Understanding Plant Nutrition: Irrigation Water As A Nutrient Source

Nutrient concentration varies dramatically between greenhouse operations (Table 1), and it has a major effect on your choice of fertilizer. Irrigation water rarely contains high enough concentrations of the primary macronutrients (nitrogen, phosphorus or potassium) to be considered significant for plant growth. However, water can contain significant concentrations of the secondary macronutrients calcium (Ca), magnesium (Mg) and sulfur (S) and micronutrients such as boron (B). The nutrients supplied to a crop are a combination of several sources, including the irrigation water, acid and chemical fertilizers. For example, it is more important to select a fertilizer that contains calcium and magnesium if your irrigation water already contains 10 ppm Ca and 2 ppm Mg than if your water contains 75 ppm Ca and 30 ppm Mg. 

Acidifying Water Can Add Further Nutrients

Adding mineral acids to the irrigation water to neutralize alkalinity can also add nutrients. Sulfuric acid (H2SO4), phosphoric acid (H3PO4) or nitric acid (HNO3) can be a significant source of sulfur, phosphorus or nitrate nitrogen (Table 2). For example, it takes about 1.7 fluid ounces of 85 percent phosphoric acid per 100 gallons of water to neutralize 100 ppm alkalinity. That concentration of phosphoric acid will also supply about 61 ppm P to the nutrient solution, which is equivalent to the phosphorus supplied by 20-10-20 at a 285 ppm N. Changing from phosphoric acid to either sulfuric or nitric acid will eliminate the phosphorus, but it will, in turn, supply either sulfur or nitrogen to the nutrient solution. 

Waste Ions

Some "waste" ions contained in irrigation water are either not needed by the plant or the plant requirement is so low that only small amounts are needed. Examples of waste ions are sodium (Na) or chloride (Cl). Generally, their presence in irrigation water at high concentration increases salt build up in the root media. Even a nutrient like calcium can be a problem for salt accumulation if its concentration is too high.

There are several ways to manage water with high salt concentration, all of which affect plant nutrition. One option is to leach more heavily than the commonly recommended 0 to 20 percent rate. High leaching rates also remove nutrients from the root medium, so leaching heavily generally means you should also increase fertilizer concentration in order to maintain adequate nutrient concentrations in the growing medium.

Changing the water source may be an option when salt concentrations are too high. If an alternative water source is not easily available or does not have a sufficient capacity, consider water treatment. For example, reverse osmosis (RO) purification will remove most salts (except for boron), leaving very pure water. Salts in a water source can also be diluted by blending the high salt water with a low salt water (like RO purified water).

If the water source is changed, then regular testing is needed, especially for EC, pH and alkalinity, until you are confident that the new water source is consistent. If the EC is greater than 0.2 mS/cm, additional laboratory testing may be required. If water quality has been changed significantly, then reevaluate your entire nutrition program (acidification, fertilizer, lime rate, media, etc.) to ensure that all nutrients are applied at adequate rates and media pH remains stable. 

Ideal Versus Manageable Nutrient Concentrations

There is no perfect irrigation water for crop production. Each water source comes with its own set of challenges. As long as the concentration of nutrients, alkalinity or waste ions (not used for growth) in the irrigation water is within a manageable range, most problems can be overcome, either through proper management of alkalinity or by supplementing or balancing the nutrients contained in the water with those supplied by chemical fertilizers. Next month, we will begin discussing chemical fertilizers and their affect on nutrition management.

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