The Importance of Water Quality and Water Testing
Water is life. Not only is it essential for all living things, the polar molecule, H2O, is one of the most important variables in your garden. Yes, many folks overlook testing it or incorrectly assume their water is safe or good for their plants. Depending on your water source, it can even change throughout the year. Because of this it is important to test your water in both the wet and dry season for the first two years of any new cultivation project.
Water tests can feel daunting at first. How do I take a sample? Is it going to be expensive? Once I get the results, how do I tell if the water is okay to use or not? How do I treat the water if I do have an issue?
Let me assure you that taking a sample is as easy and filling a disposable water bottle from your hose bib and putting it in the mail. We have been using Logan Labs for our soil and water testing for almost a decade now. Whatever lab you go with, it's important to stay with them for future testing, so you can compare your results, apples to apples. Here's a link to the water sampling instructions.
So let’s look at some of the most common water sources and what the pros and cons are of each option as well as some targets for water tests and red flags to watch out for.
Before discussing different sources of water, it’s important to understand the basic terms for understanding how to evaluate your water source.
pH - pH, or potential Hydrogen, is the measurement of how acidic or basic your water is. It’s a scale from 0-14, with 7 being neutral. Keep in mind that it’s a logarithmic scale, so a pH of 5 is 10 times more acidic than a pH of 6. To take this to a more extreme example, a pH of 2 would be 100,000 times more acidic than a pH of 7. pH can be affected by the various dissolved minerals in the water, some of which can be beneficial and others, like most heavy metals, are best avoided. Regardless of your cultivation style, you should always pH your irrigation solution. Without the correct pH your plants will not be able to uptake nutrients whether they are available in the soil or not. OLS has the best yields in a pH range of 6.8-6.9. A high pH of 7.2 or higher should be a warning to check alkalinity.
Alkalinity - Alkalinity is often overlooked and ignored. While pH will make the final distinction as to uptake capability of available nutrients in your rhizosphere, alkalinity is a measure of the liming agents in your water. Based on your alkalinity your pH may have little to no impact on your soil. Conversely, alkalinity can cause even small pH issues to become major concerns over time. pH and alkalinity give a full picture of the charge in your solution. Alkalinity controls the primary impact of your substrate pH, not pH. For example, look at two water sources both at pH 7.4. The first source has an alkalinity of 2 me/L and the second has 6 me/L. The first will do little to affect the pH of the substrate where the second would cause an unacceptably high pH. Put simply, alkalinity is the ability of water to raise pH. (It is primarily a measure of the carbonate and bicarbonate in your water-so applying alkaline water will have the same effect as adding liming agents)
EC or ppms - Soluble Salts on your Paste Test is simply your EC x 640 (approximately). This is a summation of the total mineral content of your water. The lower the EC the more pure the water is. Keep in mind many growers unknowingly get trace elements from contaminants in their water source. This can be good or bad. Keep water contaminants in mind when formulating your amendment plan. Any electrically charged salt ions dissolved in solution contribute to EC. (Noncharged molecules such as urea nitrogen will not show). 0.75 is the limit for seedlings and 1.5 is the limit for most mature plants.
Hardness - The presence of high alkalinity water dictates a look at the hardness metric for your water source. Hardness is a measure of the Ca and Mg in your water source. Hardness is expressed in me/L or as ppm of equivalent limestone (calcium carbonate). As is the case with alkalinity 1 me/L equals 50 ppm equivalent calcium carbonate. If your hardness reaches 3 me/L it is important to check the Ca:Mg. You should have a Ca:Mg of 3-5:1. Otherwise you can cause uptake antagonisms. 1.5 me/L is the caution flag for hardness. 3 me/L is the trouble flag.
Sodium- Upper limit for most crops is 50 ppm, under 100 can be ok for cannabis depending on how it balances against other cations. Regardless, it is always best to keep as low as possible. Some cultivars are very salt intolerant and will do poorly with Na levels over 50 ppm. Sodium is not an essential nutrient and we want to limit the input of sodium as much as possible. We monitor compost and inputs so we want to monitor the water as well. Since few OLS growers account for runoff, highly mobile Na tends to get trapped in the soil and can lead to osmotic stress and cation antagonisms.
Chloride - Upper limit for most crops is 70 ppm, under 100 is fine for cannabis. Chloride (Cl-)is an essential nutrient but it is needed in such low concentrations that contaminants provide sufficiency. Not to be confused with chlorine (Cl) or chloramine (NH2Cl)
Carbonates (CO3) and Bicarbonates (HCO3) increase your SAR risk. When combined with calcium and or magnesium carbonates and bicarbonates form the precipitates calcium carbonate (CaCO3) or magnesium carbonate (MgCO3). Keeping your soil moisture levels higher helps to slow this process. This locks out Ca and Mg making the percentage of Na higher by comparison than either of the aforementioned required elements yielding a much higher SAR Risk. This creates an alkalizing effect and can raise the pH.
SAR (Sodium Adsorption Ratio) is a measure of the relative amount of Na to Ca and Mg in water. SAR levels over 13 are indicative of poor native soils. SAR Risk is a determinant of usability of irrigation water.
Sources Of Water And What To Watch For:
Municipal Water can be high in chlorine and chloramine. Chlorine will off gas, even without air bubblers, in a 24 hour period. Chlorine concentrations of just 0.4ppm can cause root tip death. Chloramine requires chemical intervention however. Inexpensive dechlorinators can be added to the water to remove any chloramine. This is for the health of your microbial populations. Chlorine and chloramine are deleterious to your soil microbes, however the high organic matter in most soils will complex the chlorine/chloramine quite rapidly. Keep in mind the difference between chloride and chlorine.
Well Water can vary in quality dramatically. Well water should be checked at a minimum of two times per year. Heavy rains can push ground contaminants; nutrient and pathogens, into your well. Dry seasons can concentrate minerals to dangerous levels for your plants. pH can fluctuate widely between seasons of heavy irrigation vs dry season.
Rain Water can vary in pH but is low in alkalinity. Proper storage for greenhouse or indoor cultivation would require monitoring and aeration of some sort.
Reverse Osmosis Water. For serious water issues reverse osmosis utilizes a series of very fine filters under pressure to remove nearly all of the charged particles, minerals, pathogens and contaminants from your water source. It can be costly and it creates a huge effluence problem as it wastes upwards of 20 gallons to produce one clean gallon. This high salt water runoff is then returned to the groundwater or nearby waterways in many cases negatively impacting local flora and fauna. Since the nutrients have all been stripped from the H2O it is sometimes called “hungry water”. Typically acidic and desirous of elements to bond with R/O can strip heavy metals from anything it comes into contact from the source, fittings, hoses to storage reservoirs etc. It has no alkalinity so a buffer is needed to prevent wild swings in pH. This “hungry water” is not safe to drink and it will quickly ruin pH meters as will deionized water.
Quick Cheat Sheet On Water Quality: