July 13, 2022

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. I like to rinse the bottle several times, be sure not to drink from the bottle directly but rather pour it out so that salts from your mouth don't skew the test. Be sure to run your irrigation water for a few minutes to displace any impurities in the line.

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 7.5 or higher should be a warning to check alkalinity. I routinely hear from growers who have been advised not to worry about pH in organics. While it is true that with a large volume of substrate and loads of organic matter you can get away without managing your pH closely in many instances there is an energy tradeoff that your plant will have to expend to adjust the pH at the root tip. That energy is better spent developing biomass and secondary metabolites so don't forget to pH your water. Low pH is more of a concern than a high pH.

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 mEq/L and the second has 8 mEq/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). Without knowing your alkalinity you can't really make a sound decision as to how to manage pH. Suitable alkalinity will vary with pot size. A larger volume of soil can buffer higher alkalinity. Small plants can handle an alkalinity of 1-1.3 mEq/L while 4" and half gallon pots can handle 1.6-2.0 mEq/L. Large plants in large containers can handle a higher range, 2-2.6mEq/L. If the EC of your water is high, consider acid injection. You want to stay under 2mEq/L.

EC or PPM - 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. Non Charged molecules such as urea nitrogen and glucose (sugar) will not impact or show up in your EC reading. 0.6 is the limit for seedlings and 1.2-1.5 is the limit for most mature plants with regard to a salt feed. Organically grown plants can tolerate higher ECs. Each genetic may be different though so get to know what your specific cultivars like. Keep in mind EC readings in your substrate may not be particularly accurate without the right meters.

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. I actually prefer my metrics a little lower. Keep an eye on your Paste Reports and if your bicarbonates are climbing take action with acid injections to neutralize it. Hard water will clog emitters and drip tape. Soaking these in white vinegar is a convenient way to clean them. Har water filters are not recommended and if they are used you should ask for K not Na to be used in your exchange membranes. Na is great for washing clothes but bad for plants and people.

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. High Na will create uptake issues for Ca, Mg and K.

Chloride - Upper limit for most crops is 70 ppm, under 80 is usually safe 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). Keep in mind that chlorine filters are not efficient at removing chlorine. To remove chloramine you will need a special catalytic carbon filter or R/O to remove chloramine. Your local water provider should be able to tell you if your water source is treated with chlorine, chloramine or both. Again, due to lack of runoff chlorides will accumulate quickly requiring flushing so exclusion is key.

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. Acid treatment is used to neutralize carbonates and bicarbonates.

SAR (Sodium Adsorption Ratio) is a measure of the relative amount of Na to Ca and Mg in water. Substrate SAR levels over 13 are indicative of poor native soils. SAR Risk is a determinant of usability of irrigation water. An irrigation SAR above 2 is cause for concern.

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 or special filters however. Special catalytic carbon filters or R/O 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 and chloramine.

Well Water can vary in quality dramatically as it is highly reflective of parent material. 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. There are a variety of tests; heavy metals, water suitability, pesticide, pathogen, etc that can be utilized to check the quality of your water.

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. Add 0.5 grams of water soluble gypsum to your reservoir and keep it agitated as a safe buffer. You can also add up to 10% of your "bad water" as a buffer.

Reading a Test:

Test 2x per year for the first two years, make sure your water source does not move regionally throughout the season or test again when it does. You primarily want to see if you get a change from the wet to the dry season.

EC seedlings 0.6, 4" and 0.5 gallon 1.2 - 1.5

Salt concentrations need to be higher in the root than the soil solution or water cannot be taken up. Early signs of osmotic stress are wilting, root dieback and apical point burning.
pH is generally acceptable between 5.8 and 7.2. I prefer 6.8-7.0 to make things easy on my plants. Low pH is more of a concern than a high pH especially where heavy metals and trace toxicity symptoms are concerned.

Alkalinity controls pH of your substrate. If your substrate pH is 7 or higher check alkalinity. This is a measure of your carbonate and bicarbonate levels in your water that act as a liming input. 1.5 mEq/L (75 ppm CaCO3 equivalent) = caution. 3.0 mEq/L (150 ppm CaCO3 equivalent) is problematic and requires acid injection. 40 - 100 ppm is an acceptable range. High alkalinity will cause the pH to climb. It has a stronger impact with smaller pots and longer runs. It is dependent on irrigation frequency and rate, shoot to substrate ratio and length of crop cycle. Total alkalinity of 30-100 is safe zone in substrate.

0-1.5 no action required
1.5-3 acid and reduced lime
3-8 - acid injection
8-10 switch to R/O water or filtration

Hardness - High alkalinity dictates the need for a hardness test. This is the measure of Ca and Mg in the water.
If above 3 mEq/L check Ca:Mg ratios. 3-5:1 is correct ratio
Hardness of CaCO3 50-120 mg/L is safe range. Alternatively, you want to be below 3.5 grains/gallon (60 ppm).

TDS - 640 mg/L seedlings/960 mg/L for mature plants as a general guideline.

N - excessively high indicates surface contamination and possible pathogens. We want to see less than 5 ppm NO3 and less than 5 ppm NH4.

P - above 5 ppm can indicate surface water or septic tank contamination. P above 5 ppm can create issues for trace element uptake.

Na - should be less than 50 ppm at the highest. High Na will block Ca, Mg and K.

Chlorides - should be less than 50 ppm. Plants can look fine but have low yield with high Cl.

Chlorine - should be less than 2 ppm. Although not as impactful as some would say it will negatively impact soil biology at high levels.

Ca - safe between 40-100 ppm. Recommended ratios are K:Ca:Mg (4:2:1)

Fe - you want this less than 5 ppm

Mn - you want this less than 1 ppm, 0.5 is my cutoff typically.

Zn - you want this less than 0.3-0.5 ppm

Cu - you want this less than 0.2 ppm

Boron - you want this less than 0.3- 0.5 ppm, above 2 is toxic.

Fluorine - you want this less than 1 ppm

SAR - you want this less than 2. 2 is cause for concern.

Proper Filter Use:
TDS/EC - R/O, Deionization
Bicarbonates - Carbonates -R/O, Deionization, Anion Exchange
Ca/Mg - Deionization, Anion Exchange, water softener (acid injection preferred)
Chlorine - bubble off and or carbon filter
Chloramine - acid and or catalytic carbon filter