Water Quality and Fertilization in New Substrates: Why the Right Combination Matters

Why Water Quality Matters More in New Substrates

New substrate blends typically contain less peat and more renewable components such as wood fiber, bark, compost, or coir. These materials hold less water and have lower buffering capacity, as mentioned in our guide to the parameters to monitor when growing plants on peat-free or peat-reduced substrates. In a nutshell, as a result:

• irrigation must be more frequent,
• total water consumption increases,
• the chemical composition of the irrigation water has a stronger influence on the substrate.

This means that water quality is no longer a background parameter, it becomes a central steering tool for maintaining stable pH and EC levels.

Higher Water Consumption = Higher Impact

Because growers irrigate more often, any characteristic of the water (EC, hardness, bicarbonates, sodium, chloride) accumulates faster in the substrate. For example, irrigation water with high EC, such as well water, can lead to a gradual build‑up of ballast salts. Over time, this increases substrate salinity and reduces nutrient uptake.

Starting Point: Know Your Irrigation Water

A stable fertilization strategy begins with a clear understanding of the water used.

Rainwater

• Very low salinity
• Carbonate hardness close to zero
• Analyses usually unnecessary
• Ideal for soft‑water fertilizer programs

Municipal Water

• Typically moderate hardness
• Analyses often available online from local suppliers
• Good baseline for balanced fertilizer programs

Well Water

• Can vary significantly by season, depth, and rainfall
• Often contains higher EC and bicarbonates
• Requires regular analysis to avoid long‑term pH drift and salt accumulation

Knowing these values allows growers to choose fertilizers that counteract, rather than amplify, the natural tendencies of their water source.

Managing pH Fluctuations in New Substrates

Peat‑free and peat‑reduced substrates contain less lime and have lower buffering capacity, which makes them more sensitive to pH shifts. One of the most effective levers for stabilizing pH during cultivation is the ammonium‑to‑nitrate ratio in the fertilizer.

• Ammonium (NH₄⁺) acidifies the substrate during conversion to nitrate.
• Nitrate (NO₃⁻) has a neutral to slightly alkalizing effect.

Because new substrates already tend to fluctuate more, fertilizers with high ammonium content can intensify pH drops.

We have dedicated a full article to understanding pH stability in new substrates and you can find it here.

Choosing the Right Water‑Soluble Fertilizer

Growers often use nitrogen‑rich or balanced fertilizers in new substrates to avoid nitrogen immobilization. However, this higher nitrogen supply can unintentionally increase pH instability.

A better approach is to match the fertilizer to both:

1. the pH behavior of the substrate, and
2. the carbonate hardness of the irrigation water.

In its wide range of water-soluble fertilizers, ICL offers dedicated formulations for both hard water (high bicarbonates) and soft water (low bicarbonates). For stable pH development, choose fertilizers with low ammonium content, because each step of ammonium conversion releases an H⁺ ion:

1. Ammonium → Nitrite
2. Nitrite → Nitrate

These protons contribute directly to acidification, which is a process that is much more pronounced in low‑buffer substrates.

Interaction between Water Quality and Fertilizer Plan

Water quality, EC, hardness, and fertilizer composition are no longer isolated parameters. In peat‑free and peat‑reduced substrates, they interact dynamically and can either stabilize or destabilize the root‑zone environment. At this link you can access our full collection of articles dedicated to the peat-free and peat-reduced substrates.

The key takeaways of this article are:

• More frequent irrigation increases the influence of water quality.
• High‑EC or hard water accelerates salinity build‑up and pH drift.
• Substrates with low buffering capacity react strongly to ammonium‑rich fertilizers.
• Matching fertilizer type to water hardness is essential for stable pH.
• Regular water analysis is the foundation of any reliable fertilization strategy.

This integrated approach helps growers maintain a stable environment in new substrates, ensuring healthy growth, consistent quality, and predictable performance.

Conclusion

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