A Drop Of Clean Water

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Water

A single drop of water can contain millions of bacteria, each one impossible to see with the naked eye. The same can be said for water in the greenhouse. What looks innocent at first glance may harbor waterborne pathogens, brim with toxic salts or have alkalinity issues. Without a regular plan in place to test water quality, how is a grower to know what he or she is really feeding their plants? Monitoring water quality is one of the first steps to better plant health throughout the greenhouse.

“Testing irrigation water helps growers avoid surprises and detect changing trends so they can make small course corrections to prevent future problems,” says Paul Fisher, associate professor and Extension specialist in environmental horticulture for The University of Florida. “If you have a shallow well or water from a river or pond, testing is especially important because water quality can change throughout the year.”

In addition to a yearly analysis, Fisher recommends weekly in-house tests to check pH and electrical conductivity (EC) on clear irrigation water. Managers should also test the fertilizer solution to ensure that injectors are working properly.

Alkalinity

Rapid changes in the alkalinity level of irrigation water can wreak havoc on pH levels in the medium and negatively affect plant health. Growers need to be aware of what their water alkalinity is so they can evaluate their treatment options. 

Unlike pH, which is a measure of the hydrogen ions in the water, alkalinity measures the buffering capacity of the water or the ability of the water to neutralize acids. The two measurements are distinct, but also inherently linked on another level. Water alkalinity influences substrate pH, but the pH of irrigation water has little effect on the pH of the solution in the substrate.

Water alkalinity is the equivalent of “dissolved limestone” because dissolved bases(carbonates or bicarbonates of calcium, magnesium and sodium) react with the hydrogen ions in the water to raise pH. When this “dissolved limestone” builds up in a container over time, substrate-pH is sure to rise. Iron, manganese, zinc and boron deficiencies can occur at high pH levels because of low solubility. Plant health begins to deteriorate, often leading to secondary problems from pathogens.

Acidification, in the form of sulfuric, phosphoric and nitric acids, is a common cure for high alkalinity. The amount of acid needed depends on the level of alkalinity and the type of acid used. Two good online resources that provide alkalinity calculators are North Carolina State University (http://www.ces.ncsu.edu/depts/hort/floriculture/software/alk) and the University of New Hampshire (http://extension.unh.edu/Agric/AGGHFL/Alkcalc.cfm). Another good online resource that aids growers in sustainable and efficient water use is http://www.watereducationalliance.com, run by The University of Florida.

Other Issues

In addition to high alkalinity, growers face several other water quality problems (See Common Irrigation Water Problems and Potential Solutions, Table 1). Biological problems, such as waterborne pathogens, algae and bacteria also change water quality in the greenhouse. Pathogens lead to plant disease, while bacteria and algae can clog emitters, create safety hazards and attract pests. Filtration and sanitation are helpful for eliminating these problems. Test source water, stored water, recycled water and water at the furthest outlet when monitoring for biological problems.

Every water source is different to some degree, and treatment options vary. Growers should implement a preventative monitoring plan that includes water quality testing on a regular basis. This one small step pays big dividends, namely better productivity and better overall plant health.

Table 1–Common Irrigation Water Problems and Potential Solutions

Problem Associated media
or plant problem
Solution Potential side effect

High alkalinity
(>150 ppm)

High substrate-pH,
iron deficiency

Option 1: Acidify with a strong mineral acid

Will supply additional levels of nitrogen, phosphorus, or sulfur, depending on the acid, therefore, you may need to reduce concentration of these nutrients from your other fertilizer sources.

Option 2: Use an acidic (high ammonium) fertilizer

Fertilizer contains high levels of ammoniacal nitrogen, but not calcium.  This may lead to excess growth, or ammonium toxicity when substrate temperature is low.

Low alkalinity
(<40 ppm)

Low substrate-pH,
iron and manganese toxicity

Use fertilizer low in ammoniacal nitrogen

Substrate-pH may fluctuate or tend to drop over time.

Low calcium
(<30 ppm)

Calcium deficiency

Use fertilizers that contain calcium

Fertilizers that contain calcium also have low levels of ammoniacal nitrogen, so they tend to have a basic pH reaction in the substrate.  Therefore, track substrate-pH to ensure it does not increase too high over time.

Low Magnesium
(<15 ppm)

Magnesium deficiency

Option 1: Use fertilizers that contain magnesium nitrate

Can be used with a fertilizer that contains calcium nitrate, but these fertilizers also have low levels of ammoniacal nitrogen (see notes above).

Option 2: Use fertilizers that contain magnesium sulfate

Can be used with acidic (high ammonium) fertilizers but do not mix in the same stock tank with calcium-based fertilizers or precipitates will form in the tank.

High Boron
(>0.5 ppm)

Boron toxicity

Use fertilizers that contain calcium. Maintain substrate-pH above 6.0

Fertilizers that contain calcium also have low levels of ammoniacal nitrogen (see notes above).

High sodium
(>50 ppm)

High substrate-EC,
Calcium deficiency
Magnesium deficiency

Use fertilizers that contain calcium and magnesium.  Both higher fertilizer concentrations and higher leaching rates may be required.

Fertilizers that contain both calcium and magnesium also have low levels of ammoniacal nitrogen (see notes above).

*Reprinted with permission from Understanding pH Management for Container-Grown Crops, by William R. Argo and Paul R. Fisher, a Meister Media Worldwide publication, (Note: Potential solutions may have side effects; refer to the publication for the full table.)

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