Landscape
And
Nursery
Dialog
Mary Ann Rose
Commercial Landscape
& Nursery Specialist
The Ohio State UniversityMay, 1995
What's on Tap ? (Water Quality in the Nursery)
Keywords: water quality, irrigation, pH, nutrition, water testing, nursery, container production
I rrigation water quality plays a critical role in container plant nutrition. Understanding how water quality affects your growing medium is a key to understanding how nutrient availability in the growing medium changes over time.
Growers faced with water test results have an awful lot of numbers to look at, so which ones are really important ? While potentially any of various elements tested could contribute to a nutritional problem, the key factors we'll consider are alkalinity, calcium, magnesium, sodium, nitrate, and electrical conductivity.
Alkalinity. Alkalinity is a new term to many growers, but one they should know. Alkalinity, not pH, is the capacity of water to neutralize acidity. The pH, while given by all water tests, is not particularly useful information. Alkalinity may be expressed on your water test as either ppm bicarbonate (HCO3) or ppm calcium carbonate (CaCO3) (122 ppm HCO3 = 100 ppm CaCO3). Use of high-alkalinity water over time will elevate the pH of the growing medium; it is analogous to adding limestone. Conversely, use of low-alkalinity water may result in a decrease in the medium pH over time. High-alkalinity water is fairly common in Ohio, especially where well water is used in central or western Ohio. Over time, the elevated pH in the medium caused by high alkalinity may reduce micronutrient availability, especially iron. As a result, plants look hungry even when adequate nutrients have been supplied.
Alkalinity exceeding 100 ppm CaCO3 may be cause for concern in propagation or greenhouse production. However, in outdoor container production with larger sized pots there is probably no cause for concern below 200 ppm CaCO3. Plant species tolerate alkalinity to different degrees; those that tend to develop iron deficiency are most likely to be affected over time by high alkalinity water.
High alkalinity water is usually 'hard' water as well, but hardness is a different concept that describes the capacity of the water to form deposits. Typically, hard water will leave white (calcium or magnesium) or orange (iron) spots on leaves.
Acid injection is the most common method of neutralizing alkalinity, but has limited effectiveness in preventing hard water from clogging irrigation systems or forming deposits on leaves. The cost of acid injection may be well justified in propagation houses, but is probably cost-prohibitive in larger production sizes. If hard water spots are your problem, polyphosphate injection may be used. The polyphosphate chelates the minerals that form spots, preventing them from precipitating on leaves; however polyphosphate does not neutralize alkalinity. The cost of a polyphosphate system may be justified in sales areas where hard water spotting may be unsightly to customers.
Choice of fertilizers and media amendments also may be helpful in combatting problems with water alkalinity. Where high alkalinity water and a climbing medium pH are a problem, growers may want to consider reducing or eliminating limestone media amendments. Before doing this they will have to consider how to supply their crop adequate calcium and magnesium. In addition, most fertilizers for nursery crop production supply N in predominantly urea or
ammoniacal form (NH4), which will lower medium pH over time. The sulfur in sulfur-coated materials also decreases pH.
Calcium and Magnesium. Dolomitic limestone is commonly added to media at a rate of five to eight pounds per cubic yard to raise the pH and supply calcium and magnesium. However, if the irrigation water contains 40 and 20 ppm of calcium and magnesium, respectively, no further addition of these elements is required for plant growth. Furthermore, if the alkalinity of the water exceeds 100 ppm as CaCO3, addition of limestone to control pH is unnecessary. If you are a grower that has incorporated dolomitic lime for years but find you have water that meets these criteria, I suggest that you try cutting back on dolomitic lime addition over time until you prove to yourself that you don't need it. In the long run you may find that you have fewer hungry-looking plants.
The ratio of ppm calcium to ppm magnesium in the irrigation water also is important. Calcium to magnesium ratios of 3:1 or 1:1 are acceptable, 2:1 is optimum. An imbalance between calcium and magnesium could be counteracted by adjusting medium amendments.
Sodium. Sodium levels in the water above 50 to 70 ppm may cause leaf burn in sensitive plants; sodium also may interfere with the uptake of calcium and magnesium. The Sodium absorption ratio (SAR) is calculated from the calcium, magnesium, and sodium concentrations in water. Long-term use of water with a high SAR (greater than 6 for woody plants) may destroy the structure of field soil. A water survey of Ohio floriculture businesses conducted by Dr. John Peterson in the 1980s indicated that only a small percentage of growers had excessive SARs, however, where this problem occurs it may have a serious effect on growth.
Nitrate. Nitrate is a key plant nutrient, but may also be an environmental pollutant and a health hazard when consumed in drinking water. The EPA has determined that nitrate should not exceed 10 ppm. High levels of nitrate in your water may indicate that you have unacceptable levels of runoff from your nursery.
Electrical conductivity. The electrical conductivity (EC) is a measure of soluble salts. Problems with soluble salts can be expected to increase in the future as the demand on water supplies increase, and as more growers turn to recycling runoff. Soluble salt problems tend to increase under hot, dry conditions because water depletion from ground and surface waters concentrates the ions in solution. Moreover, the necessity of frequent irrigation during hot weather results in greater total salt accumulation in the root zone. The ions that commonly contribute to high soluble salts include chloride, sulfate, and sodium.
A high EC in the growing medium reduces plant growth by making it harder for plants to absorb water. An EC above 1.5 mmhos/cm is cause for concern for most plants, but species sensitivity varies considerably. Ericaceous plants are particularly sensitive; levels as low as 0.75 may be harmful to some azalea cultivars. Even a very low EC (0.5 mmhos/cm) will reduce quality and longevity in cut flowers.
Solving a soluble salts problem is difficult. Leaching periodically to flush salts from the medium may be necessary, but this practice wastes water and contributes to runoff. Growers who recycle irrigation water usually find it necessary to mix recycled solution with low-EC water from another source.
Testing your water. Water quality can vary a great deal depending on source and time of year. Although problems occasionally arise with water from municipal supplies, water quality problems more often occur in water from wells and irrigation ponds. If you have never tested your water before, test several times during the year. Once you have an idea of how much seasonal fluctuation there is in your water, monitor your irrigation water quality
every few years.
To collect a water sample, allow water to run at maximum flow for five minutes. Collect water in a clean, plastic pint bottle. Fill the bottle to the top leaving no airspace, and seal tightly. Water samples may be sent to private labs or the R.E.A.L laboratory at the Ohio State University, O.A.R.D.C., Wooster, Ohio 44691-4096.
