Mistakes in hydroponic culture are not uncommon among both amateur and seasoned growers. Since there is considerable distances between a successful crop and an optimal crop, growers can go a long time without noticing mistakes that are likely to be heavily detrimental to their actual crop yields. Today I am going to share with you five of the most common problems I see when consulting for hydroponic growers and why these might be costing you a lot in yields.

Sup-optimal vapor pressure deficit. Temperature and humidity play a huge role in guaranteeing a large crop production. Plants can survive under a wide array of environmental conditions but the range where they produce optimal results is dependent on several factors, including the amount of carbon dioxide in the air, the plant species and the nutrient solution used. Most growers who make mistakes regarding VPD are either growing at a temperature that’s too high or at a humidity that’s too low. During winter low humidity tends to be the largest problem while during the summer issues with higher temperatures are most common.

Bad environments around root zones. Many growers water their plants with nutrient solution without measuring the characteristics of the solution that comes out of their media. Not measuring run-off EC/pH, especially when using non-recirculating setups, is a recipe for failure since the grower is completely unaware of whether root-zone conditions are good or not. More often than not, growers who make this mistake end up with very high salinity and extreme pH values – often acidic – that can be extremely hard to correct.

No foliar spraying regimes. Plants can take a lot of nutrition through their root zones but certain additives and nutrients are taken with far more efficiency through leaves. A lot of yield can be gained if proper foliar spraying with adequate additives to enhance growth is carried out. Many growers do not carry out any foliar spraying, leaving a lot of potential growth on the table that could be gained with these procedures.

No silicate applications. Potassium silicate is a very important additive in hydroponic culture and can make the difference between a very successful crop and a crop that has been heavily affected by fungal or bacterial diseases. Silicate applications have been repeatedly demonstrated to make plants immune systems stronger and – through the prevention of diseases and the strengthening of plants – can lead to healthier plants that have stronger yields.

No tailor-made nutrient optimization. Each particular grower has a specific set of plant species, varieties, media, temperature/humidity and carbon dioxide conditions that make their particular growing situation unique. Although generic nutrient solutions can do the job well enough to provide satisfactory yields, there is a lot of potential product left on the table if proper optimization of the nutrient solution is not carried out. Some nutrients – like phosphorous and calcium – benefit greatly form being optimized to the particular conditions each hydroponic grower has. Optimization takes effort and money – as some plants need to be dedicated to research – but the results can be more than worth it.

Although the above is not an exhaustive list of potential problems, it does provide you with an idea of the things that you might be doing wrong. With this in mind you can start to do your own research to attempt to fix these issues or you can contact me and schedule a call directly so that I can help you improve your hydroponic growing results.

If you searched for the optimal P concentration for plant growth in hydroponics you will likely find very different results, ranging from low values to very high values. This is inherently contradictory and difficult to understand, why don’t we have a smaller range for optimal P conditions? Why has it been so hard to describe what the best P levels are? Today we will talk about P nutrition and why there has been so much confusion regarding optimal P levels in hydroponic culture.

Almost all books about hydroponics and flowering plants will put optimal P concentrations in solution between 20 and 50 ppm, rarely will you find any book recommending P levels outside of these values in general, since these are recognized to be safe and they play well with standard nutrient concentrations used for other elements. However you will find articles for different plants recommending P levels that can be as high as 200 ppm to as low as 10 ppm. Take for example this article on Calendula, which recommend a P application of 10ppm, while this article on Lavender suggests 60ppm. Note that optimal P might also depend on the desired result as this article on Origanum dictamnus shows that there is a movements of essential oils from leaves to bracts at higher P concentrations in these plants.

Not only is there confusion about optimal P levels, but even the effects of P and the interaction of P with micronutrients are not very well understood. There is evidence (see here) that P promotes Mn uptake in tomatoes while it suppresses Fe and Zn uptake, while we have entirely different results in barley, where P is found to actually impede manganese acquisition. The above two articles also give a lot of references to P uptake literature, which I suggest you checkout if you would like to learn more.

The P literature is quite extensive (I suggest you read this thesis and its references if you would like to get a deeper dive), but overall we know that concentrations below 20ppm are rarely optimal and we do know that levels above 60ppm can be optimal for some plants under some conditions. In the thesis mentioned above we can see that tabasco pepper plants have the highest leaf area after 90 days in a P solution at almost 120 ppm.

Optimal P levels are perhaps harder to evaluate because they depend substantially on the concentration of other elements in solution as well as solution pH and root zone temperatures. We know that lower P stimulates root growth and reduces shoot growth while higher P levels have the exact opposite effect. Therefore variations in the ratio of P to other nutrients might be the optimal path for many crops but this is very hard to generalize as it depends on the particular growing conditions of each particular crop being grown.

Sadly the answer is that we don’t have an “optimal P” that will match all growing conditions and plants. We know that growing with a P value between 30-50ppm will give you decent results on almost all crops, but we also know that there are substantial gains to be made by optimizing P under your particular growing conditions (plant, media, tempeatures, etc). In some cases 50%+ increases in yields might be possible if P is properly tuned to the exact growing conditions used.

Your optimal P might be way lower or higher than what’s recommended in the literature, so start with the ballpark literature recommendation and make experiments from there to properly adjust P to maximize yields in your crop. Also make sure you carry out leaf-tissue, media and run-off analysis while you do this to ensure you get the best possible results.

Chelates are a very important part of hydroponic nutrient solutions as they provide a reliable source of heavy metals. Without chelates, heavy metals can easily go out of solution and become unavailable, either because they precipitate as an insoluble salt or because they are captured by active surfaces with a high affinity for metals. Among the heavy metals, Fe is the most important to chelate as it’s usually present in the largest concentration and is the most easily taken out of solution by the factors mentioned above.

Commonly chelating agents such as EDTA, DTPA and EDDHA are used in solution and they do a great job in providing adequate supplies of micro nutrients to plants. These three chelators have a very high affinity for Fe and therefore ensure that Fe will remain in solution and available to plants. However, a problem all of these chelating agents share is their lack of biodegradability, they all enter plant tissue and are going to be very difficult to get rid of by the plant. They can therefore accumulate in plant tissues to some extent and can cause problems of their own.

There are however some chelating agents that are both effective at protecting the heavy metals and easily biodegradable, from these, the most largely studied is perhaps imidodisuccinic acid (IDHA) whose structure is showed and compared with the other chelates in the image above. Although this chelating agent shares some common structural features with traditional chelating agents its chemical structure makes it incredibly easy to biodegrade and therefore a nice candidate for fertilizer use.

Several papers have compared IDHA fertilization to traditional Fe chelates (here, here, here, here). Although the IDHA is usually less stable in solution – as it would be expected given its chemical nature – it tends to give better results in terms of absorption and fertilization compared with the other Fe chelates. Given that it is also completely non-toxic to the plants – while the other chelates make the plant deal with the non-biodegradable aspects – plants fertilized with IDHA can actually be healthier. The image above, showing a comparison with EDTA – shows how the IDHA plants were not affected by a fungal infection that ended up affecting the EDTA treatment.

This does not mean that IDHA is the natural best choice for an Fe chelate. Some of the above studies have shown that IDHA can easily be captured by some media and its lack of stability implies that it is not a good choice for extended use in recirculating systems. However IDHA can be a better choice if the media used allows for it and the grower is able to apply it with its biodegradable nature in mind or if the desired products needs to be free of traditional chelate contaminants. In some cases – as mentioned before – it can actually be a significant improvement over traditional chelates.

The element selenium (Se) is not commonly used in hydroponic culture – as it’s not necessary for plant life – but the fact that it’s necessary for human life has meant that plant enrichment with selenium and its effects have been studied in hydroponics. Its effects however, are more than just an increase in Se concentration in plants. In today’s post we’ll talk about Se and what its effects in plant growth are according to some of the published literature.

Different studies can use different forms of Se, so it’s important to find out whether a study uses a source of Se cations, like Se chloride or a source of Se anions, like sodium selenate. If you want to reproduce the results you will need to match the exact source used, as using a different source can lead up to completely different results. Most studies focusing on Se use it in concentrations around 0.1 to 0.5ppm, although some studies do go as far as 5-10ppm, especially when studying the effects of the salts where Se is present as a cation.

Although most studies related to Se focus on the fortification of fruits, many studies also measure yield and plant quality related parameters in order to obtain as much information as possible. In this study of Se used in tomato plants there was a substantial enrichment of Se and a delayed ripening but there were no substantial effect on plant growth. However post-harvest characteristics of fruits were significantly improved by Se. Other studies on tomatoes, like this one, have however found improvements in yields when using Se.

Other studies like this one on curly endive or this one using Se nanoparticles in pomegranate, do show significant improvements in plant characteristics from using Se. In the pomegranate study, an 1.35 fold increase in the number of fruits was achieved, a very impressive mark given the characteristics of the treatment.

Selenium can also be a defense against temperature and salt stress. This article on peppers shows that an application of foliar selenium can help reduce flower drop rates and other adverse effects of temperature stress in these plants. This article on wheat seedlings, shows that selenium can also be protective against salt induced stress, preserving root growth under these adverse conditions.

It is also worth considering that Se can also become toxic to plants at anything but low concentrations. This review, which goes significantly into the articles that had been published up until 2014, goes deeply into this particular issue. The table above is particularly useful, as it shows the ranges of applications and toxicities for some plants. It is within the conclusions of the above review – as we have seen in the articles shown before as well – that Se can be used as an effective additive, stress protector and growth promoter when used in adequate amounts and forms (remember, cationic and anionic forms are different!), while it can become toxic and damaging if used without care.

Commercial hydroponics can be extremely expensive, given the technological complexity and supplies required for a successful crop. The biggest costs are usually related with the initial setup but subsequent crops can also become very expensive, especially if you are using boutique fertilizers or additives that can get very expensive very quickly. Today I want to talk about five ways in which you can save money in a hydroponic crop from a crop-cycle perspective.

Avoid buying liquid concentrates as fertilizers. Liquid fertilizers have some intrinsic advantages – like their homogeneity – but they contain a lot of water, which means that you will need to ship more than one pound of water for every pound of fertilizer you get. This will increase the cost of the fertilizer significantly, even if you’re buying fertilizers in bulk for a commercial crop. When buying single bulk or blended fertilizers make sure you always buy solids to greatly save on these costs.

Prepare your own blend of fertilizers for macro nutrients. The most complicated part of fertilizer preparation usually deals with the micronutrient portion of fertilizers, if you want to be as simple and cost efficient as possible you can actually buy this portion – some companies specifically sell the micro part – and then prepare all the macro fertilizer blends yourself. You can then hire a consultant or read the scientific literature to get a formulation you can then use to prepare your macro portion from bulk commercially available fertilizers (which are extremely cheap).

Prepare your own foliar treatments. Foliar spraying can greatly reduce problems and increase crop yields, so it is usually a no-brainer to make sure you use foliar sprays within your crop cycle. Some of these foliar additives can be very expensive though, but it can be very cheap for you to prepare your own additives if you have the proper know-how.

Use a recirculating nutrient system. Drain-to-waste nutrient setups are extremely wasteful. If you want to have a crop that is as cheap to run as possible you will need to go to a proper recirculating setup. Once you do this you will be able to use your recirculating solutions for weeks before having to carry out a nutrient change and, even then, there are some techniques that might allow you to keep your nutrient solution for even longer. Imagine if you only needed to prepare/change nutrients once every blue moon.

Make sure you use silicon additives. Many growers fail to use silicate containing additives within their crops and generally suffer from a far greater chance of having losses due to fungi. Potassium silicate is extremely cheap and with it you can make your own silicon containing additive that you can use to greatly fortify your crop against fungal disease. A small additional expense can save you a lot of loss and heartache down the line. You can a save a lot of money by avoiding commercial hydroponic silicate products and instead making your own silicate additive yourself from potassium silicate.

When implemented, the above changes can help a commercial operation save tens to hundreds of thousands of dollars per year in nutrients, additives and crop losses. Even only implementing a couple of the above can help a mid sized operation save a ton of money in just fertilizer if, for example, in-house macro fertilizers are used, or if a recirculating system with proper nutrient management is established.

Of course, the above steps are not trivial so I would recommend anyone attempting to do them for the first time to get someone with experience in the hydroponic industry to guide their hand through the process. That could either be me or any other highly experienced consultant in the field of commercial hydroponic growing and nutrients management. If you have enough time and the inclination to do so you could also try to learn the above things yourself from scientific literature and online resources, but if you choose to do so I would advice you try to implement what you learn in smaller crops before scaling to larger projects.

The efficient use of nutrient solutions is a very important topic in hydroponics. Although some commercial growers use run-to-waste systems where solutions are not recirculated, the economics of fertilizer use often demand re-circulation in order to enhance nutrient utilization and maximize growing efficiency. However one of the biggest problems found when circulating nutrient solution continuously is the build-up of plant exudates, which can be toxic and detrimental to plant growth.

Several solution for this have been studied historically, most commonly the use of filtration systems – such as activated charcoal cartridges – to capture these exudates and prevent their accumulation. The problem with this approach is that activated carbon – or other filters – are not neutral to some of the components of nutrient solutions and might disproportionately and efficiently capture metal chelates and eventually cause nutrient deficiencies. There are some ways around this – such as changing the formulations or replenishing solutions after filtering – but both are far from ideal.

More recently a paper has been published showing how electro-degradation can actually alleviate this problem by destroying these exudates – which are commonly organic acids – in nutrient solutions. The paper talks about how they used this technique to treat recirculating solutions in strawberry, eliminating autotoxicity and increasing fruit yields substantially.

The technique is very simple, basically using either a DC or AC current passed through an electrode that the solution circulates through, destroying the problematic molecules in the process. The first image in this post clearly shows how not renewing the solution causes important problems with yields that are completely removed by the use of the AC based electro degradation.

Another advantage of this technique is that – contrary to filtering techniques – there is little loss in the amount of nutrients in solution when performing the AC electro-degradation. Since the oxidation/reduction of the metal chelates used is highly reversible, the actual concentration of these elements in solution remains practically the same after treatment. You can see this in the image above, where there is no statistically significant change for the concentration of nutrients in solution.

The paper concludes suggesting a treatment of 24 hours (for 300L in the experiments) every three weeks, to completely recover from the exudates present in solution. For this AC application they used a frequency of 500Hz at 14V with an electrode area of around 53 square centimeters, made of titanium metal. For this process you need an inert metal or conductive material that will not react at the potential values used. You can buy titanium metal tubes – which are not expensive – to build an anode/cathode pair to carry out this experiment. Note that the frequency and voltage characteristics are vital so using a proper power supply to generate them is of the highest importance.

The above technique is novel and easy to build for treating commercial hydroponic solutions. It is far easier and economic compared with filtering techniques and can be applied from smaller to larger scale growing operations.

Properly controlling the hydroponic environment is perhaps one of the most challenging tasks the modern grower must face. Either with a small grow room or a big green house, it is difficult to properly control variables such as temperature, humidity, pH and nutrient concentration, ensuring they are all kept in tight ranges with the proper controlling actions always being applied. Today we’re going to talk about some of the research done into advanced control systems and how using these could help you boost your crop yields.

Hydroponic crops are dynamic systems, with plants continuously affecting their environment and demanding control actions in order to keep conditions constant. For example plants will tend to transpire water and absorb carbon dioxide during their light cycle, so in order to keep humidity and carbon dioxide concentrations constant you might need to turn on humidifiers, dehumidifiers, carbon dioxide generators, etc. Knowing what action needs to be taken is not trivial and naive control implementations – like turning on humidifiers, AC systems, etc when some thresholds are reached – can cause problems where sensors fight each other (for example a sensor trying to increase ambient humidity and another trying to raise temperature) or even fail to trigger.

In order to provide better control, researchers have created systems that rely on machine learning – systems that can learn from examples – in order to learn what control actions are needed and execute them in order to provide ideal control to a hydroponic setup. A machine learning system will be able to anticipate things like the lag between turning an AC unit on and the temperature decreasing, so it will be able to be both more efficient and more accurate in the way it controls your environment. This use of automated control guided by machine learning is also known as “smart hydroponics”.

For example you can read this paper where growers were able to increase the yield of a crop by 66% just by ensuring they could maintain proper environmental conditions the entire time using machine learning. In this case the researchers use a probabilistic method where the system determines the probability of an action – like triggering a sensor – will cause a desired effect. As data is accumulated the system basically executes whichever action has the highest probability to lead to the desired outcome.

There are other papers on the subject. In this one a deep learning neural network is used to perform a similar control role, although the quantification of improvements in this paper is not sufficient to claim that the control method would have been an improvement over a traditionally managed hydroponic setup, as the comparison is made between a soil control, not a hydroponic control with no automated environmental management.

This paper uses a simple IoT sensor control system and a multivariate regression approach in order to control the environment in a hydroponic greenhouse, this system was created with the aim to be cheap and usable in developing countries.

Although there are now several different demonstrations of this being done in the literature there still does not seem to be a commercially mature technology to carry out this task and the implementations seem to still be tailor made to each particular situation. However the modeling techniques used are not exceedingly complex and even modest commercial growers could – nowadays – afford to setup something of this nature.

With a computer, some arduinos, raspberry pi computers, sensors and time and effort a grower could definitely setup a very nice, machine learning based control system to benefit from the above described technologies.

Calcium is often one of the most puzzling elements in hydroponic culture due to its ability to respond fairly non-linearly to nutrient concentrations in solution. This behavior is the result of its transport dynamics and its relationship to other elements that may antagonize it very effectively when they reach higher concentrations. On today’s post I will talk about calcium behavior in hydroponics and how we can play with both environmental and chemical properties to change its concentration in leaf tissue.

Imagine you’ve had a good growing season up until now, your plants are looking great, you’ve been doing everything properly. Suddenly, you start to see what appears to be a calcium deficiency on your leaves. You proceed to analyze your tissue and notice that your Ca levels are way below what you expect, yet your nutrient solution seems to be very Ca rich, at almost 150-200 ppm. You panic and increase the Ca level to 250 ppm, your following test results come out even lower. You’re not alone, you’ve just misunderstood calcium transport.

Most calcium deficiencies are actually not the result of Ca missing in nutrient solutions but they are caused by faulty Ca transport, which is often related with environmental issues. Calcium transport depends substantially on transpiration, so the solution to Ca deficiencies can be as simple as increasing your vapor pressure deficit (VPD). Ca is also absorbed more effectively when its at a lower concentration than at a higher one, so often increasing Ca will decrease its transport to leaf tissue. This study on tulips and its bibliography illustrates this fairly well, the increase in tissue with Ca follows a parabolic trajectory, where the largest Ca concentrations actually lead to lower Ca in tissue. You can push Ca to the leaves with higher VPD though, as this paper on Ca fortified lettuce shows, with the lettuce grown at Ca at 300ppm at 28C showing the highest Ca accumulation.

At lower levels Ca can actually start to show the inverse behavior and start to accumulate very heavily in tissue as its transport can become extremely favorable. If you notice a large increase of Ca in your leaf tissue at your plants ideal VPD then you might actually want to increase Ca in solution rather than decrease it, as an increased concentration in solution might actually make transport less favorable. However the most determinant factor in Ca absorption is water transport, so this excessive Ca might just be indicating you that your VPD is too high (so reduce temperature or increase your relative humidity).

This big influence of VPD explains why results of ideal Ca concentrations and Ca:K ratios are significantly disperse in the scientific literature. A recent paper for strawberries shows this ideal ratio to be around 1.3-1.4 but some papers (like the one on tulips shown before or this one) has it way closer to 1.0. The ideal ratio for your crop will also be dependent on the water transport your plants are forced to assume due to your ambient conditions so you will likely need to optimize this variable for your particular growing conditions.

A good place to start for flowering plants in terms of K:Ca is usually a ratio of around 1.2 however, you will need to do tissue analysis to figure out whether the Ca absorption is at the ideal point or whether you want to increase/decrease your Ca. Just bear in mind that increasing Ca in solution might reduce it in your leaf tissue so to reduce Ca in tissue try to play with your VPD first before you play with the concentrations in solution. Chances are that if you’re getting too much or too little your issue is that you’re too deviated from your ideal VPD situation.

Plants require specific conditions within their root zones to give optimal yields. The negative of the logarithm of hydronium concentration (a.k.a pH) is a very important measurement that tells us if the root zone is too acidic or too basic for our plants. However pH is affected by many different variables, making the calculation, theoretical understanding and modification of this variable difficult – and often very puzzling – for a significant percentage of growers. Today I want to go into how we can understand these pH changes from a qualitative perspective to get an idea of how pH can be expected to change in solution if we modify one variable within our hydroponic crop.

The final pH in the root zone of a hydroponic crop will mainly depend on the following factors: Chemical species in solution, plant absorption/secretion and media reactions. Basically we need to consider the chemistry of the solution, the way plants absorb nutrients and the chemistry of the media. The size of these effects relative to one another is also very important to consider.

In a normal hydroponic crop with a nitrate rich solution plants will tend to absorb more nitrate than any other ion, which will tend to increase pH as plants will excrete bases when they take anions in. Since the effect of nitrate in pH is very small the net effect is a substantial increase in the pH of the nutrient solution. Anions like citrate can also cause increases in nutrient solution pH due to a similar reason, they can be strongly taken in by plants and generate upside drifts.

The effect of anion absorption can be offset by acids present in solution. Phosphorus is most commonly present in hydroponics at a pH of 5.8-6.2 as H2PO4(-) which is a weak acid that is able to react with the bases excreted by plants. This means that higher P containing solutions will tend to drift less towards the upside than solutions that contain more P. This is a common reason why yields are usually better for less experienced growers when higher P solutions are used, because they are more forgiving to pH drifts due to their higher buffering capacity towards the upside at the pH used in hydroponics. When we add P the effect of the buffer is way bigger than the effect of P absorption, as P is not absorbed in very high concentrations by plants.

When you increase or decrease the concentration of an ion in solution you need to consider if it will be absorbed, how strong its absorption will be relative to other ions and how this relates to the chemistry of this ion in solution. For example citrate provides a relatively strong buffer towards both upside and downside moves at a pH of 5.8-6.2 but its absorption by plants negates this effect and generates a net drift towards the upside in solution in most cases.

Species that are not absorbed so significantly – like bicarbonate ions – are better at buffering moves towards the upside, since they provide a weak acid to react with plant secretions without providing a very significant source of anions for the plant to absorb and exchange for more basic species.

This exact same logic can be applied to positive ions that are easily absorbed by plants. Adding ammonium generally causes strong decreases in the pH due to plants exchanging ammonium with hydronium ions. However this effect can be compensated by adding things that cause upside drifts either through absorption or through buffering effects. Adding ammonium citrate to a nutrient solution causes a rather balanced effect due to both the ammonium and citrate absorption/buffering compensating.

The media is also very important to consider as non-neutral behavior will tend to strongly drift pH due to the mass of the media within the hydroponic setup. To prevent this problem it is not uncommon to treat media with a buffer before use, although this can become prohibitively expensive quickly if large volumes of media are used. It is more common to deal with the quirks of the media using nutrient solution chemistry. To get an idea of how your media affects pH you can let your media soak in nutrient solution and notice how pH evolves as a function of time. If the pH increases or decreases you’ll get an idea of what you should expect and the think about how you might want to handle it.

Media that decompose – particularly peat moss – will tend to acidify and become more and more acidic with time. This effect can be so strong that the only reliable solution is to amend with some low solubility base that can offset this effect with time (such as limestone). This is however problematic since the base will tend to run out towards the flowering stages of crops, where its help is most needed to offset potassium absorption. For this reason it’s usually more manageable to use a media like coco coir, which has a more stable pH profile and will not tend to “fight growers” through the crop cycle.

As you can see pH in hydroponics can be a complicated issue. When it starts to drift you need to immediately think about what the source of the ions is. If the pH increases you have a contribution of basic ions that exceeds the strength of the weak acids in solution, while the exact opposite is the case if the pH decreases. What is the source of these ions? Is the plant absorbing more of something? Do we need to add something to counter the effect? Is the media the culprit? Start asking and trying to answer these questions and it will become easier to understand why your pH is drifting and what you might be able to do about it.

Humidity is one of the most important metrics in hydroponic culture. In order to have a successful crop it is critical to understand what humidity actually is, how we measure it, why we need to control it and where we want it to be. Today I want to share with you five things you should know about humidity in hydroponics. These pointers should be equally useful to newer growers and those who want to get even higher performance out of their current hydroponic crops.

There is a big difference between relative humidity and absolute humidity. Although we generally understand humidity as the amounts of water in the air in reality what we usually measure in hydroponics is “relative humidity” (RH) which does not measure how much water there is in the air but what percentage of the available capacity we are using. A measure of 70% tells you that the air currently holds 70% of the water it could hold, but it doesn’t tell you anything about how much water there actually is in the air. Absolute humidity, on the other hand, tells you how much water you actually have in the air. As warmer air can carry significantly more water it is important to realize that a 70% humidity at 80F implies there is way more water in the air than a 70% humidity measurement at 50F.

Relative humidity meters are not reliable instruments. We often buy instruments to monitor ambient conditions without much thought about how they work or how good or bad they are. Relative humidity is a tricky measurement and cheap, semiconductor based relative humidity meters have not been very reliable or accurate. Usually a humidity meter will have an error of +/- 5% and it’s accuracy will not be on point if it has been exposed to very high humidity values (if there was ever any condensation on the sensor it probably was damaged to a significant extent). I often recommend buying at least 3 instruments with different chipsets. Having just one – or even several meters with the exact same chip – can be a recipe for disaster. The chipsets below are setting a new standard for precision and accuracy, so I would recommend you give sensors with those a try if you’re looking for more accurate RH measurements.

It is often better to go over than to go under. Although higher or lower relative humidity values are both sub-optimal for plants, it is often better to go with higher humidities rather than lower humidities in terms of crop yields. Lower RH values will tend to stress plants more – especially if the temperature is high – while higher RH values are often easier to deal with for tropical plants (which are the kind we often grow in hydroponics). Although higher humidity can definitely cause important issues – such as fungal diseases – we have ways to deal with this that are more effective than our ways to deal with stress caused by low relative humidity. Of course extreme values will be very detrimental to plants either way, so when I say high consider I don’t mean 95% RH at 80F.

Relative humidity can change a lot depending where you measure it. It is important to place RH meters at different places that represent what the plant is actually being exposed to. If you place RH meters in a greenhouse, far away from plant canopy, you will get a very poor representation of what the plant is actually experiencing and you might try to increase humidity substantially when this might not be needed. Ventilation is critical to alleviate this issue but in order to stay on top of it I always advice having meters within plant canopies in order to know for a fact how much your humidity values diverge between different places in your growing environment. Humidity is always bound to be higher closer to plants – as they transpire – but we should know how different it is. A big difference is a strong hint that there is not enough ventilation around the plants.

Humidifiers are often needed during the winter. Most crops that grow in warehouses during winter times require humidifiers, since the plants often do not evaporate enough water to compensate for the complete lack of water within the dry winter air (this is specially true if the volume of the growing warehouse is large). If you want to have a successful crop during winter times it will be paramount for you to have adequate humidifiers to ensure that your RH values are within what’s ideal for your plants. Depending on the size of your crop this might require significant planning and investing so make sure you always consider this when designing your winter growing cycles.

I hope you have found the above pointers useful. There are certainly many other important aspects of humidity, such as relative humidity meter calibration, judging ideal humidity using vapor pressure charts, or ensuring that plants have enough defenses if they happen to get exposed to exceedingly high humidity levels within their canopy. We will certainly discuss some more of these within future posts!