Potash fertilizers come in two main types: simple potassium fertilizers and compound fertilizers that contain potassium. Simple potassium fertilizers are those that primarily supply potassium (K) to the soil, while compound fertilizers provide multiple nutrients such as nitrogen (N), phosphorus (P), and potassium (K).
Common examples of simple potassium fertilizers include potassium chloride (KCl), potassium sulfate (Kâ‚‚SOâ‚„), and potassium silicate. Potassium chloride is available in different forms, such as granules, powders, and irregular blocks. It comes in red and white varieties depending on the source. The potassium oxide (Kâ‚‚O) content varies; imported products typically have around 60%, while domestic ones range between 57% and 60%. Potassium sulfate, usually white in appearance, is sold as crystalline particles or powder. Some products may contain impurities, causing slight discoloration. Domestic potassium sulfate from regions like Lop Nur or Taiwan generally contains about 50% Kâ‚‚O, while German production often reaches up to 54%. Potassium silicate, a byproduct of industrial processes, appears as dark gray powder. Its potassium content (Kâ‚‚O) is typically between 10% and 14%, and it has an alkaline nature.
Compound potassium fertilizers, on the other hand, contain more than one nutrient. You can identify the potassium content on the packaging, which usually lists N-Pâ‚‚Oâ‚…-Kâ‚‚O. A common example is potassium nitrate, which is white and crystalline but less commonly found in the market.
Identifying potash fertilizers can be quite complex and often requires laboratory testing for accurate results. While there are some basic methods for qualitative identification, they do not provide precise measurements of potassium levels and are limited in effectiveness.
One method involves using a red-hot iron plate. When fertilizer particles are placed on the hot iron, if they don’t melt, don’t emit any odor, and bounce slightly due to heat, they may be potassium fertilizers. Tilting the iron plate to expose the fertilizer to high heat can cause a colored flame. A golden flame indicates sodium, while a pale yellow flame with a lavender tint suggests potassium. For powdered potassium fertilizers, the color might be brick red, light red, or white. When burned on an iron sheet, potassium fertilizers do not melt or emit an odor, unlike phosphorus fertilizers, which may appear light gray and give off a smell.
If the iron melts and produces heavy smoke with an ammonia-like odor, it’s likely an ammonium-based fertilizer. If no melting occurs and there's a sour or bone-like smell, it could be a phosphate fertilizer.
Another simple test involves dipping a saturated fertilizer solution into a wire coil and burning it over a flame. Observing the flame color can help identify certain elements. However, distinguishing between potassium and sodium based on flame color can be challenging. In cases where low-quality potassium fertilizers are suspected—such as unrefined potassium feldspar—it’s best to rely on laboratory tests to determine the actual soluble potassium content. These methods are only useful for qualitative assessment, not for measuring exact concentrations.
Therefore, farmers are advised to purchase fertilizers from reputable sources to avoid counterfeit or substandard products. Always ensure the product meets quality standards before use.
Author: Cao Yiping (China Agricultural University)
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