Understanding pH Stability in New Substrates: Buffering, Oxygen, and Irrigation Water

Buffer Systems in Peat-Free and Peat-Reduced Growing Media

Buffer systems help substrates resist sudden pH changes. In traditional peat mixes, the carbonate (lime) buffer system is the strongest and most important mechanism for stabilizing pH. In new peat‑free and peat‑reduced substrates, however, this buffer system is often weaker.

Why the Carbonate Buffer Is Less Effective in New Mixes

The strength of the carbonate buffer depends on the amount and type of lime incorporated into the substrate. Many commonly used peat alternatives such as:

• wood fiber,
• bark compost,
• coco products,
• green compost,
• perlite

have a neutral pH and require little to no liming compared to acidic peat. As a result, the overall buffering capacity of the mix is lower.

Buffer Systems: Use of Coarse Lime and the Phosphate Buffer

1. To improve pH stability, coarse lime is often recommended because it dissolves slowly and provides a longer‑lasting buffering effect. However, trials over several years show only moderate success. In highly dynamic substrates, even coarse lime cannot fully compensate for rapid pH changes.
2. Substrates also contain a phosphate buffer system, which can absorb hydrogen ions (H⁺). However:

• • substrates contain only small amounts of phosphate,
  • the buffering effect is therefore weak,
  • and below pH 5.8, even this limited buffering capacity disappears.
    Once pH begins to drop, the substrate has very little resistance to further acidification.

Avoiding Oxygen Deficiency in the Substrate

Oxygen availability is a critical but often overlooked factor in pH development. Oxygen deficiency can occur in substrates that are too fine, especially in the lower part of the container where water accumulates.

What Happens When Oxygen is Limited and Consequences of its Deficiency

Under anaerobic (oxygen‑poor) conditions, certain bacteria begin producing organic acids such as:

• lactic acid,
• butyric acid,
• acetic acid.

These acids can lower pH and negatively affect root health.

In severe cases:

• sulfates may be reduced to sulfids,
• anaerobic bacteria may cause root rot,
• and pH dynamics become unpredictable.

When oxygen is lacking:

• fine roots die,
• nutrient uptake becomes impaired,
• nitrification processes stop,
• and denitrifying bacteria may convert nitrate into nitrogen gas or nitrous oxide (a greenhouse gas).

This combination can lead to sudden pH drops, nutrient imbalances, and reduced plant performance.

Water quality and fertilization strategy: influence on pH stability

• low alkalinity has limited buffering capacity,
• high bicarbonate levels can increase pH over time

To maintain a stable pH, water quality must be matched to the fertilizers used for top‑dressing or fertigation. This ensures a positive influence on pH development throughout the culture.

Why Frequent pH Checks Matter

• detect undesirable pH shifts early,
• adjust fertilization strategies in time,
• and prevent nutrient deficiencies or toxicities.

Conclusion

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