Umrechnungsfaktoren: Nährlösungen und Düngemittel korrekt bewerten

Conversion factors: plants and their nutrients

Mika Schilling

When handling and evaluating analyses of tap water, fertilizers and nutrient solutions, misunderstandings and mistakes with serious consequences often occur. The reason for this are the sometimes considerable differences in the product declaration/analysis, especially in the units of measurement used. The main problem is that the values mentioned are often given in elementary form, especially in North America and the UK, while in Germany and the EU the corresponding oxide form is much more common. However, many manufacturers based in the EU also use the units of measurement common in North America and Great Britain. This is where things get complicated for the consumer, since products and their declarations cannot easily be compared with one another. The exchange of information between gardeners, e.g. in forums (especially at international level) is full of misunderstandings.

In practice, it looks like this: Gardener A from Germany has problems with his plants and asks the community for advice. Gardener B from Canada wants to help and asks Gardener A for more information and then uses the analysis of the nutrient solution (from Gardener A) as a guide. Gardener A mentions a P value of 100 ppm, but calculates with the phosphorus pentoxide (P2O5). Gardener B, however, assumes that the P value is P in its elementary form and advises Gardener A to at least halve the P value. If gardener A were to implement this without questioning, he would probably have further problems (P deficiency), since the conversion factor from P to P2O5 is 2.2914. In other words, the P value of 100 ppm (P2O5) of gardener A corresponds to a P value of gardener B of 43.64 ppm (P).

In order to avoid problems of this kind, one should always pay close attention to the form of the respective nutrient. If in doubt, it is always worth checking again before making any adjustments to the nutrient solution.

Here is an overview of the most important conversion factors:

Calcium
Calcium (Ca) zu Calciumoxide (CaO): 1,399
Calcium Oxide (CaO) and Calcium (Ca): 0.7147

Magnesium
Magnesium (Mg) to Magnesium Oxide (MgO): 1.658
Magnesium Oxide (MgO) to Magnesium (Mg): 0.6032

Phosphorus
Phosphorus (P) to phosphorus pentoxide (P2O5): 2,2914
Phosphorpentoxid (P2O5) to phosphorus (P): 0.4364

Potassium
Potassium (K) to Potassium Oxide (K2O): 1.205
Potassium oxide (K2O) to potassium (K): 0.8302

Sulfur
Sulfur (S) to sulfur dioxide (SO2): 1,998
Sulfur dioxide (SO2) to sulfur (S): 0.501

Sulfur (S) to sulfur trioxide (SO3): 2,497
Sulfur trioxide (SO3) to sulfur (S): 0.4

Sulfur (S) to sulfate (SO4): 2,996
Sulfat (SO4) to sulfur (S): 0.334

Nitrogen
Nitrogen (N) to nitrate (NO3): 4,427
Nitrate (NO3) to nitrogen (N): 0.226

Nitrogen (N) to ammonium (NH4): 1,288
Ammonium (NH4) to nitrogen (N): 0.777
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