Can You Grow Cactus in Compost Only? [We Did It – Experiment]

Several newbie gardeners have asked if they can use pure compost in their cacti plants. To provide an actual answer, I performed a 60-day experiment of growing cacti using compost as the only planting medium.

Growing cactus in compost only harm cactus growth. An experiment conducted on cacti shows brownish-yellow burnt spots on the stem and damaged roots, after 40 and 60 days of being grown in pure compost, respectively. The drawbacks of compost include 1) high water retention, 2) being too light, and 3) poor drainage.

While compost is one of the viable organic components in cacti soil mixes, it is not ideal to use it alone as a growing medium. Read along to discover what might be the problems of using pure compost for cacti plants through an experiment.

Cacti In Pure Compost: Experiment

I conducted an experiment to assess whether using pure compost for cacti is suitable. Four cacti of two different species were used as specimens. They are all healthy and actively growing from the onset of the experiment, and I made sure they don’t have any diseases.

Cacti Specimens

These are the cacti used in the experiment:

• A pair of Ferocactus or Barrel Cactus- can handle steady watering but susceptible to diseases
• A pair of Echinopsis or Domino Cactus- prefers regular watering and is not prone to diseases

Each cacti pair consists of one control specimen and one test sample. It is essential to have a control or reference specimen to properly analyze and assess the changes that the test sample has undergone.

Experiment Setup

The test specimens were potted using pure compost (wood chips, vermicompost, and carbonized rice hulls) as the only medium. On the other hand, the control specimens were potted on the regular cacti mix (pumice, carbonized rice hull, and vermicompost).

The four specimens were set up inside the greenhouse and positioned side-by-side for easy comparison. Both the control and test samples received the same amount of sunlight. They also get the same regular watering once every week.

Growing Cactus on Compost: Our Test Results

Results show that the Ferocactus test sample exhibited yellow to brown patches on the bottom after 40 days with softened mushy root tips after 60 days. However, the Echinopsis have a slight sign of dehydration on its stem, and it developed brittle roots at the end of the test.

Ferocactus Species

After 40 days, the Ferocactus manifested brownish-yellow spots on the bottom just above the potting medium. The discolored patches appeared to be wrinkly and dry, which are signs of fungal diseases due to prolonged and excessive moisture in the compost.

On the 60th day, a week after the last watering, I uprooted both the control and test samples. Here are some of my observations.

Sample	Observation
Ferocactus (control sample)	Dry potting soil mix Healthy roots with visible new growth
Ferocactus (test sample)	Damp compost medium Main roots are brown and soft, root tips are mushy

The brown mushy roots of the test sample indicate a sign of overwatering caused by the moisture that is retained by the compost medium.

Echinopsis Species

After the experiment, the Echinopsis test sample does not develop any signs of diseases on the stem but has a minimal indication of dehydration (wrinkled bottom). The roots become brittle and withered, and only a tiny portion is left when it was uprooted.

The excess moisture in the compost damages the Echinopsis’ roots and cause them to rot and break. As the roots deteriorate, they no longer absorb water effectively which eventually leads to dehydration.

Can Cactus Survive In Pure Compost Medium?

Cacti will not last long if planted in compost alone. It will damage the cacti roots due to the excessive moisture content that leads to fungal disease. In addition, cacti will be prone to rot because the compost tends to stay moist for long, which is not favorable for the majority of cacti.

Compost is purely derived from organic materials that were set to decompose. The dark color of compost is due to the carbon produced when organic materials decompose. Since it contains purely organic matters, it has a moist property, compacts if pressed, and does not crumble easily.

Although cacti will benefit from compost, I do not recommend using it as the only potting medium because of its water-retention property. I use compost as a soil amendment to formulate my DIY cacti soil mix. Remember that cacti require a loose medium that drains well but holds a bit of moisture.

Problems Of Using Pure Compost On Cacti: 3 Drawbacks

The problems of using compost only for cacti plants are due to its 1) high water retention, 2) too light characteristics and 3) poor drainage.

I always considered compost as a “black gold” in the garden due to its several benefits for the plants. Some of my plants grow well in plain compost alone, but not my cacti collections. Why? Read more below to know what the drawbacks can be.

1- High Water Retention

One major problem with compost is its high water holding capacity. Wet compost takes a lot of time before it dries out and stays wet for an extended time. Hence, it will cause cacti roots to rot and pathogens to develop underneath.

Since compost is purely organic, it will absorb an impressive amount of water. But most cacti have delicate roots that will rot easily with prolonged exposure to moisture. Moreover, damp soil is a favorite breeding ground for pathogens and pests that damage cacti roots.

Diseases such as fungal rust and scab are likely to appear on the stem, which can spread rapidly from one plant to another. They appear as dark patches on the cacti’s epidermis.

2- Being Too Light

One physical characteristic of compost is that it is too light. This will lead to a stability problem for cacti to anchor and develop their root systems.

Most cacti roots are shallow and spread sidewards that they need something rigid where they can hold on. Compost cannot provide better support for cacti roots because it is too light to secure them in place.

3- Poor Drainage

One last thing is that pure compost has a poor drainage ability. Compost on its own tends to compact over time, filling the empty spaces in between and blocking the passage of water during rehydration. Hence, it stays wet for long, which is unfavorable for the cacti roots as they can rot easily.

While incorporating compost into the potting soil improves drainage, using it alone as a growing medium reduces its ability to promote water flow. Watering compresses compost tightly, and waterlogging is likely to happen. The ultimate cacti killer is a medium that stays wet for an extended period.

Does Watering Less Help Cacti Grow In Pure Compost?

Watering less does not help the cacti survive in a compost medium. It can reduce the risk of overwatering but cacti roots will become weak as there is not enough water to absorb.

Moreover, giving cacti inadequate water is not good for the root system because the roots will shrink when they cannot absorb enough water. Hence, watering less will restrict root development and cause dehydration to the plants.

Good Compost For Cacti

A suitable compost for cacti is consists of completely decomposed plant matter and worm castings. Compost that contains animal manure is not recommended as it may have pathogens that can harm the plant.

A good cacti potting mix must include a small portion of an organic base like compost to help with water retention. Ideally, cacti potting medium may contain 20- 30% of organic compost to hold just enough moisture for the roots. You can find below a good compost you may use for your DIY cacti mix.

Summary In Using Pure Compost For Cacti

• Cacti cannot grow well in compost only because it holds too much water that stays wet for an extended period and results in fungal diseases and damages roots.
• Using pure compost has drawbacks on cacti plants because of its high water retention, too light property, and poor drainage.
• Watering less may curb the tendency of cacti to turn mushy, but the chance of getting dehydrated and brittle roots will become the problem.

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Sources

• “Compost Increases The Water Holding Capacity Of Droughty Soils,” Michigan State University