Farm Operations Management
tipburn is not a shortage of total Ca: stop the recurrence with delivery
Articles for Farm Operations Managers
It comes back even after you add Ca. If you grow leafy greens in a PFAL vertical farm and you have had that experience, the cause may lie somewhere else. The moment you try to explain tipburn as “a calcium shortage,” it no longer squares with the reality that it keeps appearing no matter how hard you push fertilization. It is less that the total amount of Ca is short and more that the Ca is not reaching the inner leaves. The problem is not the amount itself but how it gets distributed within the plant. That said, amount and delivery cannot be cleanly separated. If the Ca in the solution drops below the floor and runs too thin, then of course it shows up as a true total shortage as well. On top of that, the kind of recurrence that does not stop no matter how hard you push fertilization will not add up unless you look at the delivery side. And delivery is not decided by Ca alone. Light, transpiration, airflow, the root zone, and the nutrient solution all link together and change where the Ca ends up. There are two things I want you to take away from this article. The order in which to suspect things when you are standing in front of an affected plant, and the line that splits the responses into “the range you can move on site starting today” and “the range you weigh as equipment.”
tipburn is not a shortage of total Ca but a question of delivery
The young leaves of a lettuce head, near the core. Their margins crisp up and die back, turning brown. This is about the symptom called tipburn. What we are dealing with is a PFAL vertical farm, leafy greens centered on lettuce. Transpiration moves differently here than in a greenhouse or with fruiting vegetables, so the same does not carry over directly. On site, the first thing said is “a calcium shortage.” So you add Ca. But it appears again.
The dieback shows up on the inner young leaves. What is interesting is that there is a difference between racks. The stronger the rack, the one getting plenty of light, the more the big outer leaves stay firm and full of life, yet the margins of only the small leaves near the core crisp up and die. Conversely, racks with weaker light do not show it as much. If the total amount of Ca were short, the whole plant should weaken, and yet only the very center of the most vigorous-looking plant dies back. This is the part that trips you up.
The fact that there is a difference between racks is already the answer. On a rack with strong light, the outer leaves transpire well. Ca flows through the xylem along with water and gets pulled toward where transpiration is high, so it piles up on the outer leaves. The young leaves at the core, meanwhile, still transpire weakly. Inside the plant the humidity is high and the air moves poorly. So the water flow itself is thin, and Ca does not reach them. The stronger the rack, the more the core loses the tug-of-war with the outer leaves. Even if you raise the bulk Ca concentration of the solution, its destination is skewed toward the outer leaves, so it has a hard time reaching the core. The reason only the center of a vigorous-looking plant dies back is exactly this mechanism. What works is not pushing the concentration number higher but moving water to the core.
This read has been borne out directly in a controlled experiment on hydroponic lettuce. Raise the light intensity, and the shoot fresh weight, the growth rate, and the number of leaves showing tipburn all increase together. The Ca uptake per plant itself does increase properly. The problem is what comes after. The Ca concentration rose only in the whole plant and the outer leaves; the Ca concentration of the wrapped young leaves on the inside did not rise even when the light was strengthened (see 1). The reason given is that the mass flow driven by transpiration is directed strongly toward the outer leaves. “It is not that the total amount is short but that it is not arriving” is not an impression; it is backed up as a problem of Ca delivery (distribution). However, this is on the premise that the floor is properly filled. If the Ca in the solution itself is too thin, it simply runs short. So it is not “it is absolutely never the amount”; the order that matters is to first check whether the floor has given way, and then move on to the delivery story (how to tell them apart comes in the second half).
the harder you push growth, the less calcium reaches the core
Deliberately weaken the transpiration of the outer leaves. At first this should feel backwards. A rack with strong light is the rack you most want to take harvest from. Weakening the airflow there, raising the humidity there — it is normal to feel both are “a waste.” Pushing water through to the core and working the outer leaves full-tilt cannot both hold at once. Is this not a trade-off?
Head-on, it is a tug-of-war. Opening the outer leaves wide and getting water to the core at the same instant is a stretch. So one option is to separate where you attack from where you defend by staggering them in time across the growing cycle. Operations that can switch airflow and humidity rack by rack in fine detail every day are not that common. On a multi-tier-rack PFAL, it is normal that neither the HVAC nor the airflow can be split per rack. So the realistic move is to switch the environment only during the period of highest risk. Only in the 3 to 5 days before harvest, drop the light a little and at the same time shift toward an environment that promotes transpiration. Without sacrificing the whole growth period, shift toward sending water to the core for just the few finishing days when the core is most at risk. It is within what you can set up on a timer, so even an operation with coarse equipment granularity can run it.
And the fact that pushing yield on a strong rack makes tipburn more likely is, in a sense, something you are better off accepting as just how it is. The faster you make growth, the more furiously the inner young leaves multiply. The faster it grows, the more new leaves appear right as you are distributing, and arrival at the core can no longer keep up (see 1). “Fast and big” and “safe all the way to the core,” when pushed to the limit, do not point in the same direction. So the harder a rack is pushed, the more you assign these few days before harvest as time to send water to the core. Not as a brake, but inserted like a wait at a traffic signal. The landing point where you cut only the crisping without dropping yield is mostly within that allocation of time.
The boundary of “the more you push, the more it appears” is shown by an experiment in a PFAL that combined temperature and light. Air temperature on the high side (28°C or above), root-zone temperature on the low side (24°C or below), PPFD on the high side (400 µmol·m-2·s-1 or above). In the combination where these three lined up, the tipburn incidence exceeded 50% from around day 6 after transplanting, and that combination had to be dropped from the analysis (see 2). It does not appear from one parameter; it appears all at once where the conditions that push growth pile up. Moreover, even at the same daily total of light, holding down the peak and spreading it over a longer time grew the plant more than applying it strongly over a short time (see 3). Keeping the instantaneous peak from going too high leaves room to avoid throwing away yield.
what to suspect first when you see an affected plant
You find one affected plant. Your hand reaches first for the Ca concentration of the nutrient solution. That is the number most immediately within reach. Even if you understand in principle that moving water to the core comes first, the moment one is in front of you, you hesitate over what to look at first. The rack position, whether that plant is on the core side or the outer-leaf side, how the humidity and airflow hit it — often the order in which to look is not settled. On top of that, whether airflow and humidity can be adjusted on the spot, or whether you cannot do it without fixing the ducts or the HVAC — that line, too, is hard to draw.
There is one thing to do first. Look at “how far” the dieback has spread. If the symptom is appearing even on the outer leaves and older leaves, that — ahead of any delivery issue — is a sign that the amount itself is not enough. In that case, check the solution Ca first. If it appears only on the core side, only on the inner young leaves, treat it as a delivery problem and move on to the next order — and at that point, measuring solution Ca directly is fine as the last check on the delivery side. It is one buffer step: just rule out the floor giving way first.
On that basis, the order on the delivery side is this. First, read “where it is appearing.” Core side or outer-leaf side. If it appears on the core side, that alone gives you a read that it is about “difficulty getting water through.” Next, which position on the rack. Is it skewed toward a corner or the back where airflow is weak? If this matches, it is nearly clear that it is a flow problem and not a question of the amount of Ca. As a rough guide: (1) the location of occurrence and the rack skew, (2) whether growth is being pushed too hard (whether light, CO2, and temperature are being raised), (3) airflow and humidity, (4) the root zone and nutrient solution and how it is taken up. Measuring the solution Ca concentration directly, once the floor giving way is ruled out, is near the end of this order. Because it is a number immediately within reach, you tend to go to it first, but at the stage of suspecting delivery, that is fine to leave for last.
The line is this. What you can move on the spot today is the direction and strength of the airflow, how you smooth out the humidity during the light period, and slowing growth a little. These three you can move by hand without touching the equipment. When you raise the humidity, the rough guide for leafy greens is 60 to 70%, and raising it beyond that makes transpiration drop too far and tends to backfire. Adjust it with that ceiling in mind. On the other hand, whether the airflow is uniform across the whole rack — the duct routing, the fan positions, the HVAC capacity itself — does not change without a retrofit. First try the three you can move by hand. If it still remains on only a particular rack, that is the task left for the equipment side. This way of dividing it is realistic.
On the point that “airflow works,” there is an experiment in a closed plant factory comparing temperature control and airflow control on lettuce. When a steady horizontal airflow was applied at 0.28 m/s or above, the tipburn symptom clearly decreased. On the other hand, the treatment that switched the daytime temperature did not work to suppress tipburn at any temperature (see 4). At least in this experiment, applying a steady airflow along the rack worked better for tipburn than raising and lowering the daytime temperature. This is not to say that the factor of temperature itself does not work — this article too treats air temperature as a driving factor of occurrence. What did not work was the operation of “switching the daytime temperature,” and you should read it as one instance where a steady horizontal airflow worked better than that. Furthermore, in the same experiment, when a steady airflow was applied, the Ca amount of the whole plant increased and the gap in Ca concentration between the inner leaves and the outer leaves narrowed (see 4). What works is not strengthening the airflow blindly. It is flowing it along the rack, horizontally, steadily, and evenly.
One cross-chapter caveat here. In the previous chapter I wrote about “weakening the strong airflow that whips up the transpiration of the outer leaves.” What you weaken is the strong, turbulent airflow that only dries the margins of the outer leaves. The steady horizontal airflow that delivers water all the way to the core, you do not weaken; rather you apply it evenly at all times. Even within the same “airflow,” a wild, strong gust and a uniform, steady airflow are different things, and although they look opposite in direction, they do not contradict. The strong, turbulent airflow only dries the margins of the outer leaves; it is a separate operation both from holding down the transpiration peak of the outer leaves and from pushing water through to the core.
telling a flow problem from a true shortage of amount
The root zone and nutrient solution are, in the flow so far, the part you look at last. But there are times when the symptom remains even after you have dealt with the flow problem. Then you start wanting to suspect a cause on the root side for why water is not reaching the core. The roots are damaged and their power to draw up water has dropped — that sort of thing. And while you accept that “just making the solution stronger has little effect,” whether raising Ca itself is entirely useless is another matter; it is not necessarily so. Situations where the Ca in the solution is simply too thin and short do occur in practice. Is the problem the flow, or the amount? Which one, and how do you tell them apart?
The first pattern to consider on the root side is one where the roots are damaged and the water-uptake power itself has dropped. Dissolved oxygen is short, the nutrient solution temperature is high, the root zone is somewhat oxygen-starved. When that happens, the vitality of the roots drops and the power to draw water weakens. Since the core is a place where water is thin to begin with, I read it as the core being the first to dry out when the roots weaken. When the root-zone temperature is too high or too low and the roots are not touching the water well, I think it follows the same line. These root-side factors — dissolved oxygen, nutrient solution temperature, root-zone temperature — straddle both the part you can reach by hand with aeration or by chilling the solution and the part that depends on the equipment capacity. So they are easy to sort out by applying the earlier line of “movable by hand, or requiring a retrofit.”
Here there is one more axis, neither delivery nor amount: a third one. How it is taken up. The Ca in the solution is at standard and the floor has not given way. When it still does not come to the core, the next thing to suspect is “how it is taken up” — whether, at the same concentration, the conditions make it easy for the roots to take Ca in. Is the pH of the nutrient solution off? Supplying nitrogen in nitrate form tilts the inside of the plant body slightly alkaline and helps Ca uptake, but if nitrogen in ammonium form acts too strongly, the inside conversely tilts acidic and obstructs Ca uptake. Is that balance off? When potassium or magnesium is in excess, they compete with Ca over the same pathway. Is K/Mg piled on too heavily? These are adjustment points that actually come into play when you build a nutrient solution for leafy greens in a PFAL. So it is not that “there is no move you can make on the solution side.” It is only that raising the bulk concentration has a hard time reaching the core; if you set right how it is taken up — pH, nitrogen form, ion competition — there is a route to make the solution work on the core from the solution side. We look at everything around the nutrient solution with these three: delivery (airflow, transpiration), amount (the floor giving way), plus this how-it-is-taken-up.
The axis for telling them apart is placed on the fact that Ca is an element that can barely move inside the body. If it is a flow problem, what is short is only on the core side. The outer leaves and older leaves stay firm and full of life. Conversely, the whole thing weakens including the outer leaves, the symptom reaches even the older leaves — this is a sign that the absolute amount is not enough. Measure the solution Ca alongside this, and if it is clearly below standard it is an amount problem, while if it is at standard it is a flow or how-it-is-taken-up problem. That gets you most of the way to telling them apart. This way of telling them apart is not something a paper has organized for me; it is a read I set up myself from the property that Ca cannot move. So raising Ca has meaning when the floor has given way and it is depleted. There, raise it without holding back. But continuing to raise it when there is enough only skews it toward the well-flowing outer leaves, and the core does not change. The step of filling depletion and the step of fixing delivery and how-it-is-taken-up once there is enough are thought of separately.
The foundation under this is the property that “Ca can barely move inside the body.” Ca cross-links with the carboxyl groups of pectin and supports the strength of the cell wall, and Mg cannot stand in for this role. And when Ca (or B) is removed from the medium, in Arabidopsis root elongation stops within an hour, with accumulation of reactive oxygen species and cell death. In tomato too, removing Ca stops root elongation immediately. Removing K or Mg does not produce that kind of immediate response (see 5). Once a place has gone short, nothing comes around to it afterward. That is exactly why the distinction “if it is only on the core side, delivery; if it reaches the whole plant and the older leaves, absolute amount” works. I read the root side as following the same line. In hydroponic systems for other crops such as tomato, the rate of nutrient uptake from the roots is largely governed by the rate of water uptake (transpiration), and environmental factors such as light, air temperature, humidity, and air velocity move uptake indirectly through transpiration (see 6, 7). It has not been quantified directly for lettuce, but the direction — that uptake drops when the root zone is hot or oxygen is short — should be the same. It lines up with the read that when the roots weaken, the core dries out.
switching cultivars or optimizing a single factor does not close it
Finally, I will touch on two things that come up a lot on the floor.
One is the cultivar. When you struggle with tipburn, the move “switch to a stronger cultivar” crosses your mind. Does it really work, and how does it connect to the story so far? The other is what happens when you move just one factor. “Lower only the humidity,” “strengthen only the airflow,” and tipburn decreased, yet something appeared somewhere else — that kind of experience. It is the flip side of the linkage where moving one thing moves another.
There is, certainly, a difference in resistance between cultivars. Compared with a type where the young leaves at the core stand densely, a leaf form that is open and has room lets water reach all the way inside more easily. A cultivar whose growth is less prone to running wild reaches the point where delivery breaks down later. But this is a move that loosens, all together and a little, the linked system we have been looking at — growth rate, airflow, transpiration, distribution — and it does not make the delivery problem itself disappear. It only raises the baseline. Even a strong cultivar, if you push too hard, will get tipburn in the normal way. So do not close the story with the cultivar.
There is, certainly, a flip side to adjusting just one thing too. Lower only the humidity, and the transpiration of the whole plant increases, and water is pulled toward the outer leaves all the more. The core dries out further, and the crisping can worsen. Strengthen only the airflow, and this time the margins of the outer leaves get damaged by desiccation stress, or uneven exposure produces plant-to-plant variation. Slowing growth is the safest, but naturally yield drops by that much. Optimize one factor, and the burden shifts onto another factor. So do not adjust them singly; look at them all together as a linkage. That is the least likely to get tangled.
The difference between racks is a place where the effect is large when you actually move it. Review the fan directions and the duct routing and even out the unevenness in the airflow, and the delivery to the core changes — it lines up with the earlier finding that “a steady horizontal airflow works on the core” (see 4).
The difference in resistance between cultivars has genetic backing too. A locus with a large effect (QTL) is involved in lettuce tipburn resistance, and one region in particular explains up to 70% of the variation in tipburn incidence in the field (see 8). Moreover, within that region a candidate gene for a calcium transporter was even found. The baseline of “how Ca is carried” certainly does differ by cultivar. But even in the same study, results came out that a single Ca-supply model cannot fully explain, such as a gene from the susceptible parent working beneficially in some regions instead. This is consistent with the view that, while a cultivar is a move that raises the baseline, it does not make the delivery problem disappear.
Even if you switch cultivars, even if you move just one factor, the “delivery” problem at the root of tipburn keeps coming back in a different form. That is precisely why you look at the linked system of light, growth rate, airflow, transpiration, the root zone, and the nutrient solution as a single connection. That, I think, is the surest stance for the long haul of dealing with this troublesome symptom.
what to do with a plant that appeared anyway
Even when you exhaust prevention, what is going to appear will appear. Finally, how to handle a plant that has appeared at the shipment stage. From the standpoint of someone who has faced lettuce for many years in a PFAL vertical farm, just one point.
Even when tipburn appears, the effect on the taste itself is limited. But the appearance is clearly inferior. Since vegetables are chosen by appearance, the effect on shipment quality cannot be ignored. So there are many situations where you are forced to decide whether to pull a lightly affected plant from shipment, or to remove the affected part before shipping it.
One thing to be careful of here is desperately trying to remove even minute tipburn. Tear leaves off recklessly, and rot can advance from the wounds. Taken overall, excessive removal can be the bigger negative. Rather than going after everything that appeared, judge whether it is at a level that affects shipment quality and then put your hand in. Not just swinging all the way to prevention, but including the line you draw after it appears — only then does it become a real on-site call.
Behind tipburn is the linkage of light, growth rate, airflow, transpiration, the root zone, and the nutrient solution that we have looked at so far, and I have gathered into one book the material for thinking it through all the way to profitability.