Understanding alkalinity

Departments - Production Pointers

Water alkalinity is a key part of understanding water quality, but growers may not be paying enough attention to it.

Fig. 1. A great analogy for alkalinity is to think of it as liquid or dissolved limestone that is added to the container each time a plant is irrigated.
Photo: Christopher J. Currey

When I talk with growers, and the discussion shifts to water quality, one of the first facts shared with me is the pH of their water source. The pH of your substrate or the root zone is important, as it influences micronutrient availability, with micronutrients more available at lower pHs and less available as pH increases. But when it comes to water quality, I am more interested in the alkalinity of their water source, not the pH. Alkalinity is the carbonates and bicarbonates dissolved in water and is a measure of the ability of water to neutralize acid.

A great analogy is to think of alkalinity like liquid limestone. Each time you irrigate, the carbonate and bicarbonate become part of the soil solution and have the same effect as the limestone in your growing media, raising pH or resisting acidification. Alkalinity is measured either in milliequivalents or milligrams per liter (meq/L or mg/L), so you may see either of these units when alkalinity is quantified. Thankfully, converting between these units is fairly easy: 1 meq/L is equal to 50 mg/L.

Now that you know what alkalinity is and how it is quantified, how much alkalinity do you need? Or how much is too much? That depends on the type of crop you are growing, including the species, container size and crop time. Generally speaking, crops that prefer a higher pH, larger containers and longer crop times can be grown with water that has higher alkalinity. Alternatively, species that grow better at a lower pH, smaller containers and short crops need to be grown with lower alkalinity.

While we can’t cover the variation in species in this article, plants that prefer a higher pH will tolerate higher alkalinity because the pH will drift up, while plants that prefer a lower pH are less tolerant of alkalinity because of the pH drift. Plugs, grown in small cells, require water with 0.75 to 1.25 meq/L (~40 to 60 mg/L), while plants in packs and flats are tolerant of a little more alkalinity, up to 1.7 to 2.0 meq/L (85 to 100 mg/L), depending on the flat size. Plants grown in containers greater or equal to 6 inches can tolerate up to 2.5 meq/L (125 mg/L) alkalinity due to the lime charge in substrate being used up in longer crop times.

There are a few different options available to measure alkalinity, and they vary in ease, accuracy and price. The simplest method is to use alkalinity test strip kits that can be purchased at pool and spa supply stores. Simply get a sample of your water, dip the test strip in sample and wait for the coloration of the strip to indicate the approximate alkalinity concentration. While this is a very quick and affordable method for measuring alkalinity, the measurement will be an approximate concentration. A similar method is using a titration kit, where reagent is added to the water sample to turn the water color, then acid is added until the color changes, with the number of drops added indicating the range of alkalinity.

For a more accurate measurement, there are alkalinity test kits that are commonly used for aquariums that can be used for greenhouse water. A reagent is added to your water sample and the vial is placed in a colorimeter that will measure the color and determine the alkalinity concentration. The accuracy of these units is greater than the test strip kits or simple titrations, while costing less than $100. The most accurate way to measure alkalinity is to submit a sample for analysis to a commercial lab, but this will cost more and take a little bit longer than in-house methods to get results back.

If the alkalinity level in your water is high, you’ll also want to check the hardness of the water as well. Hardness is a combined measurement of calcium and magnesium. These two mineral nutrients are both secondary macronutrients required by plants, but they also have an antagonistic relationship and excessive amounts of one can suppress uptake of the other.

Alkalinity is an important aspect of water quality, but it does not always get the attention that is warranted. Using the information in this article, you can measure the alkalinity in your water, revaluate how it is influencing your crop culture and consider ways you want to optimize your water’s quality.