Hydroponic Science: Hard Water, Salt Stress, and Silicon
Photo credit: Wilfredor
I have this bottle of Armor Si from General Hydro that just sits in my basement. It’s a silicon supplement for hydroponics. Sometimes I use it and other times I don’t bother. Generally, it’s had a positive effect on my hydroponic lettuce that I grow in my basement (I feel my lettuce gets a nicer crunch), but very often the effects of silicon (Si) supplementation are subtle. This piqued my curiosity about the role of silicon in hydroponics.
Out in the field, plants grow in the presence of Si; it’s a component of the soil. However, most hydroponic nutrient mixes don’t include this humble element. Actually, it’s a bit of a pain to use in hydroponic systems. For one, you must dissolve it before adding other nutrients. Second, Si supplementations generally are alkaline in nature. What this means for the hydroponic grower is that your Si supplement is going to drive up your pH levels. Get ready to break out a couple of lemons or your pH-down solution! Finally, there’s the issue of cleanup. In my experience, Si supplementation leaves a lot of residue on my containers that I need to scrub off between crop cycles. In a commercial setting, where containers are larger, more numerous, and lots of plumbing is involved, adding Si can probably impact the cost of materials and labor to a noticeable level.
So, then, what are the benefits of Si? From information found around the web, Si incorporates into the cell walls of plants as silica gel, which helps make plants more rigid, and it can be used to boost a plant’s resistance to pathogens. Another reported function of Si is its ability to help a plant with stresses, such as temperature, and drought. Looking at the scientific literature, however, there’s a lot of information on the positive effects of Si on salt stress. Salt stress is a very common plant stress in the environment, and is an active area of research, especially as it relates to water scarcity. I wanted to explore salt stress in hydroponics a bit further because the sodium ion is present in a lot of water sources, and it is especially present in salt-softened water systems, where other cations such as magnesium and calcium are exchanged for sodium. Salt-softening is a popular and inexpensive form of water softening, however because it produces sodium-contaminated water, salt-softened water is not recommended for plants. The amount of sodium in salt-softened water is directly proportional to the hardness of the pre-softened water. Nevertheless, it could be possible that somewhere on this planet, someone might have to cope with salinated water in hydroponics due to the high costs of removing salt, and so it becomes interesting to see at what point salt levels become an issue.
So we know sodium can be bad for plants. The question now becomes, how much of it starts to make plants sick? Before I go on, I must mention that scientific literature searches can be the ultimate rabbit hole. There are volumes of articles on any given subject matter, and each article can have 30-100+ references therein. Thousands of articles are published every year, and depending on the subject matter, you can have centuries of research articles at your fingertips. This makes for a great body of ammo that scientists can use to disprove their competitors’ research during Q&A sessions after conference talks (cough…GRCs…cough), but it’s also easily overwhelming to the everyday researcher. That being said, I decided to stick to lettuce in order to narrow my focus. Now, lets get the unit conversions out of the way.
Growers like using ppms, and scientists like using molarity values. Coming from a research lab, I wish that we could all just settle on moles and molarity. Oh well. We can assume for our conversion that ppm is by weight (not absolute quantities of molecules), such that 1 ppm = 1 mg/kg. This seems to be the convention for ppms, probably because absolute molecular quantities would make conversions more of a pain. One mole of sodium weighs approximately 23g. Therefore, there are about 23,000 ppm of sodium in a 1 mol/liter (1 molar, 1 M) solution of table salt (NaCl). NaCl is generally used for salt stress experiments because its cheap and readily dissolves in water. Ok, so what are relevant levels of salt in our water systems? Luckily, the US government has data on water hardness values.
US water hardness data. Source: US. Geological Survey.
One thing to note here is that these values are ppm values of calcium carbonate. The chemical structure of this molecule is CaCO3. If you used a salt-based water softener (which is essentially a cation exchanger), two sodium ions would need to exchange for every calcium. Theoretically then, the ppm values can go much higher than 250, but let’s just stick with the range given because as you’re going to find out below, it doesn’t seem to be that big of a deal:
You can see from these data that we’re not really dealing with sodium concentrations over ~11 mM. This is interesting because what I noticed is that much of the literature on lettuce uses concentrations of sodium in the >50 mM regime. This suggests to me that more mild salt stress studies merit further investigation because they may be very commercially relevant. Nevertheless, there are some interesting things that happen when plants are grown in the presence of NaCl. First of all, the salt gets absorbed by the plants and transported to the leaves and stems. Not only that, but the NaCl actually competes with other nutrients, such as potassium and calcium. In Verte lettuce, Mahmoudi et al. saw decreases over 50% in leaf potassium levels when 100 mM NaCl was used in the nutrient solution. Calcium levels were decreased about 60%. Similarly, the fresh weight of the 100 mM NaCl-treated plants decreased about 40%. Another study by Xu et al. looked at 56 butterhead varieties and found that 30 mM NaCl could reduce the fresh weight of the lettuce by 12-58%, depending on the variety. Clearly some cultivars are more tolerant to salt than others. I highly recommend you check out the Xu et al. article to see what varieties work better under higher salt concentrations. They not only looked at butterhead, but crisphead, leaf, and even wild lettuces.
Unfortunately, I couldn’t find any low-salt studies in hydroponics, though there was one study in soil-grown lettuce that yielded some very interesting results. The report by Kim et al. added NaCl to the irrigation water to investigate its effects. Under 5 mM NaCl there was no hit to the overall size of the plants, and the salt treatment provided a slight nutritional enhancement of the cultivated lettuce. Of note was the β-carotene content, which was about 1.5-2 times higher in the 5 mM NaCl-grown lettuce. Granted, this is a soil study, so Si is definitely available for the plants to uptake. That makes Si a bit of a confounding variable, along with the other soil elements. Overall, it looks like lettuce starts to show symptoms of salt stress at 30 mM NaCl (690 ppm) for many lettuce varieties. Once again, it would be very interesting to see experiments with salt concentrations in the 60-300 ppm range.
Now, let’s talk about Si, starting with the units again. Si has a molecular weight of 28g. Thus, a 1 mM solution of Si is about 28 ppm. Looking at the back of my Armor Si bottle, the recommended feed strength is about 2 ml/gallon from a solution that contains 10% silicon dioxide. That’s 2 mL for every 3.79 liters, or 0.53 ml/liter. Because the solution is 10%, you get about 53 mg of Si per liter, or 53 ppm. So we’re looking at ~2 mM Si concentrations for any effects here. I found two reports: one from Milne et al. and one from Neto et al., who both used hydroponic lettuce systems in investigating the effects of Si on salt stress. Although both groups used concentrations of Si that were relevant to what is recommended by the Armor Si bottle, there were no differences in the fresh or dry weights of the Si-treated and untreated controls when it came to salt stress tolerance. The one significant difference reported in Milne et al.’s study was a ~75% increase in fresh and dry shoot weights, using 60 mM NaCl—a high salt concentration. Overall, the results from both studies showed large variability within groups, meaning that bigger experiments (i.e. much more samples) are probably necessary to weed out the effects of Si on salt stress.
From what I can deduce, lettuce can handle the sodium levels that are commonplace in US water sources, even if passed through a softener. The detrimental effects of salt stress in lettuce start to unfold when the concentration of NaCl reaches 30-50 mM, or 690-1150 ppm sodium. It’s important to note that, in the above studies, the salt stress was applied after the lettuce plants were allowed to grow for a certain period of time. What about germination, then? Well, salt doesn’t seem to have a very big effect either. In a 2003 paper by Zapata et al., nine different varieties of lettuce were germinated in petri dishes under either distilled water alone, or with 150 mM NaCl (3,450 ppm). Aside from a ~12 hour slower germination time, no significant differences in germination percentages were detected in 7 of the 9 cultivars, and the two that were affected had a ~10% reduced germination rate (from 100% to about 90%). Granted, if these seedlings were permitted to develop into mature heads of lettuce, the complications described above would likely unfold.
So where does all this leave us? Well, it doesn’t seem that sodium is such a big deal, at least in lettuce plants. Sodium levels in hard water sources, even after salt-softening, don’t seem to approach the levels in which adverse effects occur in experiments. In other plants, your mileage may vary, of course. In some cases, such as sugar beets, sodium can even be beneficial for plant growth. As for Si, it doesn’t seem to be a game changer under lower sodium concentrations, such as that found in ordinary tap water in the US. Perhaps in recirculating systems, where sodium concentrations can increase during top offs of water levels, it may play a role, but I’m doubtful because the NaCl concentrations need to be several-fold higher than what’s present in our water systems in order for Si to start showing benefits. The effects of Si on other plant stresses is certainly interesting, and I’ll probably take a closer look at that in a future article. Stay tuned!