The Potassium to Calcium ratio in hydroponics

linkedinrssyoutubeinstagram
image_pdfimage_print

To have a healthy hydroponic crop, you need to supply plants with all the nutrients they need. One of the most important variables that determine proper nutrient absorption, is the ratio of Potassium to Calcium in the nutrient solution. These two elements compete between themselves and have different absorption profiles depending on the environment, and the plant species you are growing. For this reason, it is important to pay close attention to this ratio, and how it changes with time, in your nutrient solution. In this post, we are going to examine peer-reviewed research about this ratio and how changing it affects the growth, quality, and yield of different plant species.

Two vital elements that compete against each other. Their ratio is fundamental to maximize yields and changes depending on the plant species, environmental conditions and absolute concentrations used

Two ions with very different properties

Potassium and Calcium are very different. Potassium ions have only one positive charge and do not form any insoluble salts with any common anions. On the other hand, calcium ions have two positive charges and form insoluble substances with a large array of anions. This creates several differences in the way plants transport and use these two nutrients.

While potassium is transported easily and in high concentrations through the inside of cells, Calcium needs to be transported in the space between cells and its intracellular concentration needs to be very closely regulated. Calcium can also only be transported up the plant – from roots to shoots – while potassium can be transported up and down as it pleases.

Calcium transport – happening around cells – is heavily dependent on transpiration, which is what causes water to flow through this space. Potassium transport is not so closely related to transpiration, as it can move directly through the inside of cells in large amounts, which means it can be actively transported through the plant in an effective manner.

Note that the above is a broad over-simplification of Potassium and Calcium transport. If you would like to learn more about this topic, I suggest reading these reviews (1,2).

Competition between K and Ca

Potassium and Calcium are both positively charged, so they do compete to a certain extent. The competition is both because they compete for anions – which they need to be paired with for transport – and for the use of electrochemical potential, which they take advantage of to get transported across membranes. However, they do not have the same transport mechanisms, so the competition is limited.

Table taken from this article (3)

The table above illustrates this point. This study (3) looked into different K:Ca ratios in the growing of lettuce and the effect these ratios had on yield, tip burn, and nutrient concentrations in tissue. You can see that at low total concentrations (0.4 mS/cm EC) the K in tissue is very low when the amount of Ca is high relative to K, while at higher EC values (1.6 mS/cm EC), the K concentration remains basically unaffected, even if the Ca concentration is 3.5 times the K concentration. While Ca competes effectively with K when the absolute concentration of both is low, this competition of Ca becomes quite weak as the concentration of K and Ca increase. At very high concentrations (3.6 mS/cm EC), the potassium does start to heavily outcompete the Ca, especially when the K:Ca ratio is high (3.5x).

The above is also not common to all plants. For some plants, the competition of Ca and K actually reverses compared to the results shown above. However, it is typical for low and high absolute concentration behaviors to be different, and for the influence of K or Ca to become much lower in one of the two cases.

Optimal K:Ca ratios

The K:Ca ratio has been studied for many of the most popularly grown plants in hydroponics. The table below shows you some of these results. It is worth noting, that the results that maximized yields, often did so at a significant compromise. For example, the highest yield for lettuce came at the cost of a significantly higher incidence of inner leaf tip burn. In a similar vein, the highest yields in tomatoes, at a 3:1 ratio, came at the cost of additional blossom end rot problems. This is to say that, although these ratios maximized yields, they often did so with consequences that wouldn’t be acceptable in a commercial setup. For lettuce, 1.25:1 proved to be much more commercially viable, while still giving high yields.

RefPlant SpecieOptimal K:Ca
4Rose 1.5:1
5Tomato3:1
6Tomato1.7:1
7Marjoram0.5:1
8Strawberry1.4:1
9Cucumber1:1
10Lettuce3.5:1
Optimal K:Ca – in terms of yields per plant – found for different plant species

You can see in the above results, that fairly high K:Ca ratios are typically required to increase yields. For most of the commercially grown flowering plants studied, it seems that a ratio of 1.5-2.0:1 will maximize yields without generating substantial problems in terms of Ca uptake. As mentioned above, higher K:Ca often push yields further, but with the presence of some Ca transport issues. A notable exception might be cucumber, for which the publication I cited achieved the maximum yield at a ratio of 1:1. However, good results were still achieved for 1.5:1.

Another important point about the ratio is that it is not independent of absolute concentration. As we saw in the previous section, the nature of the competition between K and Ca can change substantially depending on the absolute ion concentrations, so the above ratios must be taken within the context of their absolute concentration. The above ratios are generally given for relatively high EC solutions (1.5-3mS/cm).

Conclusion

The K:Ca ratio is a key property of hydroponic nutrient solutions. While the optimal ratio for a given plant species cannot be known apriori, it is reasonable to assume that the optimal ratio will be between 1:1 and 1:2 for most large fruiting crops and flowering plants that are popularly grown in soilless culture. This is especially the case if the hydroponic solution does not have a low EC. An optimal value below 1:1 is unlikely for most plants, although exceptions do exist in certain plant families that have peculiar Ca metabolisms.

To obtain the largest benefit, it would be advisable to run trials to optimize the K:Ca ratio for your particular crop, by changing the K:Ca ratio between 1:1, 1.5:1, and 2:1. You will likely see important differences when you carry out these trials, which will be useful to determine the highest yielding configuration for your setup. To perform these variations, it is usually easiest to change the ratio of potassium to calcium nitrate used in the nutrient solution.

Have you tried different K:Ca ratios? What do you grow and what has worked for you? Share with us in the comments below!

Facebooktwitterredditpinterestlinkedin

21 Comments

  • Skybound
    June 8, 2021 @ 12:09 pm

    Hi Daniel, I grow Cannabis in Hempy (perlite) buckets and my current ratio K:Ca is 1:0,71. I am currently keeping my K low in hopes of preventing levels of luxury K from being too high, plus also I fear causing problems with Ca. In the past, when I used elevated K, my buds would get plenty fat, but all of the branches would have the strength of wet spaghetti.My work around thus far has been to keep the Ca high(ish) and K low(ish) and ATM, I feel I am keeping that delicate balance in check, and reading this blog entry affirms that I’m on the right track. Thank you tremendously for all that you’ve shared throughout the years and definitely for this blog entry!!

    • admin
      June 8, 2021 @ 4:53 pm

      Thanks for commenting! A ratio of 1.4:1 can be fine for cannabis in perlite. In the end, the exact ratio that works best depends on the environment you’re growing in, the strains you’re growing and the objective you’re after. Lower ratios tend to work better when you’re under environmental conditions that restrict Ca flow, while higher ratios work best when Ca movement is favored or Ca supplementation through foliar spraying is available.

      However, from the studies, the closest related plant to cannabis, taxonomically, is Strawberry – a plant in the Rosales order as well – so it is not surprising at all that 1.4 works great!

      • D
        June 25, 2021 @ 12:12 am

        Good evening! Thank you for the ratio of K:Ca in hemp, what range should the ppm’s be for Ca in coco in 65% RH under LED? Also, does the ratio change depending on what stage the plant is in?

        • admin
          June 25, 2021 @ 8:38 am

          Thanks for commenting! The media plays a role as well, media that have higher Ca absorption require higher Ca. It also depends on the amount of other elements you have in solution, as water sources and media with higher Na+ or higher NH4+ require higher Ca. Most people will tend to overdo Ca in hemp, because they have issues with Ca transport due to other problems in their environment. Usually the plants will also require lower Ca towards mid/late flower. Long story short, an in-depth evaluation of your conditions is needed to tell you what Ca level you need and how you should manage your environment and media to ensure that Ca is actually taken up effectively. I would suggest booking an hour of consulting if you’re interested in a deeper analysis.

        • Mim dal
          April 29, 2024 @ 12:53 pm

          Based on just a little more than a little chemistry knowledge i have, potassium cation hydration is independent from pH and also type and concentrations of anions in solution; It is always wild and free in nutrient solution. But calcium cation is not the same. It starts precipitating as hydroxyapatite with phosphate ions at pH above 5.8. Sulphate anions which are unfortunately in high concentration in many nutrient solutions and plants don’t need them much, accumulate in solution reducing hydrated Ca cation by one third when sulphate concentration reaches 10 mM in containerised plants by forming CaSO4 and forcing Ca to be passively absorbed by plants. Even at very low sulphate concentration of 0.3 mM which is near the minimum requirement by the plant i love, about 2% calcium and 25% sulphate exists as CaSO4 in solution. although CaSO4 is not a precipitation but i think it reduces Ca availability at high concentrations of sulphates. So sulphate, phophate and pH needs to be considered as they effect hydrated Ca cation but not potassium. I think with lowest possible sulphate and phosphate to support optimal gowth with a enough attention to pH, K:Ca becomes less matter as now both elements are in ionic hydrated form and plant can selectively choose between the two. I don’t use K2SO4 because it has potassium, i use it because my plants needs a little sulphate. To me, KNO3 or K2CO3+HNO3 or KOH+HNO3 are potassium main sources not K2SO4. I think potassium does not much more than charge balancer in plants or anion carrier in solutions. Keeping it at the same or lower than Ca is the only way to reach what i think iam doing to reach. I think just because sulphate toxicity is rare(i told the reason behind it partially) doesn’t mean having it at high concentration doesn’t have its own bad consequences.

  • Jahnavi
    June 14, 2021 @ 9:57 pm

    Hi Daniel, I have been using Howard resh formulations with help of hydrobuddy for lettuce and tomato for 2 years. I see that the K:Ca ratio of tomato formulations doesn’t match well with this article’s recommendation. The ratio is mostly 1:1 for tomato stage 3 formulation.

    • admin
      June 15, 2021 @ 6:55 am

      Thanks for commenting! A Ratio of 1:1 in tomatoes will grow healthy tomato plants and will be most effective to grow plants without any blossom end rot issues. Tomatoes’ K:Ca needs to be quite high to maximize yields – often at or above 2:1 – but then you also need to deal with many of these Ca uptake issues. You can try 1.5:1, see if you can improve your yields without too many BER issues. An easy way to do this is to add more potassium sulfate to your formulation.

      • MD. Imdadul Haque
        August 30, 2021 @ 11:43 am

        Hi Daniel, I am growing sweet peppers in my greenhouse. There is temperature 30 degree C and RH 70%. Which K:Ca ratio is best for sweet peppers ? Have a nice day.
        Imdadul Haque.

        • admin
          August 31, 2021 @ 8:00 am

          Thanks for writing! For sweet peppers, a ratio of 2:1 (K:Ca) is generally preferred under conditions like yours.

    • Mim dal
      April 29, 2024 @ 3:26 pm

      Science in hydroponics? Very good place to start learning for beginners and also intermediates like me. Among all nutrients, nitrogen uptake effect growth rate and yield the must. Plants need cation uptake rate near the same as anion uptake rate to balance the charges without effecting pH which effects solubility of nutrients. Among K, Ca and Mg, only K remains in nutrient solution as free cation ion all the time so it has the faster uptake rate than other two. So higher K promote higher Nitrate uptake rate without effecting pH which effects P, the other must limiting factor of plant growth. Thats why higher K promotes faster growth with cost of discouraging Ca and Mg uptake which have the same charge as K and are necessary for plant function. So we can’t grow our plant as fast as we wish with more K without accepting its consequences which is lowring the main function of other necessary cations and my English abilities is the must limiting elements is communication :)

  • Vo
    July 10, 2021 @ 11:51 am

    Hello there, great post as always, first one i read about K:Ca ratio… I just want to point out in your conclusion you switched K:Ca ratios, so instrad of 1.5:1 you wrote 1:1.5 etc… Keep up the good work!

    • admin
      July 11, 2021 @ 7:37 am

      Thanks! I’ve fixed the issue.

  • Diego A
    July 13, 2021 @ 2:40 pm

    Hey, i have had a couple trials on this, I grow lettuce and i have had better Ca and K absortion with a 2.5:1 K/Ca ratio! good post and very important topic!

    • admin
      July 13, 2021 @ 3:20 pm

      Thanks for commenting Diego! Interesting, have you measured Ca and K absorption by tissue analysis?

  • Alex Aguillon
    January 7, 2022 @ 8:07 am

    Hey Daniel, thank you for everything you do and information you put out. If I’m working with coco, how does one calculate the Ca absorption in the media. How do we account for CEC to find a starting point for formulating a recipe for cannabis?

    Also as we increase Calcium to account for CEC, how does this effect the interaction with the Ca:P ratio? If there is a need to increase P and P only in my formula, is an organic input such as high P bat guano or seabird guano my best option via tea or fermentation?

    • admin
      January 7, 2022 @ 4:44 pm

      Thanks for commenting Alex. Your questions are significantly complex, so I wouldn’t be able to answer them through a comment. I would suggest booking an hour of consulting if you’re interested in discussing this in detail.

      If you are an organic grower, you might want to consider amending your coco with rock phosphate instead, this can solve both of your issues simultaneously.

    • Mim dal
      April 29, 2024 @ 2:34 pm

      I think true CEC of coco is not as mush as many of us think. Coco holds lot of water and water holds cations and also anions. Its interaction with water is very slow at room temp. Untreated coco has less or more 30-40 meq K and 10-15 meq Na and also sum of them as Cl per litre when it is hydrated. Lab experiments have shown If you soak a coco in the same volum as it has with RO water for 3 days it releases near 1/2 and 4/5 of its K and Na respectively into water and that interaction is still probably ongoing. Na is lighter so it moves and releases faster. Why 1/2? Because of you squeeze it, it still holds half of water it had soaked in and that water holds K. I personally soak coco with fresh water as mush as its own volum for 2 days and then i squeeze it and soak it again for 1 or more day and i squeeze it again and i soak it again for another day or two till the EC reaches 0.2 mS/cm and i take it as near ideal inert substrate. 0.2 mS/cm has no more than 2 mM KCl. I think This coco is not ready to be used yet as a hydroponic substrate as it is saturated with water which has almost no nutrients and it dont hold more water which contain nutrient. so i Soak it in nutrient solution containing as double as normal feeding for a day before planting. I think Buffering coco with calcium and magnesium nitrate is a way to speed up K release and adjusting Ca:K. Although I haven’t done any lab analysis but as i saw no differences in plant grown between only this water treatment method with water treated combined with 20 or even 40 meq/kg Ca for buffering, i believe that this water treatment have given me an near ideal inert medium which soon enough will have Ca:K as nutrient solution.

  • Facundo Samaniego
    June 28, 2022 @ 1:54 pm

    Hi Daniel, just a simple question, is the K:Ca ratio 2:1 in ppm or in mmol? For example, if I have 200 ppm of Ca I need to have 400 ppm of K? or is it 5 mmol of Ca and 10 mmol of K? Thanks a lot.

    • admin
      June 29, 2022 @ 12:38 am

      Thanks for commenting, the molar masses of K and Ca are very close (39.0983g/mol Vs 40.079g/mol) so 2:1 ratios in terms of moles or mass are both basically equivalent.

  • Gustavo
    August 17, 2022 @ 7:00 am

    HI Dr Fernandez, I believe in the 1st paragraph of your Conclusion, you meant to type “2:1” instead of “1:2” for optimal ratio.

  • Ga
    December 6, 2024 @ 5:51 am

    cannot be known apriori, it is reasonable to assume that the optimal ratio will be between 1:1 and 1:2 for most large fruiting

    I think there is a typo here,

    You wanted to write 2:1

Leave a Reply

Your email address will not be published. Required fields are marked *