Tony O’Neill, gardener and author of the popular “Composting Masterclass” and “Your First Vegetable Garden,” combines lifelong passion and expert knowledge to simplify the art of gardening. His mission? Helping you cultivate a thriving garden.
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A member of the asparagus family, spider plants are herbaceous perennial houseplants that originate from tropical West Africa to South Africa.
Spider plant watering frequency depends on the plant’s size, growth activity, environment, container size and type, and soil composition. Regularly check the soil for dryness and then water deeply, allowing water to seep through the container to ensure no salts accumulate in the soil.
In its natural environment, the spider plant (Chlorophytum comosum) thrives on volcanic or sedimentary soils made of sandstone, shale, dolerite, or granite. Typically, the soils are mildly acidic, winter weather fluctuates from hot to cool, and frost is mild.
Managing your spider plant’s hydration needs depends on:
- How often you water
- How much do you water
- But mainly, how the potting soil manages available water
Introduction to Potting Soil Basics
You have several options to replicate your spider plant’s natural soil environment. While the spider plant is native to forested moist river valleys, the succulent roots enable the plant to cope during extended dry spells.
Even though this article is focused on watering the spider plant, the guidelines can be applied to any houseplant. We’ll be reviewing the technical aspects of potting soil’s water and air management capacities, different substrates, and everything that contributes to growing healthy indoor plants.
In growing any plant indoors, six factors relating to potting soils need consideration:
- Moisture management – a balance between drainage and moisture retention
- Air Management – avoidance of anaerobic conditions
- Nutrient management – an ability to store and release essential plant nutrients
- The role of microorganisms in plant resilience and needs
- Plant requirements for acidic or alkaline soil (pH requirements)
- Plant anchorage – ensuring the media isn’t so light that the plant cannot remain reasonably erect in winds
The Essential Elements of a Good Potting Soil
Our physiological composition allows us to go without water for a maximum of three days and without food for three weeks (depending on our physical reserves). Plant roots, like humans, need access to water and air; if submerged in water for an extended period, roots will drown.
Balancing Moisture Retention and Aeration
A carefully graded soil allows the water to drain to the bottom of the pot, where it builds up a little before escaping from the drainage holes. The fractional difference between added and drained water amounts to the water retained in the soil.
If the soil is healthy, it contains micropores that trap air even when flooded, allowing it to retain air essential for the plant’s continued health. These two soil attributes are field capacity and saturation porosity.
- Field Capacity is the water content remaining after saturation and drainage.
- Saturation Porosity is the air content remaining in the soil after saturation and drainage.
Measuring a Potting Mix’s Field Capacity and Saturation Porosity
Your spider plant wants access to water without being drowned, i.e., a balance between water and air availability. To measure these interplaying factors, test the soil mix before planting, using a container of known volume and manageable draining holes.
Steps to Measuring Field Capacity
- Start with a reasonably sized empty plant pot, inserting a mesh inside to prevent potting soil loss during the procedure.
- Seal the pot’s drainage holes with duct tape, allowing you to unseal them from the outside without affecting the screens. Initially, the plant pot needs to be watertight.
- Weigh the empty pot and record the weight (M1). This allows you to tare out its weight in later calculations.
- Fill the plant pot with water and weigh it (M2). The difference between the full and empty container is the weight of the water and, thus the volume (if a metric system is used, where 1 liter = 1 kilogram).
- Empty the water, and dry the container out. Now fill it to the brim with dry soil mix and weigh it (M3). The mix must be dry. By subtracting the container’s weight from your last reading, you can determine the weight of the soil mix added.
- Add water to the brim with the dry soil mix in the pot, waiting for the rising air bubbles to stop. Before weighing, bring the water level to the brim (or the same level as the previous water weight measurement). This is your fourth reading (M4).
- Drain the container by removing the duct tape, ensuring no soil solids are lost, waiting up to five minutes for drainage to complete, and weighing again (M5).
Using the above procedure, let’s use the following example data:
| Reading | Element | Weight | Tared Weight (kg) |
|---|---|---|---|
| M1 | Empty Pot | 250g | 0.25kg |
| M2 | The volume of the Pot | 5.25kg | 5kg (5 liters) |
| M3 | Dry Weight of Potting Mix | 1.25kg | 1kg |
| M4 | Saturated Potting Mix | 5.75kg | 5.5kg |
| M5 | Drained Potting Mix | 4.75kg | 4.5kg |
Using the tared weight and volumes above, let’s determine:
Potting Soil Density
A substance’s density is expressed as its volume-to-weight ratio. Water has a density of 1.0, and ice has a density of 0.9 – the reason an iceberg floats. Measure your soil’s density by dividing its dry weight by its volume. (this is easier if the metric system is used as weight and volume of water are proportionally related)
- M3 divided by M2 gives us a density of 0.2.
Field Capacity (FC)
The Field Capacity is the difference between the drained wet weight and the dry weight of the mix divided by the volume. Let’s assume the following:
- (M5 minus M3) divided by M2
- (4.5 – 1) / 5 = 0.6 or 60% field capacity
- Typical farm soil is in the region of 50% field capacity
Saturation Porosity (SP)
The Saturation Porosity is the difference between the fully saturated mix and the drained mix as a factor of its volume. Let’s assume the following:
- The SP (the fractional air content) is (M4 – M5) / M2
- (5.5 – 4.5) / 5.0 = 0.20 or 20% saturation porosity
- Typical farming soil ranges between 2 to 7%. An SP below 6% indicates anaerobic conditions where roots cannot breathe.
Saturation Porosity and Field Capacity of Typical Potting Soil Components
Inexpensive yet effective components with good drainage and water absorption include perlite, calcine clay, bark, pea gravel, and granite grit.
| Material | Saturated Porosity (SP) – Air | Field Capacity (FC) – Water |
|---|---|---|
| Calcine Clay (Turface or Haydite) | 28% | 40 – 60% |
| Expanded Shale | 30% | 38% |
| Granite Grit | 31% | 20% |
| Pea Gravel | 27% | 10% |
| Perlite | 22% | 40% |
| Pine Bark | 35% | 30% |
By concocting these ingredients, an ideal potting mix can be created for any plant, including succulents, bonsai, orchids, and, you guessed it, spider plants. But what else is needed to meet the demands of your plant?
The Value and Risk of Including Compost in Potting Soil Mixes
Some spider plant growers believe it can be grown without using organic components. They claim that drainage and aeration are the two key essential components of a healthy mix and that it is your duty as the grower to provide your plant with whatever nutrients and moisture it may need.
This, of course, is doable, but is it not similar to hydroponics? I prefer adding compost at a minimum ratio of one part of cured compost to four parts of other ingredients. The advantages are well established:
The Advantages of Compost in Potting Soil Mix
The addition of compost to potting soil introduces a soil food web to the root environment. There are hundreds of benefits to its inclusion, but here is a list of three of them:
To Facilitate Optimized Soil pH Variability
Aerobic composting has a pH buffering function able to adapt soil acidity to environmental requirements. The microorganisms cause the pH to fluctuate between 5.5 and 7.2.
Optimize Nutrient Cycling and Micronutrient Uptake
During composting, organic materials do not merely undergo processes of degradation. Microorganisms reduce complex compounds into monomers (monosaccharides, amino acids, etc.), reconstituting these into bioavailable compounds. The activity also involves re-synthesis and polymerization reactions to produce a product that is intricately beneficial to soil health and plant life.
Increase soil gas and water holding capacity
Microorganisms help form aggregates (clusters of soil particles) that benefit soil structure and reduce erosion. Tiny channels and pores are formed, increasing the soil’s capacity to retain air, moisture, and nutrients.
The Risks of Too Much Compost in Potting Mixes
Including compost in potting soil can present two problems. Firstly, uncured compost can leach nitrogen, and compaction is the other risk.
Compost and Nitrogen Leaching
Adding fresh compost to the soil will reduce the nitrogen available to the plant as the organisms use it to degrade the carbon content. This will stabilize after about six weeks but can be avoided using cured compost.
You should also avoid using anything that might be too “hot” (too much nutrient) for tender young roots, such as cow manure or fresh compost.
Compost and Soil Compaction
The particle size of compost, as is the case with all partially decomposed organic matter, reduces over time. As the particles become smaller, the surrounding area collapses and can cause anaerobic conditions – an absence of oxygen.
Growing Spider Plants
Chlorophytum comosum thrives indoors and is well-liked because of its attractive, slenderly ascending-spreading to recurved leaves in a center rosette. Spider plants are straightforward to propagate.
It can also be cultivated outdoors in warmer regions, commonly used as ground cover. Wherever spider plants are grown, they must be shielded from direct sunlight. Usually, it takes plants a year to achieve flowering size.
Spider plants can also be grown from seed, which should be sown in warm, shady conditions on sandy, mildly acidic soil in the spring or summer. Keep wet and cover with a thin layer of sand. In most cases, germination occurs within three weeks.
It is advisable to place seedlings outside as soon as they are large enough to handle since they grow quickly. Plants respond favorably to organic nutrition (compost or liquid fertilizer). The ideal time of year to water Chlorophytum comosum is all year round.
Allow the plant soil to dry out between watering, and then water thoroughly. It is always better to be guided by the soil’s moisture levels rather than a timed schedule.
In Summary
Generally, a composition of 75 percent inert aggregate and 25 percent organic material is a good starting point for potting soils. Take the time to measure your mix’s field capacity (water retention capacity) and saturation porosity (air availability) using the guidelines provided.
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