Measurement of concentrations

First we look at the nutrient solutions, some of which have been around for over a hundred years. This shows us in which concentrations the measurement must take place. 

This serves as an initial orientation as to what nutrients or elements must be contained in a solution. A further step is to closely observe plant growth in order to be able to identify deficits as such.

The next step is to get an idea of ​​which elements, and therefore which compounds, are in the end product. Unfortunately, such an analysis (the plant is put into a blender and additional chemicals are added depending on the compounds we are looking for) has the disadvantage that it doesn't really reveal everything that interests us. This is because the chemical compounds can rarely be found in the plant in the form in which they were originally added. This is where biology comes into play. The only example that we would like to mention here is the citric acid cycle, which we do not want to withhold from you. It illustrates the complexity of metabolism.

Citric acid cycle

 

Nutrition of hydroponic plants

When grown in containers, the plants are nourished by an aqueous solution of inorganic nutrient salts. Since the chemical properties of the soil differ greatly from their natural state due to the lack of fine organic soil components, normal plant fertilizer is only partially suitable for hydroponics.
A special hydroponic fertilizer can help, which uses additives to buffer the pH value of the solution in a range suitable for many plants. So-called ion exchange granules are also used for this purpose, which supply the plants with nutrients through ion exchange and at the same time bind minerals such as lime that are present in the water in excess and are incompatible with the plants.
The microbial conversion of ammonium ions into nitrate ions consumes oxygen that is lost to root respiration. Hydroponic fertilizers therefore use less ammonium salts as nitrogen fertilizer and more nitrates.
In hydroponics, the electrical conductivity of the nutrient solution is usually constantly monitored. If the concentration of dissolved substances increases (for example through exudates or extraction from soil), the solubility for oxygen in the nutrient solution decreases. If solutions are too concentrated, it becomes more difficult for the plants to absorb water (see also osmosis). Different stages of the plant also require different conductivity of the nutrient solution depending on the variety, cuttings around 0.2-0.4 mS/cm, which can increase to 2.4-2.6 mS/cm until fruit formation . The morphology of plant growth also depends on the concentration of the nutrient solution, for example whether squat plants grow or stretched ones. If the nutrient solution is too concentrated, it can be diluted with deionized water or rainwater.
Depending on the nutrient composition, the expected concentrations are in the following orders of magnitude:
 

Compounds and trace elements / orders of magnitude in nutrient solutions

 K

Potassium

0.5 - 10 mmol/L

 Approx

Calcium

0.2 - 5 mmol/L

 S

Sulfur

 0.2 - 5 mmol/L

 P

Phosphorus 

0.1 - 2 mmol/L

 Mg

Magnesium

0.1 - 2 mmol/L

 Fe

Iron

2 - 50 µmol/L

 Cu

Copper

0.5 - 10 µmol/L

 Zn

Zinc

0.1 - 10 µmol/L

 Mn

Manganese

0 - 10 µmol/L

 B

Boron

0 - 0.01 ppm

 Mo

Molybdenum

0 - 100 ppm

 NO2

Nitrite

0 – 100 mg/L

 NO3

Nitrate

0 – 100 mg/L

 NH4

ammonia

0.1 - 8 mg/L

 KNO3

Potassium nitrate

0 - 10 mmol/L

 Ca(NO3)2

Calcium nitrate

0 - 10 mmol/L

 NH4H2PO4

Ammonium dihydrogen phosphate

0 - 10 mmol/L

 (NH4)2HPO4

Diammonium hydrogen phosphate

0 - 10 mmol/L

 MgSO4

Magnesium sulfate

0 - 10 mmol/L

 Fe-EDTA

Ethylenediaminetetraacetic acid

0 – 0.1 mmol/L

 H3BO3

Boric acid

0 – 0.01 mmol/L

 KCl

Potassium chloride

0 – 0.01 mmol/L

 MnSO4

Manganese (II) sulfate

0 – 0.001 mmol/L

 ZnSO4

Zinc sulfate

0 – 0.001 mmol/L

 FeSO4

Iron(II) sulfate

0 – 0.0001 mmol/L

 CuSO4

Copper sulfate

0 - 0.0002 mmol/L

 MoO3

Molybdenum oxide

0 – 0.0002 mmol/L
 
In order to convert the quantities (mg, ppm, mol, etc.) we have created some articles for you here. You can also find corresponding "stoichiometry" calculators online, such as here:  https://www.omnicalculator.com/chemistry/ppm-to-molarity
 
 
 

 

 

Here are some recipes for nutrient solutions...

 
Nutrient solution according to Wilhelm Knop
One liter of finished solution contains:
1.00 g Ca(NO 3 ) 2  calcium nitrate
0.25 g MgSO 4  * 7 H 2 O magnesium sulfate
0.25 g KH 2 PO 4  potassium dihydrogen phosphate
0.25 g KNO 3  potassium nitrate
traces of FeSO 4  * 7 H2O iron(II) sulfate
Medium according to Pirson and Seidel
One liter of finished solution contains
1.5 millimol KH 2 PO 4
2.0 mM KNO 3
1.0 mM CaCl 2
1.0 mM MgSO 4
18 μM Fe-Na-EDTA
8.1 μM H 3 BO 3
1.5 μM MnCl2 _
 
Culture medium according to Epstein
One liter of finished solution contains
1 mM KNO 3
1 mM Ca(NO 3 ) 2
1 mM NH 4 H 2 PO 4
1 mM (NH 4 ) 2 HPO 4
1 mM MgSO 4
0.02 mM Fe-EDTA
0.025 mM H 3 BO 3
0.05 mM KCl
0.002 mM MnSO 4
Trace elements:
0.002 mM ZnSO 4
0.0005 mM CuSO 4
0.0005 mM MoO 3
 
Trace element additive according to DR Hoagland (1884–1949)
One liter of finished solution contains
55 mg Al 2 (SO 4 ) 2
28 mg KJ 28 mg
KBr
55 mg TiO 2
28 mg SnCl 2  · 2 H 2 O
28 mg LiCl
389 mg MnCl 2  · 4 H 2 O
614 mg B(OH ) 3
55 mg ZnSO 4
55 mg CuSO 4  · 5 H 2 O
59 mg NiSO 4  · 7 H 2 O
55 mg Co(NO 3 ) 2  · 6 H 2 O
 
 
ID:
 
URL
Category: Technology
Also available: German (Hochdeutsch)

Technology