pH Fluctuations

The pH in a hydroponic system fluctuates because a reservoir is a dynamic, living chemical environment. As plants grow, they constantly interact with the water, changing its chemistry every hour.

Here are the primary reasons why your pH levels refuse to stay still:

1. Plant Nutrient Uptake  

Plants do not just passively drink water; they actively take in individual nutrient ions.

• When plants absorb negatively charged ions (like Nitrate for leaf growth), they release a hydroxide ion to balance their internal charge. This raises the pH.
  

• When plants absorb positively charged ions (like Potassium or Ammonium), they release a hydrogen ion. This lowers the pH.
  
  

• Because vegetative plants use nutrients at different rates than fruiting plants, your pH will naturally drift up or down depending on the plant’s growth stage.

2. Fluctuating Water Volumes

As plants drink water and evaporation occurs, the volume of water in your reservoir drops.

• The physical nutrients and minerals do not evaporate; they stay behind.

• This makes the remaining solution highly concentrated, which naturally pushes the pH away from its original reading (usually driving it upward).

3. Dissolved Oxygen and Carbon Dioxide

The amount of gas dissolved in your water directly changes its pH.

• Air pumps and air stones pump oxygen and carbon dioxide into the water.

• Dissolved carbon dioxide reacts with water to create carbonic acid. As your air stone drives carbon dioxide out of the water through surface agitation, the acid disappears, causing the pH to rise naturally.

4. Algae and Bacteria 

If light leaks into your reservoir or root zone, microscopic life will take hold.

• Algae during the day: Consumes dissolved carbon dioxide via photosynthesis, causing a massive pH spike.

• Algae at night: Releases carbon dioxide back into the water, causing the pH to crash.

• Rule of thumb: If your pH bounces wildly between morning and evening, you almost certainly have an hidden algae issue.

5. Poor System Buffering 

• Small reservoirs have very little chemical "buffer" capacity. A tiny change in nutrient levels will cause a massive, immediate jump in pH.

• Larger reservoirs are far more chemically stable and hold their pH targets for much longer.
  
  

An upward drifting pH is the most common issue in hydroponics. In a healthy system, a slow drift upward (e.g., from 5.8 to 6.2 over a few days) is perfectly normal because plants naturally release alkaline ions as they eat.

However, if your pH is spiking too fast, it is usually caused by one of these four issues:

Why Your pH is Spiking Up

• Algae Growth: If light enters your water, algae will consume dissolved carbon dioxide (CO₂) during the day, causing a massive upward spike in pH.

• Low Water Levels: As plants drink, water evaporates but nutrients stay behind. This concentrated solution drives the pH up.

• New Growing Medium: Materials like unwashed clay pebbles, rockwool, or concrete-based gravel are naturally alkaline and will continuously leak minerals into your water, pushing pH up.

• High Carbon Dioxide (CO₂) Outgassing: If your tap water has a lot of dissolved CO₂, your air pump will bubble it out of the water over the first 24–48 hours, causing a natural upward climb.

How to Fix It and Lock It Down

• Block All Light: Ensure your reservoir lid and tanks are completely opaque. Use black plastic or wrap containers in aluminum foil to starve potential algae of light.

• Top Off with Fresh Water: Never let your reservoir drop below 80% capacity. Top it off regularly with plain, pH-adjusted water to keep the mineral concentration stable.

• Pre-Soak New Mediums: Always wash clay pebbles thoroughly and pre-soak rockwool in slightly acidic water (pH 5.5) for 24 hours before introducing plants.

• Increase Reservoir Size: Upgrading to a larger water container provides a stronger chemical buffer, slowing down how fast the pH can rise.

A downward drifting pH is less common than an upward drift, but it is a much bigger warning sign. While a slow upward climb means your plants are eating healthily, a sudden or consistent drop usually means your root zone is in trouble.

If your pH is crashing below 5.5, it is almost always caused by one of these four issues:

Why Your pH is Crashing Down

• Root Rot (Most Common): If your water lacks oxygen, pathogenic bacteria and fungi will attack your roots, causing them to decay and turn brown or slimy. This rotting organic matter releases acidic compounds directly into the water.

• Overfeeding / Water Drop: If your nutrient mix is too strong, plants will drink water much faster than they consume the nutrients. The remaining water becomes highly concentrated with acidic fertilisers, causing the pH to plummet.

• Dead Plant Matter: Accumulations of dead leaves, old root debris, or decaying organic material sitting in your reservoir will rot and naturally acidify the water.

• Ammonium-Based Nutrients: If your hydroponic fertiliser relies heavily on ammonium for nitrogen rather than nitrate, plants will naturally excrete hydrogen ions as they feed, driving the pH down.

How to Fix It and Stop the Crash

• Inspect the Roots Immediately: Lift your plants and look at the roots. Healthy roots are crisp and bright white. If they are brown, slimy, or smell like pond water, you have root rot.

• Increase Oxygen Levels: Add a larger air pump or extra air stones. Pathogenic bacteria thrive in stagnant, low-oxygen water. Highly oxygenated water prevents rot and keeps pH stable.

• Dilute Your Reservoir: If your roots look perfectly clean and white, your nutrient concentration is likely too strong. Top off your reservoir with plain, un-nutriated water to dilute the mixture.

• Clean the System: Flush your entire system, scrub the reservoir walls, rinse out any debris, and refill it completely with a fresh, clean batch of water and nutrients.