Introduction to Field Capacity for Mushroom Substrate
Field capacity is one of the most important — and most misunderstood — concepts in mushroom cultivation. Whether you’re working with coco coir, CVG mixes, manure-based substrates, or ready-made bulk substrate, getting field capacity right is critical to success.
In simple terms, field capacity describes how much water a substrate can hold while still allowing enough air for healthy mycelial growth. Too much moisture creates oxygen-poor conditions where contamination thrives. Too little moisture causes stalled colonisation, weak pinsets, and poor yields.
Many growers struggle not because of genetics or equipment, but because their substrate hydration is slightly off. This guide explains exactly what field capacity for mushroom substrate means, how it should feel, what it looks like in practice, and how to test it accurately.
What Is Field Capacity for Mushroom Substrate?
Field capacity is a term borrowed from soil science. It refers to the maximum amount of water a material can retain after excess water has drained away.
In mushroom cultivation, field capacity describes substrate that is:
Fully hydrated
Moist throughout
Holding water internally
Not dripping or soggy
This balance becomes especially important once grain spawn is transferred into bulk substrate, a step explained in detail in our bulk substrate recipe guide.
Why Field Capacity Is Critical for Mushroom Growth
Mycelium Needs Oxygen
Mycelium is a living network that requires oxygen to grow. Oversaturated substrate collapses air pockets and slows colonisation, particularly in enclosed systems like monotub mushroom grows.
Excess Moisture Encourages Contamination
Overly wet substrate creates ideal conditions for bacteria and competitor moulds. Many issues blamed on spores or grain are actually hydration problems, a topic we explore further in contamination in mycology.
Water Drives Fruiting
Mushrooms are mostly water. If substrate lacks sufficient moisture at fruiting time, yields suffer — even if colonisation looked healthy.
Should Field Capacity Substrate Feel Moist?
Yes — field capacity substrate should feel moist, but never wet.
At correct field capacity:
The substrate feels cool and damp
Your hand may feel slightly wet after squeezing
Water does not stream out
If you’re unsure, compare your mix to examples shown in our bulk substrate & casing materials section.
What Does Field Capacity Look Like in Practice?
Visually, substrate at field capacity:
Appears evenly hydrated
Has a darker, richer colour
Lightly clumps when squeezed, then falls apart
Shows no pooling water
If water collects at the bottom of a tub or bag, field capacity has been exceeded — a common issue in improperly hydrated all-in-one mushroom grow bags.
How to Test Field Capacity: The Squeeze Test
The squeeze test is the simplest and most reliable method.
Take a handful of mixed substrate
Squeeze firmly
Observe the moisture released
Streams of water = too wet
A few slow drips = correct
No moisture = too dry
This test is used universally, including when preparing substrate for monotub kits and trays.
Common Field Capacity Mistakes (And How to Avoid Them)
Overhydrating Coco Coir
Coco coir absorbs water rapidly, especially when hydrated with boiling water.
Avoid this by:
Adding water gradually
Mixing thoroughly
Comparing against properly hydrated pasteurised bulk substrate
Uneven Hydration
Dry pockets and wet pockets lead to uneven colonisation.
Avoid this by:
Breaking up all clumps
Allowing substrate to rest, then re-testing
Field Capacity for Popular Substrate Types
CVG (Coco Coir, Vermiculite, Gypsum)
CVG is forgiving but easy to overhydrate. Vermiculite increases water retention, so always perform a squeeze test before use in bulk substrate monotub setups.
Straight Coco Coir
Coir-only substrates require precise hydration. Many beginners benefit from starting with pre-hydrated bulk substrate bags to eliminate guesswork.
Manure-Based Substrates
Manure holds moisture longer and compacts easily. Slightly drier field capacity often produces better results, especially in large monotub grows.
Ready-Made Pasteurised Bulk Substrate
Prepared substrates from SporeBuddies bulk substrate products are hydrated to correct field capacity using closed-system pasteurisation. This ensures consistent moisture levels and reduces contamination risk — ideal for growers who want predictable outcomes.
Field Capacity vs Pasteurisation vs Sterilisation
These terms are often confused:
Field capacity = hydration level
Pasteurisation = microbial reduction
Sterilisation = complete microbial elimination
Hydration comes first. Substrate must reach field capacity before pasteurisation or sterilisation, a process explained in our how to pasteurise mushroom substrate guide.
How Field Capacity Affects Monotub Performance
In monotubs, field capacity directly influences:
Colonisation speed
Surface conditions
Evaporation rate
Pinset formation
Improper hydration is one of the most common causes of failure in monotub mushroom cultivation.
Troubleshooting Field Capacity Problems
Symptoms of too-wet substrate often overlap with issues covered in preventing contamination in mushroom cultivation, while overly dry substrate leads to stalled growth and constant misting.
Most of these problems originate before spawning — not during fruiting.
Key Takeaways: Field Capacity Explained Simply
Field capacity balances water and air
Moist, not wet, is the goal
A few slow drips = perfect
Overhydration causes more failures than underhydration
Prepared substrate removes guesswork
If consistency matters, mastering field capacity — or using ready-to-use pasteurised substrate — is one of the biggest upgrades you can make.
Frequently Asked Questions About Field Capacity
What is field capacity for substrate?
Field capacity is the amount of water a substrate can hold while still allowing excess moisture to drain, leaving enough air for healthy mycelial growth.
Should field capacity substrate feel moist?
Yes. It should feel moist and cool, but never wet or dripping.
What does field capacity look like substrate?
Evenly hydrated, slightly darkened, lightly clumping substrate with no standing water.
Legal & Educational Disclaimer
This article is provided for educational purposes only. Always comply with local laws and regulations. SporeBuddies.com supplies materials intended for lawful research and educational use.