Many growers are familiar with this situation.
The plants suddenly show light-colored leaf veins.
The leaves lose their color.
Growth slows down.
The diagnosis seems clear: nutrient deficiency.
So what happens is what almost every grower does at this moment.
The amount of fertilizer is increased.
But instead of getting better, the problem gets worse.
The leaves are turning yellow.
The plants appear stressed.
And at some point the question arises:
Why is my fertilization schedule suddenly not working anymore?
The answer often lies not in the fertilizer.
But in something that many growers hardly pay attention to:
the tap water.
Because water is not just a transport medium for nutrients in growing.
It is itself an important part of the nutrient solution.
And this is often where the problem begins.
Why tap water plays a bigger role in growing than many people think
Many growers invest a lot of time in their setup.
They optimize:
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Light output
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humidity
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temperature
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Fertilization schedule
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PH value
But one factor is often overlooked:
the chemical composition of the tap water.
Tap water already contains numerous dissolved substances, including:
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Calcium
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magnesium
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sodium
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Hydrogen carbonates
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Sulfates
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Trace elements
This composition is usually unproblematic for humans. Drinking water in Germany is among the most strictly controlled foodstuffs.
However, it can be crucial for plants.
Because all these substances have a direct effect on:
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the stability of the pH value
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the balance of important plant nutrients
In other words:
Tap water is already part of your fertilizer.
And that's precisely why its composition can determine whether your grow runs smoothly – or whether inexplicable problems suddenly arise.
The typical problem with many German tap waters: high calcium levels.
In many regions of Germany, the tap water is relatively hard .
This means that it contains a high proportion of dissolved minerals, in particular:
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Calcium
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Bicarbonate
This so-called water hardness is not a problem for humans.
However, it can have an impact on plants.
The reason lies in the relationship between two important plant nutrients:
Calcium and magnesium.
Both belong to the so-called secondary macronutrients and play an important role in plant metabolism. Calcium stabilizes cell structures and is involved in numerous physiological processes, while magnesium is a key component of chlorophyll and thus directly involved in photosynthesis.
However, for these processes to function optimally, the ratio of these two nutrients must be correct.
And this is precisely the critical point.
When too much calcium suddenly blocks magnesium
Plants absorb nutrients through their roots. In this process, various dissolved ions compete for transport pathways within the root.
Calcium and magnesium both belong to the so-called cations , i.e., positively charged ions.
If the water contains a lot of calcium, this can make it difficult for the plant to absorb magnesium. This doesn't mean that magnesium is completely absent – but rather that the plant can no longer absorb it efficiently.
The result is symptoms that many growers immediately interpret as a classic magnesium deficiency:
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Light patches between the leaf veins
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yellowish older leaves
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slowed growth
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reduced vitality
The problem is, however:
The deficiency is not necessarily caused by too little magnesium.
It is caused by an unfavorable calcium-magnesium ratio in the water.
Why many growers misdiagnose the problem
When plants show such symptoms, many growers automatically resort to more fertilizer.
That's understandable. After all, the symptoms look like a classic nutrient deficiency.
However, if the underlying problem is the water, additional fertilization can actually worsen the situation.
A typical scenario looks like this:
The plant is showing initial deficiency symptoms.
The grower increases the amount of fertilizer.
This also increases the total amount of calcium in the nutrient solution.
The result:
Magnesium is blocked even more strongly.
This phenomenon is known as nutrient lockout .
The nutrients are present, but can no longer be effectively absorbed by the plant.
For many growers, it then seems as if the fertilizer suddenly "no longer works".
Soil, coco or hydroponics: Why water plays a role everywhere
The effects of water chemistry differ depending on the growing medium.
Organic components and clay minerals in soil can buffer some of the nutrients. This often means that problems only become apparent with a delay.
In coco coir, the calcium-magnesium ratio plays a particularly important role. The substrate itself can bind calcium and thus further influences the nutrient balance.
Problems usually become apparent most quickly in hydroponic systems. Since there are hardly any buffering effects, any change in the nutrient solution has an immediate impact on the plant.
Regardless of the medium, one central insight remains:
If the water doesn't match the fertilization schedule, the grow will become unstable.
A frequently overlooked factor: water softening systems in the home.
Another point that surprises many growers is the use of so-called water softening systems in the home.
Such systems are often installed to reduce limescale deposits in pipes and household appliances.
Many of these systems use a so-called ion exchange process . In this process, calcium and magnesium ions are removed from the water and replaced by sodium.
The water may seem "softer" afterwards, but it contains significantly more sodium.
For plants, an increased sodium level can be problematic, as sodium can interfere with the absorption of other nutrients and lead to additional salt stress.
That means:
Even if the water initially appears "better" after softening, it may be unsuitable for plants.
In such cases, simply adjusting the fertilization schedule is often insufficient. It is frequently more effective to use reverse osmosis water and then selectively build up the nutrient composition yourself.
Here's how to find out what's really in your water
The first step is to know the composition of your tap water.
In many cities, water suppliers publish their drinking water analyses directly on the websites of the municipal utilities or communities. Alternatively, this data can usually be easily requested from the local supplier.
Values such as the following are particularly interesting for growers:
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Calcium
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magnesium
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sodium
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Total hardness
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Bicarbonate
This information makes it much easier to assess whether your water is generally suitable – or whether adjustments might be advisable.
Conclusion: Many growing problems do not begin with the fertilization schedule.
When plants show signs of deficiency, many growers first think of the fertilizer.
But in many cases, the real problem lies in a place that hardly anyone checks:
the tap water.
An unfavorable calcium-magnesium ratio, high water hardness, or sodium from water softening systems can prevent plants from optimally absorbing nutrients.
The symptoms then look like classic deficiency symptoms – even though the nutrients are actually present.
Anyone who truly wants to understand and manage their grow stably should therefore not only keep an eye on the fertilization schedule.
But also take a look at what comes out of the tap every time you water the plants.
Because sometimes it's not the fertilizer that causes problems.
But the water that accompanies him.
