So, if you know that you have a 20-20-20 fertilizer, and you need a 10-10-10, you can generally dilute the fertilizer down to half strength.  It also helps to do a soil analysis to check the levels of the chemicals that are already present in your soil.  If you know that you need a 5-10-10 fertilizer, but your soil is already have in potassium, then maybe you would be looking for something more like a 5-10-5 or a 5-10-2.

Speaking of 10-10-10… a lot of “General purpose” fertilizers will use this balanced formula to try to cover all of their bases, but it’s been said that a ‘balanced’ fertilizer like a 10-10-10 looks better on paper than it does in the soil.  I don’t have enough experience to speak on this first hand, but have enountered enough people mentioning it to be cautious about fertilizers that are very high in nitrogen.  I’m guessing that might be helpful for a pot with stale potting soil, a plant with already well established roots, and that is getting cut back and trimmed occasionally… but the danger of high nitrogen content is that it can spur overly rapid growth, which can weaken the plant to pests and disease; or, alternately, can build up in the soil if not absorbed and be harmful to beneficial bacteria and other microorganisms.  That’s where testing your soil helps.  If you have already been using a high nitrogen fertilizer regularly, or if you are testing very high for nitrogen in the soil, you can take steps to try to avoid potential negative outcomes by watering heavily to try to flush the fertilizer salts out of the soil.

The most common fertilizer you’ll see is the Miracle-Gro “blue powder” that you dissolve into your watering can.  If you look at the label (or use the site below to look it up) – you’ll see it’s ratio is 24-8-16.  That’s really heavy on nitrogen!  That’s probably good for things like my very large potted schefflera, or a pothos that’s all vine, or something like a dieffenbachia or a dracena.  For the things I’m working on currently, like trying to root cuttings, it’s maybe not the best choice since it’s ratio is pretty low in phosphorus – and until the roots are established, it’s not going to be using much nitrogen to fuel new growth up top.  (Not that I would likely use a recommended strength anyway, but even at a dilution it’s probably not as good as using the slow-release all purpose Miracle Gro pellets, which are a 10-10-10.)  For that, I might be better to use something like a bonemeal or a general fertilizer labelled as being formulated for tomatoes.

There are other elements that various plants will need.  Calcium, magnesium, sulfur are the most common.  The advantage of these slightly more rare chemicals is that its much harder to burn a plant with too much of these nutrients.  

Calcium and sulfur can be added by adding amounts of gypsum to the soil; calcium can also come from lime, eggshells or “shell meal”; magnesium and sulfur can be obtained by adding amounts of epsom salts.  You can find the needs of your specific plants with some quick google searches.  Tomatoes, for instance, have a specific need for calcium — and a shortage of calcium in tomatoes can cause a condition called “Stem rot” that manifests exactly as its name implies.  A lot of them you will find connected to various “Farmer tricks” for plants, like putting egg shells in the hole beneath your tomato plants, or match sticks in the hole beneath your peppers plants.

There are numerous sites online that talk about using epsom salts and gypsum for gardening if you’d want to learn more about that.

On a final note, I wanted to share this site, which lists some of the more common household fertilizers and the ratios associated with each if you are curious to look up the fertilizers you have in your cupboard or shed.


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