Feed your cacti plants wrong, and you’ll end up killing them. Despite my years of experience, I decided to help educate gardeners about feeding cactus with a 36-days long experiment on cactus fertilization. The results are just amazing!
The rule of thumb, potted cacti need to be fertilized a maximum of twice a year with a balanced fertilizer. If more fertilizer is applied, cacti can show effects such as 1) burning of leaves, 2) withered stems, and 3) burnt roots as early as a month from the fertilizer administration.
Quite a few cactus collectors asked me several questions regarding fertilizers. Like, why should I fertilize my cactus? Or how and when should I apply fertilizer? If you also have the same dilemmas, I got it all answered here.
Table of Contents
- 1 The Experiment Preparation
- 2 Overfertilization Experiment Results – String of Tears
- 3 Overfertilization Experiment Results – Zebra Plant
- 4 When To Fertilize Cacti?
- 5 How To Apply Fertilizer to Cacti?
- 6 Can Cacti Live Without Fertilizer?
- 7 Why Should You Feed Your Cacti?
- 8 What Nutrients Do Cacti Need?
- 9 Do Indoor Cactus Need Fertilizer?
- 10 Is Tomato Feed Suitable for Cacti Use?
- 11 What Fertilizer Do Cacti Use?
- 12 Takeaways
- 13 Sources
The experiment required four actively growing arid plants that hadn’t been fertilized for almost six months. They spent all their time inside the greenhouse.
The four potted plant specimens are:
- One pair of Senecio herrianus ‘String of Tears’ – prefers low nutrient fertilizer once every year
- One pair of Haworthia Fasciata ‘Zebra Plant’ – prefers low concentration fertilizer once or twice a year.
Each pair of plant specimens are identical. One is used as a test sample, while the other is the reference plant. Having each sample in pairs helps determine possible changes and evaluate the effects of overfertilization.
The arid plant samples are alongside each other, remaining on the same spot in the greenhouse until the end of the experiment. Each pair requires fertilizer with low nutrient concentrations.
All cacti received the same sunlight exposure. The amount of water they get once a week is also the same. Hence, the only difference is in the fertilizer provided.
Every week, the test samples will receive a pinch of pellet-type fertilizer, while the reference sample will have no fertilizer in the entirety of the experiment.
The fertilizer I used was the ICL Osmocote complete slow-release pellet fertilizer like the one below from Amazon.
To monitor any changes, I snap photos of each sample every week or in instances when I observed sudden change.
The experiment used the complete (NPK equal to 14-14-14) slow-release Osmocote pellets. These were added on top every week. Since the fertilizer is slow-release, the effects were not visible until the 30th day. The result shows 1) Brown wilted leaves, 2) withered stem, and 3) burned root system.
After the experiment, I uprooted the String of Tears and found that all the roots were brownish-black, not brittle, and soft to touch (lower right photo above). The roots absorbed too much nutrients released by the coated synthetic fertilizer, causing it to burn and die down.
As you can see from the photo above, some coated pellets are in contact with the stem and leaves of the plant. This is something that must be avoided.
The prolonged direct contact of fertilizer pellets with the leaves and stem of any plant will cause burning.
It is also apparent that the test sample has no new growth, while the reference sample is profusely growing. There is no growth development on the test sample due to the damaged roots, impeding the transport of essential growth factors.
The effect of improper fertilizing on the String of Tears causes burning after 30 days. However, in the case of the Zebra plant, the burning of leaves was observed earlier.
The zebra plant test sample developed a hint of burnt leaf after ten (10) days. The tip of its pointy leaf slightly turned brown and became more notable on the 36th day.
The timelapse results on the test sample manifesting burnt leaves (red circles photo above). The burning intensifies until the 36th day where the leaves appear blackish-brown and wrinkly (see magnified view).
As early as ten days, the leaf shows a speck of wilting tip that becomes more obvious as I keep on feeding the plant. Although wilting of the tip may sometimes be caused by underwatering, this is not the case as the plant receives regular watering.
To gather more evidence, I uprooted the zebra plant after the experiment to check the situation of the roots. Like the string of pearls, the root systems of the zebra plant are almost dead. Although intact, it looks stressed and blackened.
In conclusion, the experiment has clearly shown that too much nutrients and improper application of the pellet fertilizer can harm the plant by burning the plant tissues, from the root systems up to the stem and leaves.
The right time to feed cacti is when they start to show new growth. This depends on the variety as every species has a different growing season- ideally, one dose of phosphorus-rich fertilizer before spring. Cacti, in general, should not be fertilized more than twice a year to avoid burnout.
When my shop customers ask when to fertilize their cactus, my answer would usually feed them with the right amount of suitable fertilizer when the plants are beginning to grow actively. If you want to see your cactus bloom profusely, provide them with phosphorous-rich fertilizer before spring.
I would always recommend fertilizing at the start of the growing or flowering season rather than at the end. In this way, the cacti can take in the nutrients efficiently throughout the season. If you apply fertilizer when the season is nearing an end, the cacti would not have enough time to absorb and use them before the season of dormancy and slowed growth.
The feeding frequency depends on the fertilizer you are using, so make sure you follow the recommended dosage at the back of the packaging. As a general rule of the thumb, fertilize your cacti once or twice a year. When you are using a product that must be diluted, lower the concentration than the product recommends so you won’t burn your precious cacti.
To fertilize cacti opt for a complete or balanced slow-release with lower nutrients, one that contains all the three primary nutrients NPK in its label (like a 10-10-10 or a 2-7-7). Fertilizer can be diluted, sprinkled, mixed, or buried depending on the label instruction of the manufacturer.
Generally, your hint when applying fertilizer is the growth speed of your plant – fast-growing cacti tend to require a lot of nutrients to support their growth. However, slow-growing cacti are likely to take in steady but lower nutrients than fast-growing ones.
Liquid and powder form fertilizers need to be diluted as per label indications. This is recommended for fast-growing cacti.
Before fertilizing your cacti with diluted fertilizer, you need to water them thoroughly. A saturated medium allows the nutrients to be absorbed easily by the roots. While fertilizer brands indicate the proper dosage in the label, I recommend diluting it slightly more for a weaker concentration to be on the safe side.
Suppose you are using this fertilizer on Amazon. The label indicates seven drops of the liquid to one quart of water. You need to add a bit more water, say a quarter more, to make it a little less concentrated.
Organic fertilizer is applied by either incorporating it in the soil mix or sprinkling a pinch above the soil mix. Synthetic granules and prills are used on top or buried near the roots.
A small portion of organic fertilizer can be mixed in the potting medium. Please be careful when adding organic fertilizer, as too much in the potting mix may retain more moisture making the substrate damp for an extended period.
The slow-release synthetic prills can be either sprinkled around the cacti stem or buried just below near the roots. If you use the complete triple 14 slow-release prills, apply 5-10 pellets on top of the soil or bury them underneath. Although they are slow-release, an excessive amount can lead to burning. Do not stick the prills on the stem and roots as it will also cause burns, just like in the experiment.
Cacti grow just fine without getting additional supplements from fertilizers. However, cultivated not-wild cacti would appreciate a nutritional booster for healthier growth, bud formation, disease prevention, and fruit development.
There are seemingly two sides of opposing ideas regarding fertilizing cactus – to apply and not. In their natural desert habitat, cacti thrive in a dry and nutrient-poor environment. They only get infrequent nourishment during rainfall. Most people believe that cacti do not need a nutrient-rich growing medium to survive. And this is the reason why so many people are against fertilizer” for cacti.
However, based on my years of experience in growing cacti, it was clear that they thrive with sporadic fertilization. Cacti in cultivation or those planted in pots have different growing demands from those growing in desert lands. You can’t expect cultivated cacti to be the same as their wild counterparts.
Understanding why you should fertilize and how fertilizers work is the better approach here so you can decide whether you should apply them to your plants or not.
While cacti don’t technically need much nutrients to survive, feeding them with the proper nutrients at the right season and in the appropriate way will 1) replenish lost nutrients and 2) Boost their overall health.
In the long run, the nutrients in the potting medium will be depleted and absorbed. Meaning, if the cacti have been planted on the same container for some time, chances are the soil probably runs out of nutrients.
If this is your case, think about feeding your plant to rejuvenate their soil with fresh nutrients. Another way of supplying new nutrients for your cacti is to re-pot them on a fresh medium once they have outgrown their container.
However, if you have slow-growing cacti, it will take longer before they fill up their pots, and that they might be just needing an infrequent dose of fertilizer.
Supplementing cacti with suitable nutrients will improve their outward appearance and give them the vigor to endure environmental stress like harsh temperature and light levels and pests or diseases.
Cacti are very resilient plants and are very well known to cope with environmental stress very easily. Extreme temperatures and light levels can cause them stress, like developing vibrant colored stems. With a small level of fertilizers, they can handle stress easier, even more increasing their resilience.
The nutrients provided by fertilizers can strengthen cell walls, making the plant stronger against disease and pests. Other nutrients contribute to photosynthesis and help the cacti to develop bright, lively colors. If your cacti aren’t active, even if it’s already their growing season, it might be helpful to dash them with balanced nutrients to encourage growth.
Furthermore, if you want your cacti to produce stunning flowers, add nutrients to enhance their blooming.
Cacti can survive with little to no nutrients. However, potted cacti, as well as any other plants, need the following macronutrients:
- Nitrogen (N),
- Phosphorous (P), and
- Potassium (K)
Other nutrients are equally necessary but are used in much smaller quantities.
Aside from Carbon, Hydrogen, and Oxygen that your cacti receive from the air and water, they also need other essential nutrients on their soil for their proper growth. The most important ones that cacti use are Nitrogen (N), Phosphorous (P), and Potassium (K).
Calcium (Ca), Magnesium (Mg), and Sulfur (S) are equally vital but only required in much lesser amounts. Another class of nutrients is called micronutrients. They are essential but needed in a relatively low concentration.
Cacti can grow normally until they run out of at least one of these nutrients, limiting their growth. If there is an excess of these nutrients, the problem will arise, leading to the deficiency of other elements.
Here is a table of the three essential nutrients, their functions, and the effects of excess and deficiency in cacti.
|Nitrogen (N)||– Promotes stem growth|
– Aids in cell formation & photosynthesis.
|– Soft tissues|
– High risk to diseases and pests.
– Burning on stems and leaves
|– Reduced growth|
– Lightening usually at the base
– Woody stems
|Phosphorous (P)||– Important for seed germination|
– Promotes root growth, flower, and fruit development.
|– Damage roots|
– Potassium deficiency
|– Absence of blooms|
– Poor root growth
– Early bloom drop resulting in no fruit
|Potassium (K)||– Important for storing water|
– Vigorous growth and stability
– Resistance to diseases/pests
– Necessary for adaptation to stress (drought, cold, temperature)
|– Calcium & Magnesium deficiency||– Stunted growth|
– Paleness/ yellowing
In reference to the above photo, the liquid cactus plant food has an NPK of 2-7-7, which means that the fertilizer has 2% nitrogen, 7% phosphorus, and 7% potassium. The plant food ideal for houseplants contains a balanced amount of NPK, which is 10-10-10.
These liquid plant food samples are suitable for cactus use because all three primary nutrients are present. The only key here is to look at the back label for recommended dilution.
Whether or not to feed indoor cactus depends on the condition of the cactus. If it is starting to grow, give it a dash of suitable fertilizer. In contrast, if the indoor cactus is dormant (often during winter), do not feed the plant.
Before introducing any plant food, consider monitoring and checking the situation of your indoor cactus. Upon checking, and you observed that it’s dull and needs a boost, consider feeding the cactus. Try applying a bloom booster fertilizer (high in phosphorus) if you want to see it blooming.
Tomato feed with higher Potassium and Phosphorus are suitable for cacti use. However, they must be diluted at ¼ to ½ concentration. Since tomato feed is rich in potassium and phosphorus, cacti will likely bloom and bear fruit if properly applied. The best application of tomato feed is from spring to fall when the cacti are actively growing.
Fertilizers can be natural or synthetic origin materials applied to cacti in lower dosages. Natural origin fertilizers are those obtained from nature like compost, bone meal, manure, etc., whereas synthetic origin fertilizers refer to synthesized compounds available in granular, liquid, pellets, etc.
Cacti need a few nutrients to boost growth and strengthen their system. These nutrients are naturally available in the air, water, and soil, but they may not be accessible to potted cacti in particular. You will need to supply these nutrients manually through fertilizers, so your cacti will grow properly.
Natural fertilizers are organic materials extracted from living things or nature. They are obtained from plants or animals.
Examples of natural fertilizers are:
- Animal manure (poultry or cattle)
- Bone meal
- Worm Castings
The amount of N-P-K in natural fertilizers tends to be lower than the synthetic fertilizer and feeds the plant for a long time. They release nutrients slowly, which could be good or bad depending on the variety of cacti you have. Natural fertilizer is just suitable for cacti that need a small concentration of nutrients.
Aside from providing nutrients, they also improve the cacti soil in some way if properly used. Organic fertilizers supply active microbial life, which conditions the potting medium.
Refer to the natural fertilizer items below from Amazon.
Synthetic fertilizers are synthesized chemicals that usually contain only a few nutrients such as nitrogen, potassium, phosphorus, and micronutrients, either compound or single.
Inorganic or synthetic fertilizers have two types: 1) Single/Straight or 2) Compound/Complete. These types of fertilizers come in various forms, such as 1) liquid, 2) powder, 3) granular, or 4) prills. The first three forms are water-soluble, and they are taken by the cacti almost immediately, while the prill form is either applied on top of the soil or buried a little bit near the plant’s roots.
I recommend the synthetic fertilizers below for your cacti.
Unlike natural fertilizers that provide nutrients for a long time, synthetic fertilizers do not last long in the cacti soil. They are fast-acting and absorbed rapidly by the plants. For this reason, you would need to re-apply depending on the recommended dosage and the plant’s requirements.
Single or straight fertilizers contain a single amount or only one source of the primary plant nutrient. They are excellent fixers for specific deficiencies. Like when your plant is deficient in nitrogen, you may apply urea fertilizer. As much as possible, avoid using this type of fertilizer in cacti as there might be a danger of applying too much and may burn your beloved cacti.
Compound or complete fertilizers are those that contain two or three primary plant nutrients combined chemically. They are usually produced in granular or prill/pellet form and are often used by cacti collectors. Complete fertilizers provide the cacti macronutrients and micronutrients that it needs. This type of fertilizer is the most ideal for cacti because you address all the needs.
- Cacti can get by with just a little nutrient. However, feeding is required when you are growing cacti in containers. Over time, nutrients in the potting medium will all be used by the cacti and get depleted. So you need to apply fertilizer to replace the lost nutrients required to support cacti growth.
- Too much fertilization will harm the plant leading to burning leaves, stems, and roots.
- The best time to fertilize cacti is when they are in their active state. Feeding them during this will give the plant a chance to have their optimum growth while building up their immune system.
- Avoid feeding the plant during dormancy state because they won’t utilize the nutrients you provided. Also, avoid fertilizing dry soil.
- “Cactus,” Colorado State University
- “Synthetic Vs. Natural Fertilizer,”Chicago Botanical Garden
- “Nutrient Management: Fertilizers,” Tamil Nadu Agricultural University
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