Soil with a high concentration of organic matter, like peat moss or compost, is made up of materials that have been broken down over time. As a result, soil with high levels of organic material tends to retain more moisture than soil with a low concentration of organic matter. Cactus soil is primarily made up of inorganic materials, which also provides it with an additional benefit: preventing root rot.
The primary mediums used in cactus soil are crushed rock, such as pearlite or perlite, and sand. These dense materials allow for good drainage and aeration.
A good potting mix for cactus plants contains two-thirds inorganic material, such as sand or perlite, and one-third organic ingredients, such as peat moss or compost. To make an ideal cactus soil, combine regular potting soil with sand and perlite to provide drainage and air circulation.
Organic matter – compost, peat moss, or shredded bark – is necessary for the soil of a cactus plant because it retains just enough water to keep the roots from drying out, but not so much that the soil becomes soggy.
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What is Peat Moss?
Pronounced “peat moss,” this dark brown material comes from peat bogs. Peat bogs are wetlands in which the natural process of decaying plants and animals creates waterlogged soil that has a high moisture content. Its fibrous, crumbly texture makes it a good material for use in gardens and on plant beds, where it retains water. This is because peat moss contains lignin, a substance that helps the plant cells bind together.
“You could compare peat moss to a sponge because its large cell structure helps it to absorb water.”
By Amanda Buynak
Alternatives to Peat Moss those will not Retain Water
Here are some of the best alternatives to using peat moss:
Coco coir — As an environmentally responsible gardener, you’re careful about the products you use in your landscaping. You may even have stopped using peat moss because it’s harvested from wetlands, where it can destroy plant communities. But now you’re looking for a good alternative to peat moss and wondering what else you could use as a soil conditioner.
Coco coir is one of the best alternatives to peat moss. The fibrous husks of coconuts are waste products from coconut processing. This durable substance is so similar to peat moss that it is also called coconut peat. And because cocoa coir comes from a renewable resource, it’s an eco-friendly alternative to the fossilized remains of plants that are used in peat moss manufacturing.
Compost – Compost is a natural, organic product that improves the soil by providing nutrients and aerating the soil. It can be used in combination with peat moss or alone. The best compost is made at home from your own yard waste such as leaves, grass clippings, and vegetable scraps. Compost is easy to make at home and you save money since compost is less expensive than peat moss.
Wooden Materials— Wood mulch is available in various forms, including shredded, chipped, and whole chips. Wood can be a great addition to a garden because it contributes nutrients to the soil, helps with drainage and aeration, and makes beds look tidy.
The Physical Structure of the Soil Retains Water.
Porosity is the easy part. Total soil porosity is simply the percentage of the soil volume which is air. Here are some typical values for various soil components:
| Soil Components | Porosity percentage |
| Peat | 80%-95% |
| Clay | 50%-70% |
| Loam | 40%-45% |
| Sand/grit | 25%-40% |
| Perlite | 80%-95% |
| Pumice | 70%-85% |
| Calcined clay | 60%-70% |
| Calcined diatomaceous earth | 70%-80% |
To understand how much water is needed to sustain plant life, it helps first to understand the role of porosity in a soil’s ability to hold water. Porosity refers to the amount of space between soil particles and can be thought of as the “open spaces” in soil. Soil with high porosity drains quickly because there are many open spaces between tiny chunks of soil or minerals.
Porosity is often expressed as a percentage—in a low-porous soil, holes between particles make up only 5% of the available volume, while in a high-porous soil, openings occupy 50% or more of the volume.
In reality, the amount of water retained in the soil is dependent on the suction pressure applied to the water. As suction is applied, increasing amounts of water are drained away until a set amount remains even at high pressures.
This is shown in a curve of water retention. From the left side to the right, is displayed the proportion of water present in the soil in terms of volume. The vertical axis displays the suction or pressure on the water, which means that at the bottom, there is saturated soil, while at the top is soil that is completely draining (but not completely dry).
In soil, water that remains with very high pressures is unable to be extracted by the plant roots. This pressure is referred to as the wilting threshold because once soils are at this point of dryness, the leaves of plants begin to wilt. It usually occurs around 1000 on the graph of water retention.
The coarse, fluffy peats shown in this graph also contain huge amounts of water, while the finer and decomposed peats act more like clay soil.
Fine, claylike materials are the least able to hold onto the water; these are known as “sands.” Loamy, sandy soils are called “clays.” Coarse, fluffy materials can hold large amounts of water—these are called “humus” or “muck.” Finally, peat is a unique material that can be either coarse and flaky like soil or fine, friable, and crumbly like hummus.
In open ground, the typical average water retention corresponds to the 10kPa mark on the graph, but in a pot, they correspond to somewhat below the 1kPa mark.
5 Components of Cactus soil: Does not Retain Water
- Pumice
Pumice is another volcanic substance ideal for use in a cactus soil mix. Pumice is created when lava rapidly cools and shatters into tiny pieces. It can be used on its own, but it works best as part of a mixture—for example, it’s often combined with perlite to create a soil that drains well while still retaining moisture. Up close, small pieces of pumice look porous due to hundreds of tiny holes. This means that water easily moves through the spaces between the tiny pieces and drains out just as quickly.
- Perlite
Cactus plants are native to dry environments so they require well-drained potting mixes that allow for proper aeration. Perlite allows for this by providing a large number of small air pockets within the potting mix.
Perlite will help keep the soil at the surface dry while also keeping the roots wet, which prevents the roots from rotting. Perlite will not stop the plants from soaking up water when they need it, but it will make sure that they have proper drainage so that too much water does not stay at the surface of the soil for too long.
- Sand
River sand is fine-grained sand that is typically used as a drainage medium. It can be found under several names, including river sand, sand from the Rhine, the Loire, or the Seine. The sand must be of large grain size, at least 0.4 mm or a little more. Coarse sand and fine gravel improve drainage.
- Vermiculite
Vermiculite is a mica-like mineral that expands when heated. It has been used in agriculture and horticulture as a soil conditioner. The vermiculite layer lightens the substrates and therefore the aeration. This has the effect of improving root development.
- Gravel
Gravel and grit are essential ingredients for cactus potting mix because they provide drainage and air circulation—two important factors in keeping a cactus healthy while preventing root rot. Without adequate drainage, your cactus roots will sit in water for long periods of time, which will cause them to rot. Without air circulation, your cactus roots will sit in stagnant air for long periods of time, which will also cause them to rot.
“ Ideally, you can add any inorganic substance if it doesn’t retain water and creates loose soil.”
Regular Potting Soil Retain Water
Cacti are a unique type of plant that requires special care. Using potting soil for cacti is not an ideal choice because it stays wet for too long and retains too much moisture. Most potting mixes contain peat, compost, or bark that holds moisture. Cacti are susceptible to root rot and disease if they grow in too much moisture.
The best soil for cacti is a cactus mix or cactus soil, which contains extra perlite to absorb excess water and drainage holes to allow excess water to drain. Perlite is a lightweight volcanic rock that helps with aeration, the movement of air in the soil. It is an essential component of most modern potting mixes because it improves drainage and reduces compaction.
Another reason not to use regular potting soil for growing cacti is that it compacts quickly. The dense structure of the soil takes longer to dry out, and the wet soil begins to rot the roots of the cactus. In addition, compacted soil is poorly drained.
Why Is My Cactus Soil Not Draining?
If you notice that the roots of your cactus are circling around the edge of the soil, it’s a good idea to repot it soon. The plant is choking itself and not getting enough water or oxygen. Plus, if you leave it in there, the roots will continue to grow and circle until they’re at least 1″ from the edge of the soil. The plant can’t survive like that because there will be no room left for new roots to develop.
“Bound roots can prevent proper drainage.”
Summary
- Don’t use peat. It will retain water in your soil mix. And finally, it will cause root rot.
- More organic materials in cactus soil will lead to more water-retaining in your mixture.
- Commercial potting mix retains water too much because of peat moss and compost.
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