Chapter 11

Irrigation Water for Greenhouses

Alkalinity Control for Greenhouse Irrigation Water

Alkalinity levels up to 2 meq/L (or 1.5 meq/L for plug production) will probably not require any preventive action against substrate pH rise for most crops. If the alkalinity levels range from 2 to 3 meq/L, consider adding less limestone to the substrate and/or using acid-reaction fertilizers. In severe cases, when the alkalinity level of water is 3.0 meq/L or higher, it is often necessary to inject acid into the irrigation water to prevent the substrate pH level from rising too high by the end of the crop. If the alkalinity levels are higher than 8 meq/L, consider treating your irrigation water by reverse osmosis.

Reduce Limestone Added to Potting Media

If water is moderately alkaline the amount of limestone added to potting media can be reduced. Most greenhouse substrates require limestone in their formulation. Therefore, less limestone can be used to compensate for the lime that will be applied in the alkaline water.

Fertilizer Selection

Use of an acidic fertilizer may be all that is required to counteract alkalinity in moderately alkaline water. As will be discussed in Chapter 15, Fertilizers for Greenhouse Crops, some fertilizers are basic (alkaline) in that they raise the potting media pH, while others are acidic. Fertilizer acidity/basicity can be used to manipulate moderate pH shifts in the potting media.

Acid Treatment

When the alkalinity level of water is 3.0 meq/L or higher, it becomes necessary to take stronger action by injecting acid into the water supply to prevent the substrate pH level from rising too high by the end of the crop. This action should only be taken when reducing limestone in the medium and using acidic-reaction fertilizers have proven inadequate. Bicarbonate (HCO3) and carbonate (C032)  in water cause the substrate pH to rise because bicarbonate and carbonate combine with substrate acidity (H+) to form carbonic acid (H2C03), which then converts to water (H20) and carbon dioxide (C02). In these processes, the acidity and the bicarbonate are consumed.

Types of Acids

The acids commonly available to growers include phosphoric, sulfuric, and nitric. Table 11.5 lists criteria for choosing the right acid for your situation: relative safety, neutralizing power, cost, and nutrient content. For moderate alkalinity levels, many growers opt for phosphoric acid, as it adds phosphorus to the final fertilizer solution and is safer and inexpensive than most other acids. Phosphoric acid is suitable when only 1 or 2 meq/L of alkalinity needs to be neutralized. The amount of acid required to neutralize higher alkalinity levels provides phosphorus in concentrations far above those needed by plants. For extremely high alkalinity, sulfuric (or battery) acid has the highest neutralizing ability but is more hazardous to handle.

Nutrients from Acids

With the exception of citric acid, acids used for water acidification also supply essential plant nutrients. The nutrient supplied can be beneficial to plant growth (if not supplied in excess), but it can also react with fertilizer salts in concentrated stock solutions or with pesticides if mixed into spray solutions. Growers who acidify their water should adjust their fertilization program for the nutrient supplied by the acid.

Acid Injection

Acid is introduced into the water stream in various ways. Growers with low­volume wells pump water into a large holding tank. Water is then pumped at a much higher flow rate from this tank to the crops during the few hours of the day when irrigation is required. In this situation, acid may be pumped directly from the drum in which it was purchased to the holding tank any time water is being supplied to the holding tank. The two pumps that separately deliver water and acid to the holding tank are activated by the same water level sensor in the holding tank.

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