The Very Basics Every Grower Should Know
Before you jump in the deep end, it is good to get an idea of what’s in the water! Knowing the characteristics of the irrigation source is an absolute necessity in container plant production. Complex interactions between water, substrate and fertilizer determine the availability status of various nutrients that are essential for normal plant growth. Water quality can vary significantly depending on factors such as geographic location, season of the year, and source. For example, rainwater and most surface water supplies are pure as they contain relatively few minerals. In contrast, deep aquifers in coastal Southeastern areas can contain high levels of minerals. During dry seasons wells tend to have a higher mineral concentration compared to rainy seasons. Various mineral elements are always present in water, whether in almost undetectable or excessive amounts. Both situations can create problems for the greenhouse grower, and this is why prior knowledge of water quality before the crops are grown, is crucial. Often signs of a particular problem that has developed in the plants could point to a likely culprit, like whitish or reddish brown deposits on the leaves after water applied from overhead dries out. In the first case, white deposits are usually associated with water high in calcium and/or magnesium carbonates, and in the second, brown spots indicate excessive iron in the water. When designing the nutrition program, you should take into consideration the presence or absence, and the relative quantities of various chemical elements in your water source. Irrigation water affects the pH of the soil solution, which in turn controls nutrient availability.
Having the water source tested is but a first step. You also need to understand the results. Here’s a brief overview of water quality characteristics and their meaning. We’ve also included information on what steps to take to correct a specific problem.
Alkalinity, pH and Hardness
When growers are asked what their water is like, the answer is often: “I know I have high pH.” Conversely, the answer is “low pH.” But pH is not the most important thing about the water source! By far the most crucial factor to know about your water source is alkalinity. Let us look at both.
A pH reading is a measurement of the acidity or basicity of a solution and indicates the concentration of hydrogen ions. The range is 0 (most acid) or 14 (most basic). Lime juice has a pH of about 2.0, drinking water ranges from 6.5 to 8.0, and a strong lye solution reaches 14.0. The recommended ranges of irrigation and substrate solution pH depends on the crop grown; generally 5.4 to 7.0 for the irrigation water and 5.2 to 6.3 for the substrate solution.
Alkalinity is a total measure of the substances in water that have “acid-neutralizing” ability. Two ways to think of alkalinity are that it is the buffering capacity of water and that it is like lime in the water.
Alkalinity is attributed mostly to calcium and magnesium carbonates and bicarbonates, which are major components of limestone. Alkalinity should not be confused with pH. While pH of a solution is the concentration of hydrogen ions in it and measures the strength of an acid or a base, the alkalinity reflects the solution power to react with acid and keep the solution pH from changing. The alkalinity, then, indicates how well a solution is buffered. Alkalinity sounds very much like “alkaline” but beware because they are not the same thing! Alkaline is a term applied to solutions with a pH higher than 7.0.
The alkalinity level has far-reaching implications because of its strong effect on the substrate pH. Of two water sources, one with a pH of 9.0 and alkalinity of 50, and the other with a pH of 7.0 and alkalinity of 300, the first will raise substrate pH very little, while the second will cause a much higher raise in the substrate pH. Judgment of the quality of the two water sources would have been wrong if based on the pH alone, and correct if based on the alkalinity levels.
Since carbonates and bicarbonates are the major components of water alkalinity, most laboratories equate Total Carbonates [TC = carbonates (CO 3 2- ) plus bicarbonates (HCO 3 – )] with alkalinity. Other laboratories assume that bicarbonates are the sole contributors to alkalinity. Alkalinity is expressed as parts per million (ppm), milligrams per liter (mg/L), or milliequivalents per liter (meq/L) of equivalent calcium carbonate or bicarbonate alone. Various sources prefer to use one or another of these units, and unless you are familiar with the conversion factors, it could be rather confusing:
50 ppm CaCO 3 =50 mg/L CaCO 3 =1 meq/L CaCO 3 =61 ppm HCO 3 – = 61 mg/L HCO 3 – =1 meq/L HCO 3 –
How does the water from a well get a high alkalinity? The underground aquifer is associated with large rock layers made of limestone (calcium carbonate) and dolomite (calcium and magnesium carbonate), calcium and magnesium bicarbonates. Water comes in contact with the rocks and dissolves some of the component minerals. The longer the duration of contact, the more minerals are dissolved (this is why after prolonged periods of drought the alkalinity of a well may rise and the opposite may occur during rainy periods). When the calcium and magnesium carbonates and the calcium and magnesium bicarbonates are dissolved, they dissociate as calcium (Ca), magnesium (Mg), carbonate, and bicarbonate ions:
Ca 2+ + Mg 2+ + HCO 3 – + CO 3 2 –
The substrate pH rises because the carbonate and bicarbonate ions react with the substrate acidity (H + ) to form carbonic acid, which in turn converts to water and carbon dioxide.
HCO 3 – + H + Þ H 2 CO 3 Þ H 2 O + CO 2 , and, CO 3 2 – + 2H + Þ H 2 CO 3 Þ H 2 O + CO 2
In these reactions, the acidity (H + ) and the carbonates and bicarbonates are consumed. The loss of hydrogen ions in the substrate results in a higher pH level. This is the mechanism through which alkalinity in the water affects the substrate pH.
But wait! What about the calcium and magnesium ions that are left in the substrate solution? They are the major contributors to the waterhardness . Hardness is a measure of the combined content of calcium and magnesium in water. Hardness is expressed as parts per million (ppm), milligrams per liter (mg/L), or milliequivalents per liter (meq/L) of equivalent calcium carbonate. As you might expect, hard water would generally be associated with high alkalinity, but not always. If there are high levels of calcium and/ or magnesium chloride in the water, then the water may not have high alkalinity, even though it would be considered “hard.” In this case you have to measure the level of chloride, as it may pose potential problems. If you have hard water, you have to look at the levels of Ca and Mg and the ratio between them. Proper balance should be 3 to 5 ppm Ca to 1 ppm Mg, if expressed in meq, or 5 to 1, if expressed as ppm Ca and Mg. Calcium levels higher than these can interfere with the uptake of magnesium, causing magnesium deficiency in the plant. If you have hard water with high levels of Ca and Mg, it may be wise to lower the amount of limestone added to the growing substrate. Make sure that you monitor the substrate pH to make sure that it is in the proper range!
Bottom line on the water pH, alkalinity, and hardness: A high water pH (over 7.2) should be considered a warning to look at the alkalinity level. Hardness can be used only to estimate alkalinity. A specific test for alkalinity is required.
Not all water sources have high mineral content and high alkalinity. In fact, if you have very pure water and especially if you are using acidic fertilizers, the substrate pH may gradually decrease of over time. This may affect the availability of nutrients, and cause micronutrient toxicities in susceptible crops. For this reason, some growers with water that has very low levels of carbonates and bicarbonates, add potassium bicarbonate (KHCO 3 ) to increase the water buffering capacity. Suggested minimum alkalinity levels range from 0.66 to 0.8 meq/L for plug production, and 1.2 to 1.98 meq/L for plants in 6” pots.
Steps to take to correct high alkalinity
What level of alkalinity should you consider “best”? Precise upper critical alkalinity level cannot be assigned. An excessive alkalinity will cause a substrate pH to rise to unacceptably high level by the end of the crop cycle. Three factors decide the upper critical alkalinity level: the length of the crop period, the plant to substrate ratio, and the upper substrate pH level that the crop can tolerate. Water alkalinity causes substrate pH to increase over time as more water is added with each irrigation . The longer the crop cycle, the higher the pH can get. A short-term crop may tolerate a high alkalinity level in the water, while a long-term crop may not. Also, the smaller the growing container and the larger the plant shoot , the faster the changes in pH can occur. This situation develops in plug production, where the seedlings are grown in very small volumes of soil. As their shoots grow, they use large quantities of water. If the irrigation water has a high alkalinity level, the substrate pH can quickly rise, because there is little substrate to neutralize the carbonates and the bicarbonates. Lastly, crops that need low substrate pH for normal growth will not tolerate high alkalinity.
Alkalinity levels up to 2 meq/L (or 1.5 meq/L for plug production) will probably be safe for most crops. If the alkalinity levels range from 2 to 3 meq/L, you should consider adding less lime to the substrate and/or using acid fertilizers. Acid injection is required if the alkalinity levels are above 3.0 meq/L. If the alkalinity levels are higher than 8 meq/L, you should consider treating your irrigation water by reverse osmosis. The precise quantities of acid to add can be determined using an alkalinity calculator developed by researchers at North Carolina State University and can be found on the following website: https://www.ces.ncsu.edu/depts/hort/floriculture/software/
Salinity is the total quantity of dissolved salts in the water. Since all salts are charged ions, the solution that they are dissolved in, conducts electricity when an electric current is applied to it. We obtain a measure of the electrical conductivity or EC of the Total Dissolved Salts (TDS), and it gives us the salt levels. The higher the EC, the more salts are dissolved. However, there is no indication of which salts are present. A common conversion factor derived from the average of many water samples is: 1 mmhos/cm = 640 ppm TDS. The soluble salt level should ideally be less than 0.75 mmhos/cm for plug production, less than 1.0 mmhos/cm for other greenhouse crops, and less than 2.0 mmhos/cm for nursery crops. High levels of salts can accumulate in the plant tissue and cause burns, and/or they could build in the growing substrate and prevent plant roots from absorbing water. The latter case could cause wilting and stunted growth.
Sodium is another salinity factor, which if found in high levels, could reduce water movement into the plant and growth retardation. It could also interfere with the uptake of nutrients and thus lead to various macro- and micronutrient deficiencies. Sodium levels are reported as Sodium Adsorption Ratio (SAR). SAR reflects the amount of sodium in relation to the amounts of calcium and magnesium present in the water and helps determine if the sodium is the dominant cation. SAR levels higher than 4 meq/L could result in excessive amounts of sodium absorbed. This situation could be alleviated by addition of calcium. It is recommended that water containing more than 3 meq/L sodium, should not be used for overhead irrigation because of the danger of excessive foliar uptake of sodium and leaf marginal burn.
Chloride is the final salinity factor of concern. Similarly to sodium, high levels could interfere with the water absorption and cause wilting and stunted growth. Chloride can accumulate in leaf tissues, resulting in leaf burns. Chloride could become a problem if levels are higher than 2 meq/L.
Macro- and Microelements
Nitrogen (N), Phosphorus (P), Potassium (K), Calcium (Ca), Magnesium (Mg), and Sulfur (S) are macroelements that are essential for plant growth. In moderate levels they will not create any production problems. However, their levels in the water should be evaluated as an indicator for potential water contamination, and in relation to the nutrition program. We already discussed Ca and Mg relationship with respect to water hardness. Acceptable levels of various nutrients and other water quality parameters are listed in Table 1. Aluminum (Al), Boron (B), Copper (Cu), Fluoride (F), Iron (Fe), manganese (Mn), Molybdenum (Mo), and Zinc (Zn) are microelements that also are essential for plant growth. Among the micronutrients boron and fluoride could pose problems. Boron-sensitive crops could show toxicity symptoms if levels are above 0.5 ppm. Fluoride in levels higher than 1 ppm pose a special problem to crops in the Liliaceae, Agaveceae, and Marantaceae families.
Iron fixing bacteria present in some water sources, especially wells and basins, can lead to bluish sheen on leaf surfaces and brown stains on plants. The blue and the brown deposits have two different causes, both related to high iron content in the water source and overhead irrigation. The bluish sheen is caused by iron fixing bacteria that keep the iron in the water from settling out so when water is applied overhead, the bluish iron deposit is left on the leaves. The red-brown stains are caused by iron. If you suspect high iron levels or iron fixing bacteria in your water source, have your water analyzed by a lab.
The quality of the irrigation water source should be analyzed and carefully evaluated. The major factors to consider are the alkalinity, hardness and pH. Water hardness should be used as an indicator of alkalinity. If hard water is present, alkalinity, chloride, and Ca to Mg ratio have to be evaluated. Both pH and alkalinity are important in making decisions concerning proper water treatment. Water salinity should also be taken into account, especially if high sodium is suspected. Water analysis should be done regularly. Recommendations are to obtain a water report:
- every time a new water source is added;
- at least twice a year, once during the dry season and once during the rainy season;
- as often as every three months.
Table 1. Recommended upper limits of nutrients and chemical capacity factors for water used for greenhouse crops and for containerized nursery crops.
|Capacity Factor||Upper Limit for greenhouse use||Upper limit for nursery use|
Substrate pH Factors
|pH||5.4 to 7.0 is acceptable||5.4 to 7.0 is acceptable|
|Alkalinity||2 meq/L||2 meq/L|
|Total Carbonates (as CaCO3 )||100 ppm||100 ppm|
|Bicarbonate (HCO 3-)||122 ppm||122 ppm|
|Hardness (Ca + Mg)||150 ppm CaCO 3||150 ppm CaCO 3|
|Electrical Conductivity (EC)|
|for plug production||10.75 mmhos/cm||2 mmhos/cm|
|for general production||1 mmhos/cm|
|Total Dissolved Salts (TDS)|
|for plug production||480 ppm|
|for general production||640 ppm||1280 ppm|
|Sodium adsorption ratio (SAR)||4||10|
|Sodium (Na)||69 ppm (3 meq/L)||69 ppm (3 meq/L)|
|Chloride (Cl -)||71 ppm (2 meq/L)||71 ppm (2 meq/L)|
|Total Nitrogen (N)||10 ppm (0.72meq/L)||10 ppm (0.72meq/L)|
|Nitrate (NO 3-||44 ppm (0.72 meq/L)||44 ppm (0.72 meq/L)|
|Ammonium||10 ppm (0.56 meq/L)||10 ppm (0.56 meq/L)|
|Phosphorus (P)||1 ppm (0.03 meq/L)||1 ppm (0.03 meq/L)|
|Phosphate (H 2PO4-)||3 ppm (0.03 meq/L)||3 ppm (0.03 meq/L)
|Calcium (Ca)||0 to 120 ppm (0 to 6 meq/L) is normal range||0 to 120 ppm (0 to 6 meq/L) is normal range|
|Magnesium (Mg)||0 to 24 ppm (0 to 2 meq/L) is normal range||0 to 24 ppm (0 to 2 meq/L) is normal range|
|Sulfur (S)||20 to 30 ppm (0.63 to 0.94 meq/L) is suggested for most plants||20 to 30 ppm (0.63 to 0.94 meq/L) is suggested for most plants|
|Sulfate (SO 4-)||60 to 90 ppm (1.26 to 1.88 meq/L) is suggested for most plants||60 to 90 ppm (1.26 to 1.88 meq/L) is suggested for most plants|
|Aluminum (Al)||0 to 5 ppm is normal range||0 to 5 ppm is normal range
|Boron (B)||0.5 ppm||0.5 ppm|
|Copper (Cu)||0.2 ppm||0.2 ppm
|Fluoride (F-)||1 ppm||1 ppm|
|Iron (Fe)||0.2 to 4 ppm||0.2 to 4 ppm|
|Manganese (Mn)||1 ppm||1 ppm|
|Zinc (Zn)||0.3 ppm||0.3 ppm
Organisms to Test for
|Iron fixing bacteria|