Taking Care of Plant Nutrition in Your High Tunnel – Water Hardness and the Removal of Unwanted Ions

It is essential, especially in hydroponics to start with a laboratory analysis of your source water. It is also important to do follow-up analysis throughout the year. Water quality can change especially where the water source is a well or a pond. In the article Taking Care of Plant Nutrition in Your High Tunnel-Water Alkalinity (Issue 627), we have discussed the importance of water alkalinity and how to correct high alkalinity levels. Additional elements of importance are Ca, Mg, S, Na, Cl, Fe and Mn. Knowing concentrations of these ions can help you to determine the need to purify water, leach or bleed more frequently, as well as to avoid these contaminants by choosing the appropriate fertilizer.

Hard water might be generally associated with high alkalinity, but it is not always the case. Water hardness is a measure of the amount of dissolved calcium (Ca) and magnesium (Mg), expressed as if it were calcium carbonate (CaCO3). Hard water contains high amounts of dissolved Ca and/or Mg and the upper limit for high quality greenhouse irrigation water is 150 ppm CaCO3 (Table 1). When the hardness is greater than 150 ppm, it is important to check the amount of dissolved Ca and Mg in the water and determine the Ca:Mg ratio. The ratio should be 3 to 5 ppm Ca to 1 ppm Mg. If the ratio is higher, Ca can prevent or block the uptake of Mg. Conversely, if the ratio is lower Mg can block the uptake of Ca. The most common problem is a low level of Mg relative to Ca. This problem can be corrected with the addition of MgSO4 (Epsom salt). Hardness at levels above 150 ppm could result in equipment clogging and foliar staining (deposits of scale) problems. Some types of water softening equipment replace the Ca or Mg in the water with sodium (Na). These treatment options make the water unsuitable for use in high tunnel and greenhouse production systems. Softening of water for greenhouse use is not recommended unless the process is followed by reverse osmosis treatment.

Table 1: Targeted range for high quality greenhouse irrigation water

Parameter Target range (ppm)
Hardness 100-150
Calcium 40-120
Magnesium 7-24
Iron <5
Manganese 0.2-0.7
Sulfate 24-240
Sulfur <100
Sodium <50
Cholride <70

 

The sodium (Na) and chloride (Cl) concentration in the water should be less than 50 and 70 ppm, respectively. Sodium can interfere with the uptake of Ca and Mg, and can cause foliar burns due to poor water uptake and Na accumulation in plant tissue. The Sodium Absorption Ratio (SAR) indicates the relative concentration of Na to Ca and Mg in water. SAR greater than 4 could result in root uptake of toxic levels of Na. High Na and Cl levels are a much bigger problem in coastal areas. If you do experience excessive levels of Na and Cl dilute the water with rain water or any other source that has lower Na and Cl levels. Reverse osmosis or distillation treatment of the water is also recommended in very bad cases. It will be advantageous to irrigate larger water volumes at a time to ensure leaching of salts from the soilless substrate. When producing in soil you may also want to uncover your high tunnel during the winter in order for the snow and rain to leach the excess salts from the soil.

Iron (Fe) and Manganese (Mn) is soluble in their  reduced states (Fe2+ and Mn2+). High levels of soluble ferrous iron and manganese may be found in well water. When oxidized, Fe3+ precipitate as an insoluble red substance and Mn2+ precipitates as MnO2. When water containing high Fe and Mn concentrations are used for drip irrigation, the ions are oxidized and these insoluble salts will block drippers. Apart from oxidization due to aeration, chemotrophic ferric and manganese bacteria can also contribute to the oxidization of Fe and Mn. These bacteria cause the oxidized residues to accumulate among the bacterial waste, creating a slimy residue that blocks drippers. Specialized equipment can be purchased for iron removal. After oxidation it can be filtered out through a resin bed. Large-scale removal is most efficient using a settling pond and a sand media filter. Various oxidizing filters can also be used depending on other water chemistry. Removal of manganese utilizes the same treatment described for iron, but removal is more difficult and may require additional pH adjustment (Table 2). Characteristics of water quality in terms of risk of blocking drippers are listed in Table 3.

Table 2: Aerobic oxidation time as affected by pH

pH level Aerobic oxidation time for Fe (minutes) Aerobic oxidation time for Mn (minutes)
<6.0 > 180 > 1000
6.6 < 60 > 1000
7.2 < 10 > 1000
7.8 < 6.6 > 1000
9.0 < 3 < 200
9.5 < 2 < 45

 

Table 3: Water classification indicating relative clogging potential in drip irrigation systems

Clogging Hazard Maximum Fe (ppm) Maximum Mn (ppm) Maximum hydrogen sulfide (ppm)
pH <7.0 7.0 – 8.0 >8.0
Minor <0.2 <0.1 <0.5
Moderate 0.2 – 1.5 0.0 – 1.5 0.5 – 2.0
Severe >1.5 >1.5 >2.0
1. Bicarbonate concentrations exceeding about 2 meq/liter and pH exceeding about 7.5 can cause calcium carbonate precipitation.

2. Calcium concentrations exceeding 2–3 meq/liter can cause precipitates to form during injection of some phosphate fertilizers.

3. High concentrations of sulfide ions can cause iron and manganese precipitation. Iron and manganese sulfides are insoluble, even in acid solutions.

Source: Circular 573. New Mexico State University

Sulfur and hydrogen sulfide is rarely a parameter of concern in Indiana. Sometimes sulfate in well water converts to hydrogen sulfide. Hydrogen sulfide occurs in concentrations of less than 10 ppm, but can be as high as 50 – 75 ppm. Its commonly found in ground water, especially where wells are drilled in shale or sandstone, or near coalfields. Much of the groundwater in northwestern and northeastern Indiana has noticeable levels of hydrogen sulfide . High levels of hydrogen sulfide occurs in smaller sections of the state. The biggest threat to agriculture is the possible clogging of drip irrigation emitters. Sulfide can precipitate to clog emitter flow passages. Precipitation problems will generally not occur when hard water, which contains large amounts of hydrogen sulfide, is used. Hydrogen sulfide will minimize the precipitation of calcium carbonate (CaCO3) because of its acidity. Several removal techniques can be considered. The same iron and manganese removal filter can be used to remove low to moderate amounts of hydrogen sulfide. The filter oxidizes the hydrogen sulfide, converting it to insoluble sulfur, which are removed through the filtering process. An activated carbon filter can also be used to remove sulfur sediment (Extension bulletin WQ-11, Purdue University).

This is the second article in a 7 part series that look at soil fertility and nutrient solution management for high tunnels. In the next issue, we will concentrate on ‘Water Soluble Fertilizer Calculations’.

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