Greenhouse Water Treatment and Filtration
Neutralizing Alkalinity/pH Adjustment
The main problem that arises from a high alkaline water is its effect on pH. Alkaline water can raise the pH of substrate resulting in nutrient deficiencies if the pH of substrate is raised above the ideal range for nutrient solubility. The management actions and corrective procedures taken to optimize fertility and combat the other adverse effects of high alkalinity/pH will vary based on the amount of alkalinity of the water and the crops you are growing. Water with alkalinity between 1.0 and 1.5 meq/L (50 and 75mg/L) is considered high-quality irrigation water.
Fertilizer Selection
Fertilizer selection is one method to manage pH in soilless production systems when using irrigation water with moderate alkalinity. Every fertilizer is labeled with a potential acidity or basicity value in calcium carbonate equivalents (CCE, lb/ton). A fertilizer formulation with a high potential acidity can balance the effects of moderately high alkalinity, keeping the substrate pH within an acceptable range. This is especially true in systems where soluble fertilizers are added frequently during irrigation. Acidifying fertilizers include ammonium sulfate, diammonium phosphate, monoammonium phosphate, urea, and ammonium nitrate.
Reduce Limestone Added to Potting Media
If water is moderately alkaline the amount of limestone added to potting media can be reduced. Most greenhouse potting media requires limestone to raise the pH within a range acceptable for plant growth. Applying water with moderately high alkalinity can be viewed as applying liquid lime.
Acid Treatment
In commercial greenhouses, the most common method for minimizing the liming effect of high alkalinity is to add a strong mineral acid (usually sulfuric acid or phosphoric acid) directly to the irrigation water. As the pH of the water decreases, some of the alkalinity is neutralized.
Types of Acids
The acids commonly available to growers include phosphoric, sulfuric, nitric, and citric. Table 19.1 lists criteria for choosing the right acid for your situation: relative safety, neutralizing power, cost, and nutrient content. When selecting an acid, try to specify food grade or technical grade. This will avoid undesirable impurities such as lead, arsenic, or selenium that may be present in some industrial formulations.
Pre-Dilution of Acid/Base Concentrates
Pre-dilution helps with mixing since the specific gravity is reduced and the acid is less concentrated and viscous. Pre-dilution can improve the distribution of acid along the moving water stream, since more pulses of dilute acid will be added for a given effect.
pH Reaction Time
A common problem when adjusting pH is the process reaction time. Typically, it can take several minutes or even hours after the addition of a pH amending substance for the pH to fully stabilize. This is because the added acid or base materials take time to react fully and come to equilibrium with the other constituents in the water.
Nutrients from Acids
As well as providing pH correction, each acid supplies an essential plant nutrient (phosphorus, sulfur, and nitrogen). Nitrate nitrogen is supplied by nitric acid, phosphate phosphorus by phosphoric acid, and sulfate sulfur by sulfuric acid.
Click on the following topics for more information on greenhouse water treatment and filtration.
