Does potassium phosphite react with Ca?

Potassium phosphite fertilizer is characterized by good water solubility, high phosphorus content, and growth-regulating properties. Long-term use can activate the crop's active defense mechanisms and enhance its disease resistance. However, many farmers mix it with calcium fertilizer to reduce labor intensity. This raises the question: Does potassium phosphite react with calcium? If so, what substances are formed? How can we avoid the formation of precipitates from the reaction with Ca? This is a complex question requiring a certain understanding of inorganic and organic chemical reactions. Some people in China even use the reaction between K₂HPO₃ and Ca to determine the authenticity of potassium phosphite. This is a very wrong method. We will explain this from several aspects below.

1. Does potassium phosphite react with inorganic calcium?

Commonly used inorganic calcium salts in fertilizers include calcium sulfate, calcium carbonate, calcium chloride, calcium nitrate, tricalcium phosphate, and some organic calcium salts. Some of these calcium fertilizers have good water solubility, while others have very low solubility. When mixed with potassium phosphite, potassium phosphite almost always forms an insoluble calcium phosphite precipitate, reducing the fertilizer's effectiveness. Recently, some companies have been producing ion-retaining agents that supposedly prevent the reaction between K₂HPO₃ and Ca. However, our extensive testing and research have revealed that the effectiveness of ion-retaining agents or ion stabilizers is only effective for certain mixtures of inorganic or organic calcium with potassium phosphite. If the formula contains calcium sulfate or calcium phosphate, precipitation is almost impossible to prevent.

2, does potassium phosphite react with chelated calcium?

Chelated calcium forms stable complexes by coordinating calcium ions with chelating agents (such as EDTA, citric acid, lactic acid, malic acid, amino acids, etc.) through coordinate bonds. The chelated calcium ions are encapsulated by the chelating agent. Due to the strong coordinate bond between the chelating agent and Ca, the concentration of free calcium ions in the solution is extremely low, making it impossible to form an effective precipitate with phosphate ions. Therefore, almost no reaction occurs. However, it's important to note that if our formulation system has a high pH or contains strongly alkaline substances, this alkaline environment can disrupt the stability of the chelating agent's binding to calcium, causing a large amount of calcium ions to be released and react with phosphite to form calcium phosphite precipitate.

Through the above analysis, we can clearly see that potassium phosphite and inorganic calcium salts will definitely react to form calcium phosphite precipitate or other insoluble calcium salts. During the reaction with chelated calcium, if we set the formulation system to weakly acidic conditions, it will hardly react with chelated calcium. However, under alkaline conditions, it may react with chelated calcium to form calcium phosphite precipitate. Therefore, when using potassium phosphite, avoid mixing it with calcium fertilizer unless absolutely necessary. If mixing with calcium fertilizer is unavoidable, please use an organic acid to lower the pH and create an acidic environment, which can significantly reduce the phosphorus-calcium reaction.