Handling hardness

May 22, 2023

The effect of water alkalinity, and its subsequent effects on substrate pH, is an important topic for greenhouse growers. However, elevated alkalinity doesn't just affect substrate pH — it can also result in having to deal with hardness. This article will focus on water hardness — what it is, how to measure it, and how to manage it in production.

First, let's define hardness. Hardness is the combined calcium (Ca) and magnesium (Mg) in water, and is expressed as the equivalents of calcium carbonate (or limestone) in me/L or ppm (1.0 me/L = 50 ppm for converting between the two). When the alkalinity in a water source is high, it's a good idea to also measure hardness, as hardness can accompany high alkalinity. If hardness measures more than 150 ppm or 3 me/L, additional measuring of the Ca and Mg concentrations is useful. Both Ca and Mg are cations, or positively charges ions, and are taken up by the plant in bulk flow. However, Ca and Mg interact and can cause antagonisms, where an excess of one nutrient can inhibit the uptake of another. Ideally, the ratio of Ca:Mg should be 3 to 5 ppm Ca to 1 ppm Mg to keep the uptake of each nutrient sufficient to avoid deficiency symptoms from developing.

How can you measure hardness? There is a range of options for measuring hardness in irrigation water ranging in price, ease of use or implementation, and resolution of information generated. The simplest approach to quantifying hardness in irrigation water is using color-changing test strips. While test strips are an inexpensive, simple and rapid approach for measuring hardness, the resolution of the color-coded ranges is too low to be of much use. Next, inexpensive titration kits are available that offer results with much better resolution. Hand-held colorimeters available at a modest price point (approx. $50 to $75) and desk-top photometers ranging from a several hundred dollars offer excellent resolution for measuring total hardness or Ca or Mg concentrations in-house. Finally, water samples can always be submitted to a commercial laboratory for analysis, and total hardness and Ca and Mg concentrations are reported in the results.

For residential water, a water softener is used to reduce Ca and Mg in water by passing water over or through sodium-containing salt or columns. Since Ca and Mg are both positively charged ion, are exchanged for positively charged sodium. While this is acceptable for human use, using water with elevated sodium is not appropriate for plants and residential water softening should not be used for irrigation water. Water softeners using potassium (another positively charged ion) are also available for commercial irrigation water, though these systems can be costly to install and maintain. Thankfully, there are other steps that can be taken to deal with hard water.

When hardness results in magnesium more than the recommended Ca:Mg ratio of 3 to 5:1, pre- and post-transplant steps should be taken to re-balance these two nutrients. First, if possible, consider using a different source of lime to adjust pH for soilless substrates. Use calcitic limestone, composed of calcium carbonate, instead of dolomitic limestone. Dolomitic limestone is the most common type of limestone used to adjust soilless substrate pH, and there are a few important things to keep in mind when switching to calcitic limestone. First, it is faster-acting than dolomitic lime and, as a result, the amount added to substrate to adjust pH is less than the amount of dolomitic lime that would be added to achieve the same change in pH. Additionally, while calcitic lime is faster-acting than dolomitic lime, it also has a shorter residual of one to two months. This shorter residual may pose a challenge for crops with longer production times. For post-transplant correction of elevated Mg relative to Ca, adjust fertilizers accordingly by reducing magnesium and increasing calcium provided. Keep in mind that increasing calcium in concentrated fertilizer solutions can increase the chance of it reacting with sulfur from magnesium sulfate (MgSO4) or micronutrient-containing sulfates, precipitating, and falling out of solution. A two-tank system — similar to the A and B tanks commonly used in hydroponics — allows for increasing Ca concentrations in fertilizer stock solutions while avoiding precipitation.

When Ca is elevated, there are a few steps that can be taken to restore the balance of calcium and magnesium. First, use dolomitic limestone as the lime source for adjusting substrate pH, since dolomitic limestone is comprised of both calcium carbonate and magnesium carbonate. Additionally, pre-plant additions of MgSO4 to growing substrates may be used. In addition to modifying pre-plant strategies, there are some post-transplant approaches to hardness with elevated calcium. First, look for opportunities to reduce calcium in fertilizers. Perhaps you are using a high-calcium fertilizer formulation that may be exacerbating the elevated Ca in the irrigation water. Or use the two-tank approach mentioned above and increase Mg concentrations by adding additional MgSO4. For one-time applications, up to 32 oz./100 gallons of MgSO4 can be applied as a substrate drench.

Having appreciable hardness in your water certainly doesn't making crop production any easier. Hopefully the steps outlined in this article describing hardness and its potential effects on nutrient management, how to measure hardness, and correcting for Ca and Mg imbalances will make growing high-quality greenhouse crops easier.

Christopher is an associate professor of horticulture in the Department of Horticulture at Iowa State University. [email protected]

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