Phosphorus is not as active as nitrogen in the soil. Generally, the volatilization and leaching loss of phosphorus is not as much as that of nitrogen. Most crops require phosphorus in the early stages of growth. Therefore, phosphate fertilizer should be applied before or during planting. In the production stage, if signs of phosphorus deficiency are found, injecting phosphorus fertilizer into the drip irrigation system can also supplement the phosphorus deficiency.
Injecting phosphate fertilizer may clog the drip tape irrigation system. Due to the reaction of water and phosphate fertilizers, solid precipitation in water tends to cause clogging. Solid phosphate fertilizers such as superphosphate, agricultural monoammonium phosphate, and agricultural diammonium phosphate cannot be injected into the irrigation system due to their low solubility. The industrial grade monoammonium phosphate, diammonium phosphate, potassium dihydrogen phosphate, polyphosphate and other phosphate fertilizers are soluble and can generally be used for drip irrigation.
Superphosphate injection into irrigation water can often cause precipitation, which may cause blockage. The formed precipitate is very difficult to dissolve. When phosphorus and calcium ions meet in the solution, divalent or trivalent calcium phosphate is formed. The solubility of this salt is very low. Similarly, phosphorus, magnesium and zinc will form magnesium phosphate and zinc phosphate, which are insoluble in water, which can easily block the drip irrigation belt system.
Sometimes it is necessary to inject phosphoric acid into the drip irrigation system. In addition to applying phosphorus to crops, it can also lower the pH value of irrigation water. Lowering the pH value can avoid sedimentation. The method to lower the pH value is to mix and add appropriate sulfuric acid and phosphoric acid, and the pH value can be lowered to less than 4.0.
However, if phosphoric acid is injected for a long time, it will lead to zinc deficiency in crops. Generally, only when the combined concentration of calcium and magnesium in the water is lower than 50 ppm and the bicarbonate concentration is lower than 50 ppm, it is injected into the drip irrigation belt system.