Aleksandrow Agar is a selective and differential culture medium developed in 1925 by Aleksandrov for isolating potassium-solubilizing microorganisms (KSMs) such as bacteria and fungi from soil and rhizosphere samples. This agar medium is essential in agricultural microbiology and biofertilizer research due to its unique ability to detect microbes that can release potassium from insoluble minerals, thereby enhancing soil fertility and promoting plant growth.
What is Aleksandrow Agar?
Aleksandrow Agar is designed to screen for potassium-solubilizing bacteria (KSB) and fungi by using insoluble potassium-containing minerals like mica or feldspar as the sole potassium source. Microorganisms capable of dissolving these minerals secrete organic acids or chelating compounds, creating clear halo zones around their colonies on the otherwise hazy agar plate. These halos serve as visual indicators of potassium solubilization.
Principle of Aleksandrow Agar
The medium contains insoluble potassium aluminosilicates (e.g., mica or feldspar), which most organisms cannot utilize directly. Potassium-solubilizing microorganisms (KSMs) produce organic acids like citric, oxalic, tartaric, and gluconic acid. These acids help dissolve the mineral matrix, releasing soluble potassium ions (K⁺) into the medium. The appearance of a clear halo around microbial colonies signifies potassium solubilization activity. The larger the halo, the more efficient the organism is at releasing potassium.
Composition of Aleksandrow Agar (per liter)
| Ingredient | Amount (g/L) | Function |
| Glucose | 5.0 | Energy and carbon source |
| Calcium carbonate (CaCO₃) | 0.1 | pH buffering agent |
| Ferric chloride (FeCl₃) | 0.006 | Trace element |
|
Potassium aluminosilicate (mica/feldspar) |
0.2 | Insoluble potassium source |
| Magnesium sulfate (MgSO₄·7H₂O) | 0.5 | Enzyme cofactor |
| Manganese sulfate (MnSO₄·H₂O) | 0.006 | Trace element |
| Agar | 15.0 | Solidifying agent |
How to Prepare Aleksandrow Agar
- Dissolve all soluble components (except agar and potassium minerals) in distilled water.
- Add mica or feldspar powder as the potassium source. It remains suspended, not dissolved.
- Add agar and heat gently until fully dissolved.
- Adjust pH to 7.0 ± 0.2 using 1N NaOH or HCl.
- Autoclave at 121°C for 15 minutes.
- Aseptically pour into sterile Petri dishes and allow to solidify.
- Plates appear hazy due to suspended mineral particles.
Microorganisms Growing on Aleksandrow Agar
| Microbe | Type | Colony Appearance |
| Bacillus mucilaginosus | Bacterium | Mucoid, creamy colonies with halo |
| Bacillus subtilis | Bacterium | Creamy white colonies with clear zone |
| Pseudomonas putida | Bacterium | Greenish colonies with transparent halo |
| Aspergillus niger | Fungus | Black, powdery colonies with large halo |
| Penicillium chrysogenum | Fungus | Blue-green colonies with clear zone |
The presence of a halo zone indicates positive potassium-solubilizing activity.
Uses of Aleksandrow Agar
- Isolation and screening of potassium-solubilizing microbes from soil and plant rhizosphere.
- Development of biofertilizers containing effective KSM strains.
- Studying plant-microbe interactions for potassium uptake enhancement.
- Environmental microbiology to improve soil nutrient cycling.
- Useful in qualitative analysis of potassium solubilization potential.
Precautions During Use
- Ensure even distribution of the potassium mineral powder in the medium.
- Maintain sterile conditions during preparation and inoculation.
- Do not overheat the minerals to preserve their solubilization properties.
- Store plates at 4°C and use within 7 days for best results.
- Record halo formation after 5–7 days of incubation for accurate assessment
Limitations of Aleksandrow Agar
- Only provides qualitative, not quantitative, data.
- Results may vary with temperature, pH, and incubation time.
- Some KSMs may not produce visible halos.
- Fast-growing microbes may mask slower potassium-solubilizing ones.
- In vitro performance does not always reflect in vivo efficacy in field conditions.
Conclusion
Aleksandrow Agar remains a valuable tool in agricultural and environmental microbiology for identifying and studying potassium-solubilizing microorganisms. It plays a critical role in the development of eco-friendly biofertilizers, supporting sustainable agriculture by enhancing potassium bioavailability in soil. While it has some limitations, its ability to visually demonstrate microbial potassium solubilization makes it indispensable for initial screening and research purposes.
