The Quick Basics

Numbers? Nitrogen-Phosphorous-Potassium or N-P-K

Nitrogen: vegetative growth, base for proteins

Phosphorous: root/flower/seed development, hardiness

Potassium: water/nutrient transport, blooming, hardiness

Other Essentials: Mg, Ca, S, Fe, Mn, B, Mo, Cu, Zn, Cl, Ni, Co, Na, Si

Use a complete fertilizer that contains all the essential nutrients your plants need. Only having to use one product will make your life much easier. 

Organic fertilizers:

• organisms convert to usable forms
• generally, slow release
• less likely to burn/damage plants
• generally, not the best in soilless media

Synthetic Fertilizers:

• nutrient forms are immediately available
• more likely to burn/damage plants
• more frequent applications
• can be used in any growing media

Most Common Problems

• nitrogen deficiency: leaves yellowing from oldest to newest leaves,
• overfertilization (burn): leaf margins browning and crisping up usually first sign, root rot, yellowing/wilting leaves, brown spots to follow

The Aroid Addicts Recommendation

Dyna-Grow Foliage-Pro, FEED ME! MSU Orchid Fertilizer, and General Hydroponics Flora Series are my favorites (complete fertilizers and easy to use)

I like to use a "loading concentration" first at a higher concentration. Then, I maintain nutrients with a lower concentration "maintenance concentration". At the first watering, use the loading concentration. 2nd and 3rd are maintenance, and 4th is flush. This helps to make sure your plants are getting enough nutrients, but you are also resetting every 4th watering to make sure there isn't too much building up. I use a pH meter and GH's pH control kit to adjust pH. I also use a TDS meter to check the amount of minerals in my solution.

Loading Concentrations

Choose one of the following, dilute in filtered water, adjust pH (5.5-6.5)

• 2-3 tsp/gallon DGFP
• 1 tsp/gallon Orchid
• 1 tsp/gallon of each GHFS, more info on GHFS below

Maintenance Concentrations

Choose one of the following, dilute in filtered water, adjust pH (5.5-6.5)

• 1/4 tsp/gallon DGFP
• 1/2 tsp/gallon Orchid
• 1/4 tsp/gallon of each GHFS, more info on GHFS below

Note: these concentrations are diluted and can be used with most waterings

Water by flooding pots with diluted fertilizer and let drain

Flush (flood and let drain) growing media with plain filtered water 2-3 times a couple minutes apart around every 4th watering (1-2 times/month for me) to prevent salt build up/algae formation, do not fertilize at this watering, can use Florakleen with flushing to further eliminate any mineral buildup

Can use Miracle Grow's Orchid Plant Food Mist for orchid and Hoya foliage

Links: any commission earned is used to create more educational content

Dyna-Grow Foliage Pro- FYI, I can't get commission on this

Amazon.com : Dyna-Gro DYFOL008 Foliage Pro, 8 oz, White : Fertilizers : Patio, Lawn & Garden

FEED ME! MSU Orchid Fertilizer

https://amzn.to/3KaBs2I

General Hydroponics Flora Series

https://amzn.to/33ejewo

Orchid/Hoya Foliar Mist

https://amzn.to/3fWrfZP

pH Control Kit by GH

https://amzn.to/3qmSKBO

Florakleen by GH

https://amzn.to/3r9nKo9

Water filter- removes over 99% of minerals and comes with water testing device

https://amzn.to/3fOxG13

The Complete Guide

Essential Nutrients

Overview

The 16 essential nutrients required for plant survival are carbon (C), hydrogen (H), oxygen (O), nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S), iron (Fe), manganese (Mn), boron (B), molybdenum (Mo), copper (Cu), zinc (Zn), chlorine (Cl), nickel (Ni), cobalt (Co), sodium (Na), and silicon (Si). They can be categorized as micronutrients and macronutrients. Carbon, hydrogen, and oxygen are taken in through the air, and the rest come from the soil. Nutrients serve numerous roles within the plant and are a vital part of metabolism. Some roles include osmotic pressure regulation, membrane permeability, enzymatic activity, maintaining cell-sap concentration, electron transport system, and buffers to name a few. The 3 nutrients that we are most familiar with are nitrogen, phosphorous, and potassium... NPK. These are the 3 numbers displayed on fertilizer labels. Those values are percentages of each contained within the concentrate. The ratio of each nutrient to the others is more important than the actual value because the concentrate can be diluted as desired. Nutrients must be available in forms that plants can actually absorb. Organic fertilizers typically do not contain nutrient forms that are ready for uptake. They have to be broken down by organisms in the soil to the forms needed. Synthetic fertilizers generally supply nutrients in forms that are ready for uptake. This is important to consider when thinking soil vs. soilless media. Generally, soil contains organisms and soilless media does not (initially). Soilless media can develop microbes over time through water and air and by inoculation. Once the nutrients are available in the right forms, they can be absorbed through multiple mechanisms. Each essential nutrient plays a specific role, and a deficiency in any one of them has dire consequences. 

Nutrient Uptake

Plants can uptake nutrients through a couple different mechanisms. Most nutrients are absorbed through mass flow. Mass flow delivers nutrients to the roots through the movement of water. With this mechanism, water moves through evaporation, percolation (water draining through the soil when watering), and transpiration (plant exhaling water vapor).  Another way is root interception. Root interception uptakes nutrients through direct contact with the roots and is important for nutrients that don't move well through the soil. Things like mycorrhizae (fungi) that further develop the root mass can increase the amount of root interception. This mechanism is especially important for phosphorous uptake. The last mechanism that I will mention is diffusion. Nutrients move from high to low concentrations... when fertilizer is added to the soil, it is present in higher concentrations than that of the plant, so it moves towards the roots to be absorbed until an equilibrium (equal concentrations within soil and plant) is established. 

Micronutrients

Micronutrients are required in lower quantities. They include iron, manganese, copper, molybdenum, zinc, boron, chlorine, and nickel. Most of these elements assist with important enzymatic reactions.

Macronutrients

Macronutrients are required in larger quantities. They include carbon, hydrogen, oxygen, nitrogen, phosphorous, sulfur, potassium, calcium, and magnesium. These elements are needed for normal growth and reproduction. Macronutrients can be further subcategorized into primary and secondary macronutrients. 

Primary macronutrients include nitrogen, phosphorous, and potassium.

Nitrogen is utilized in protein synthesis and energy metabolism. It is available in a few forms, and the important one to remember is nitrate as this is the form plants can absorb. Nitrogen is also needed for photosynthesis and production of chlorophyll. Lastly, nitrogen assists with cellular division. Due to nitrogen's functions, a deficiency causes loss of strength and color. This is seen in an ascending pattern from the bottom to the top of the plant... stunted growth and yellowing to dead leaves. 

Phosphorus encourages root growth and flowering. It is needed for overall plant health but is more vital during a plant's flowering phase. It also is used in energy transfer/storage, increasing a plant's hardiness to withstand environmental changes, and photosynthesis. A deficiency leads to improper flowering and brown, wilted leaves. 

Potassium is used in water management, chemical transport, photosynthesis, cell vigor, blooming, glucose and enzyme synthesis, and nitrate uptake. Due to these functions, it also is important in the plant's hardiness to withstand environmental changes. Potassium deficiency causes less tolerance to change in situations like drought, frost, and disease/pest attacks. It also decreases the uptake of other vital nutrients and causes dark stains to form on leaves. 

Secondary macronutrients include magnesium, calcium, and sulfur.

Magnesium is needed for photosynthesis since it is the foundation of chlorophyll. It also aids in the uptake and transport of phosphorus, carbohydrate storage, and activating enzymes. A deficiency results in weak stems, older leaves turning pale, and yellow/brown spots on leaf veins while veins continue to be green. 

Calcium strengthens and encourages the formation of cell walls. It also is vital for cell production and growth and root development. Lastly, it neutralizes harmful chemicals and stabilizes various reactions. Deficiency causes yellow/brown spots and slow growth. 

Sulfur assists in the production of chlorophyll, protein synthesis, and tissue development. It is also important in maintaining defense mechanisms. Sulfur deficiency is not common. The main sign being pale leaves.

Organic vs. Synthetic Fertilizers

Organic

Organic fertilizers are generally considered slow release. They require microbes within the soil to breakdown nutrients over time in a process called mineralization to forms that the plant can absorb. For example, microbes can convert the organic nitrogen in manure to inorganic forms such as ammonia and nitrites/nitrates that are usable forms. These fertilizers are less likely to cause damage from overfertilization because the organisms regulate the breakdown of nutrients. Organic fertilizers are generally more suited for growing media that contains soil. Since soil is natural, it already contains beneficial organisms like bacteria and fungi. Soilless media does not initially contain the microbes needed to breakdown organic fertilizers.

Synthetic

Synthetic fertilizers are immediate release. They do not require microbes within the soil because the nutrients are already available in usable forms. Due to this property, they are well suited for soilless media. They are more likely to cause damage from overfertilization because there isn't an intermediate step before being able to affect the plant.

(More on overfertilization below)

Diagnosing and Managing Problems

Deficiencies can occur by 3 related issues: incorrect usage of fertilizer (either concentration or frequency), incorrect pH, and incorrect TDS (total dissolved solids). Deficiencies can manifest as chlorosis (lightening), necrosis (dying cells), slow growth, and falling leaves/buds. They can appear in an ascending pattern that affects old growth first, a descending pattern that affects new growth first, or can affect the plant overall. Whether old or new growth is affected first is dependent on the mobility of the nutrient. Some nutrients like nitrogen are able to move freely within the plant (mobile), and other nutrients like calcium can't (immobile). This is important because it provides the reasoning for deficiency patterns. Since nitrogen is mobile, it is able to move from older leaves to newer ones as a protection mechanism. Unlike nitrogen, calcium is immobile, so new growth tends to be affected first.

Nutrients also affect one another in antagonist and synergistic ways. This is shown in Mulder's chart below. If the amount of one nutrient is too high or too low, it can have an antagonist effect on other nutrients (decrease ability to uptake) or a synergistic effect on other nutrients (increase ability to uptake). For example, I previously mentioned that magnesium aids in the uptake and transport of phosphorous; magnesium has a synergistic effect on phosphorous. I like to keep this complex interaction in the back of my mind. It's important in theory, but its practical use is limited. There is not a great way to keep track of nutrient levels and their effects on one another. Just know that if one level is too high or too low, it's most likely affecting other nutrients as well. This is another reason why flushing is important. If you think there is a nutrient imbalance, flushing is a good reset button. Then, you can add a good pH balanced complete fertilizer to correct the issue.

The overall amount of salts/minerals within the soil is important to keep in mind. Having a buildup of minerals over time (or overfertilizing) can lead to fertilizer "burn". The term “burn” comes from the leaves burning from light. When the concentration of chemicals in the soil is too high, water will actually flow out of the plant into the soil/just not be taken up by the plant. This is caused by osmotic pressure. Osmotic pressure is the force that drives the movement of water. Water tends to move until it reaches an equilibrium state (equal concentrations). If the concentration of chemicals within the soil is higher than that of the plant, water will move out of the plant into the soil to reach equilibrium. I'll give some other examples. This is the way salt increases blood pressure. Increased concentration of salt in the blood draws water into the blood and increases the pressure. This is why things absorb water when soaked. Putting sugar on strawberries makes like syrup... the water from the strawberries gets drawn out and mixes with the sugar… the leaves burn because the leaves lose water and don’t have that protection against evaporation and light drying/damaging it.

Overfertilizing/salt buildup kind of looks like the signs of overwatering. This could be root rot, brown tips/edges, brown spots, yellow/wilting leaves, slow growth, mineral build up on top of the soil. The leaf margins are generally affected first as these areas are the first to lose water. 

Nitrogen Deficiency

Nitrogen deficiency is important to learn about because it is the most common. The main sign is yellowing leaves that begin with the oldest leaves and ascend towards the new growth over time. If caught in time, adding a synthetic fertilizer (immediate release!) will correct the issue. Using a weak complete fertilizer with each watering or using organic fertilizers (in soil) should prevent this issue. 

Other Deficiencies

Most of the other deficiencies are hard to distinguish from one another. A soil analysis can be performed to confirm deficiencies. After nitrogen, calcium and magnesium are common deficiencies. Some people supplement with CalMag, but this isn't completely necessary if using a complete fertilizer. Supplements of other nutrients can be given if other deficiencies are suspected. Start low and go slow if not sure. The below picture illustrates the various deficiencies pretty well.

Incorrect pH

The pH of your fertilizer water should be between 5.5 and 6.5. If the pH is outside this range, the plant will not be able to absorb nutrients correctly. Even if you have the correct amount of fertilizer and use frequency, a deficiency can occur if the pH is off. The pH can be adjusted using General Hydroponics' pH control kit. The kit comes with the solutions pH up, pH down, and indicator. The pH indicator solution turns colors based on the pH of the solution it's applied to. The color is compared to the pH color chart to estimate the pH. Adjustments can be made until the correct color is shown (yellow). You can alternatively use a pH meter which may be easier.

Incorrect TDS (Total Dissolved Solids)

The amount of minerals dissolved in your watering solution is important. This gives you an idea about the amount of nutrients available for your plant to uptake. The TDS comprises the minerals you started with in your water plus anything added.  A basic TDS meter can be purchased on Amazon or local hydroponic stores for less than $10 (link above). A TDS of 800-900 is good for vegetative growth in hydroponic systems. Above that, there will be too many nutrients available which will change how nutrients are absorbed. Below that, there will be a deficiency. I honestly don't know the best value for other systems but measuring your TDS at baseline in the water your using is helpful.  You can also use a TDS meter to compare different fertilizers and their concentrations. Once you find a TDS value that works for you, you can measure your nutrient solution to check if it’s made the same every time. Then, you can let others know what your value is! Using water with a low TDS is important because there is more ability to add the correct nutrients before overloading the solution's appropriate mineral capacity. 

Using Fertilizer

My 3 favorite fertilizers on the market are Dyna-Grow Foliage Pro, FEED ME! MSU Orchid Fertilizer, and General Hydroponic Flora Series. They can be diluted to the measurements mentioned above. Use filtered water, if possible, to minimize unwanted minerals. Flood your pot with the mixture and allow it to drain completely (remove any standing water after). You can use your pH meter to compare the water that drains to your fertiliizer solution. If the pH is higher within the runoff, there's most likely mineral build-up. For GHFS, follow instructions on package closely. Flora micro has to be mixed into your water first to prevent nutrient lock out. Mix solution after each nutrient solution is added. Correct the pH for all fertilizer solutions. I recommend using GH pH control kit to do this. Flush your growing media about every 4th watering with plain water. To do this, flood the pot with plain water and wait for it to completely drain, repeat 1-2 more times. This removes any excess minerals and prevents damage from mineral buildup. Florakleen can be used at this time as well as an additional mineral cleanser. It's generally best to water/fertilize in the morning, so that your plants can utilize the water and nutrients to the best of their ability. Fertilizers can be used as foliar sprays for plants to uptake nutrients through their leaves instead of their roots. This results in a quicker uptake of nutrients. Some plants absorb nutrients through their foliage better than others. I don't use foliar sprays on my aroids, but they could be helpful when correcting deficiencies. I use Miracle Grow Orchid Plant Food Mist on my Hoya and orchids about once every week or two, and they seem to love it. 

Hydro/Semi-Hydroponic Systems

Hydroponic systems generally use the highest concentrations of fertilizer when comparing growing media. Semi-hydroponic systems and systems without recirculating water use a bit less but still higher than non-hydro systems. Most fertilizers have different instructions based on use. As previously stated, the TDS is a good measurement for checking if the minerals in your nutrient solution are at the correct concentration. Shoot for around 800 for vegetative growth. As a side note, this takes into account any other supplements added like super thrive. If using supplements containing minerals, the concentration of your complete fertilizer should probably be a bit lower so that your TDS is still in a good range. As plants uptake nutrients, your TDS meter can be used to check when more nutrients need to be added and how fast/much uptake is occurring. As the TDS gets down to 400-600, more nutrients can be added to get back up to 800. After doing this 2-3 times, (let’s say on the 4th time) you can flush the system with plain water or Florakleen and replace your nutrients. If there is algae/bacteria/fungal overgrowth, you can sterilize the system with a diluted hydrogen peroxide solution (100-150ml of 3% hp per gallon water). You can also add 10-15ml of 3% hp per gallon of water in your hydroponic solution to prevent microbial growth (don't use if inoculating beneficial bacteria). You also should be sanitizing all equipment at about the same time frame.

Tips, Tricks, and Takeaways

The NPK ratio is more important than the concentration, higher N and K is better for foliage growth (aroids)

Water weakly weekly (dilute correctly) ... It's a common saying but it's true 

Flush like every 4th watering or so to prevent salt buildup (it's easy to forget and end up with leaf damage)- especially in growing media like moss where minerals accumulate easily

Look out for signs of improper fertilizing but be aware that they may look similar to watering issues and some pests/diseases 

Use a complete fertilizer as previously mentioned

pH test/adjust your fertilizer water if you have the means to

Use distilled or filtered water if available

Test your soil if you aren't sure 

FAQ

Should I use less fertilizer in the winter?

This depends on the conditions your plants are in. If your plants are getting less light, their growth will slow down. They probably won’t properly utilize the fertilizer added and are more likely to burn. You can either stop fertilizing or dilute your fertilizer to ¼-½ strength when you notice growth slowing down and/or the plant requiring less frequent watering. If you notice signs of fertilizer burn, stop fertilizing or dilute further. 

Does the pH matter?

Yes, the pH shows how acidic or basic your solution is. Most tropical plants like their pH to stay around 5.5-6.5. If the pH is outside this range, plants will have a hard time utilizing nutrients.

What about the kind of water?

Distilled water is usually the best but can get expensive especially as your collection grows. Rainwater and reverse osmosis water are your next best option. Then, filtered water through a Brita or my favorite zero water filter. If using tap water, test the TDS (total dissolved solids) level and check what is added to your local water. Chlorine will evaporate from tap water if set out for about a day. Some water facilities add chloramine instead of chlorine, and this is does not evaporate and can be toxic to plants. I do not recommend using tap water if it has a high amount of minerals. I'm not going to give an exact cutoff number, but the more minerals, the less fertilizer that will be able for your plant to use. You can use a TDS (total dissolved solids) meter to get an idea about the amount of minerals in your tap water. Most foliage plants thrive around 800-900ppm. That number results from the combination of the baseline minerals in your water plus any additives. If your baseline from the tap water is already at 500ppm, you have less room to add fertilizer to reach that 800-900ppm goal, and the proportions of the nutrients would be off as well. 

Do all plants need fertilizer?

No, there are some plants that are very sensitive to fertilizer and don't do well with it. Carnivorous plants come from nutrient poor bog environments and are very sensitive to fertilizer. Do not fertilize your carnivorous plants; they get nutrients from bugs and other organisms. Begonias tend to like less fertilizer than other plants. Hoyas and orchids do well with foliar fertilizers (I like orchid mist)

Is it a fertilizer issue or something else?

Overfertilizing can look like a lot of other things (watering issues, pests/diseases). I would generally say that overfertilizing is less common than other issues. My approach would to be to rule out other issues before determining a fertilizer issue. I would check the soil for root rot/watering issues and check the leaves and petioles over with a magnifying lens. If other causes are ruled out, then nutrient issues can be further investigated. 

Some information from this post was sourced from Macronutrients in Plants: Role and Functions (collegedunia.com) and Essential Nutrients for Plants - How do nutrients affect plant growth? (tamu.edu)