Fertigation is the method of applying dissolved fertilizers directly to crops through an irrigation system, most commonly drip or micro-sprinkler systems. This practice significantly increases the efficiency of nutrient delivery by placing dissolved minerals directly into the plant’s root zone. Because nutrients must pass through small emitters and irrigation lines, specialized fertilizer compounds are required. These compounds must possess specific characteristics to ensure they remain dissolved and do not clog the system.
Essential Characteristics of Fertigation Fertilizers
Fertilizer materials selected for irrigation systems must exhibit high water solubility to prevent clogging of filters and emitters. Nutrients must dissolve completely and rapidly, sometimes without continuous agitation in the mixing tank. Solubility depends on the temperature of the water, as cold water reduces the maximum concentration that can be dissolved.
These fertilizers must also be highly pure, containing minimal inert materials or insoluble residues that could settle out. Purity helps prevent system blockages, which disrupt nutrient timing. Furthermore, the compounds must be compatible with the irrigation water, especially when it contains high levels of minerals like calcium or magnesium. Mixing incompatible salts, such as sulfates with high-calcium water, can cause precipitation that forms insoluble compounds like gypsum.
Primary Nitrogen and Potassium Compounds
Nitrogen (N) and Potassium (K) compounds form the foundation of most fertigation programs because they are required in large amounts and are highly water-soluble. Urea is a commonly used nitrogen source due to its high nitrogen concentration and non-ionic nature, which minimizes unwanted chemical reactions. It dissolves easily, though the process is endothermic, meaning it cools the water, which can slightly reduce maximum solubility.
Liquid nitrogen solutions, such as Urea Ammonium Nitrate (UAN) 32%, are popular, particularly in large-scale agricultural systems. UAN provides nitrogen in three forms—urea, ammonium, and nitrate—offering a readily available and sustained source. These liquid products offer convenience and eliminate the need for on-farm mixing of dry materials.
For potassium, Potassium Nitrate (KNO3) is highly favored because it provides potassium and nitrate-nitrogen in a single, highly soluble compound. This compound introduces no chloride or sulfate, avoiding common incompatibility issues. Conversely, Potassium Chloride (Muriate of Potash) is the most economical and highly soluble potassium source. However, its use is often restricted in chloride-sensitive crops due to potential toxicity from the chloride ion.
Specialized Phosphorus and Micronutrient Sources
The application of Phosphorus (P) through fertigation presents a challenge because most standard P fertilizers are not adequately water-soluble and can react with calcium in the water to form insoluble precipitates. To overcome this, specialized, highly soluble, and often acidic sources are necessary. Monoammonium Phosphate (MAP) is a common choice, as it provides both nitrogen and phosphorus in a soluble form.
An effective solution is the use of Phosphoric Acid (H3PO4). This liquid product is a strong acid that supplies phosphorus and lowers the pH of the fertilizer solution. This acidity is beneficial because it helps dissolve existing mineral scale in the irrigation lines. It also prevents the formation of calcium and magnesium precipitates, keeping the emitters clean.
Micronutrient Sources
Micronutrients, which include elements like iron, zinc, and manganese, are required in very small quantities but must be equally soluble. These elements are typically applied in chelated forms, such as Fe-EDTA or Zn-EDTA. Chelation involves surrounding the metal ion with an organic molecule. This protects it from reacting with other ions in the water solution and the soil. This safeguard ensures the micronutrients remain dissolved and bioavailable to the plant.