If you clean the tip of your moisture meter and dip it in a cup of distilled water, the meter will tell you the water is dry; this because the meter actually measures electrical conductivity. So if you have a given volume of water in a given soil, the higher the TDS (total dissolved solids or roughly - salts) the wetter the moisture meter will tell you the soil is. A wooden dowel, used as a 'tell' is more reliable. If I do heavy root work on tropicals, I habitually reduce the canopy by a very large fraction to keep the plant from A) collapsing, or B) from shedding branches that might be an important part of the composition I'm building. Example: From this root mass to this:You can see the huge roots I severed, and that was only the first rootwork session. Since then, I've pruned the root mass back to just about even with the soil level in the image below. We actually have a tool designed specifically for this chore, called (what else?) a root cutter. The tree is F benjamina. The foliage mass, on the same day, went from this: To this: I'm not sure what this means: There is no other location that's suitable for this big tree. Given this condition, should I supplement the lack of light with the Dyna-Gro in a month or so? You can't counter or remedy a lack of light by supplying fertilizer. In fact, you need to be careful about not over-fertilizing in low light conditions. Dyna-Gro's Foliage-Pro 9-3-6 is a good choice for low light conditions though, because it's nitrogen sources tend to reduce likelihood of the coarse growth that occurs when using fertilizers that get their N from urea. More about using a 'tell' please forgive the weird formatting, which happens on all the word documents I write and save to share): Over-watering saps vitality and is one of the most common plant assassins, so learning to avoid it is worth the small effort. Plants make and store their own energy source – photosynthate - (sugar/glucose). Functioning roots need energy to drive their metabolic processes, and in order to get it, they use oxygen to burn (oxidize) their food. From this, we can see that terrestrial plants need air (oxygen) in the soil to drive root function. Many off-the-shelf soils hold too much water and not enough air to support good root health, which is a prerequisite to a healthy plant. Watering in small sips leads to a build-up of dissolved solids (salts) in the soil, which limits a plant's ability to absorb water – so watering in sips simply moves us to the other horn of a dilemma. It creates another problem that requires resolution. Better, would be to simply adopt a soil that drains well enough to allow watering to beyond the saturation point, so we're flushing the soil of accumulating dissolved solids whenever we water; this, w/o the plant being forced to pay a tax in the form of reduced vitality, due to prolong periods of soil saturation. Sometimes, though, that's not a course we can immediately steer, which makes controlling how often we water a very important factor. In many cases, we can judge whether or not a planting needs watering by hefting the pot. This is especially true if the pot is made from light material, like plastic, but doesn't work (as) well when the pot is made from heavier material, like clay, or when the size/weight of the pot precludes grabbing it with one hand to judge its weight and gauge the need for water. Fingers stuck an inch or two into the soil work ok for shallow pots, but not for deep pots. Deep pots might have 3 or more inches of soil that feels totally dry, while the lower several inches of the soil is 100% saturated. Obviously, the lack of oxygen in the root zone situation can wreak havoc with root health and cause the loss of a very notable measure of your plant's potential. Inexpensive watering meters don't even measure moisture levels, they measure electrical conductivity. Clean the tip and insert it into a cup of distilled water and witness the fact it reads 'DRY'. One of the most reliable methods of checking a planting's need for water is using a 'tell'. You can use a bamboo skewer in a pinch, but a wooden dowel rod of about 5/16” (75-85mm) would work better. They usually come 48” (120cm) long and can usually be cut in half and serve as a pair. Sharpen all 4 ends in a pencil sharpener and slightly blunt the tip so it's about the diameter of the head on a straight pin. Push the wooden tell deep into the soil. Don't worry, it won't harm the root system. If the plant is quite root-bound, you might need to try several places until you find one where you can push it all the way to the pot's bottom. Leave it a few seconds, then withdraw it and inspect the tip for moisture. For most plantings, withhold water until the tell comes out dry or nearly so. If you see signs of wilting, adjust the interval between waterings so drought stress isn't a recurring issue. Al

Your plants' issues, relative to the spoiled foliage, are most likely related to over-watering, a high level of dissolved solids (salts) in the soil solution, or an ammonium toxicity that develops when roots are asked to live in saturated soils. I'd suggest that you start using a 'tell' to determine when your plants actually NEED water, and only then supply it. A 5/16" wooden dowel rod, ends sharpened in a pencil sharpener, makes a good 'tell. Read more below, and please forgive the formatting. It's from something I wrote and saved. The leaf shedding can be from a decrease in light during the last 6 weeks or so, from nutritional issues; it can be a drought response to a high level of dissolved solids in the soil solution, under-watering, or over-watering. It also appears there might be some mechanical damage (like a bruise would be to a human or animal) and some oedema symptoms on the calathea. Using a 'tell' Over-watering saps vitality and is one of the most common plant assassins, so learning to avoid it is worth the small effort. Plants make and store their own energy source – photosynthate - (sugar/glucose). Functioning roots need energy to drive their metabolic processes, and in order to get it, they use oxygen to burn (oxidize) their food. From this, we can see that terrestrial plants need air (oxygen) in the soil to drive root function. Many off-the-shelf soils hold too much water and not enough air to support good root health, which is a prerequisite to a healthy plant. Watering in small sips leads to a build-up of dissolved solids (salts) in the soil, which limits a plant's ability to absorb water – so watering in sips simply moves us to the other horn of a dilemma. It creates another problem that requires resolution. Better, would be to simply adopt a soil that drains well enough to allow watering to beyond the saturation point, so we're flushing the soil of accumulating dissolved solids whenever we water; this, w/o the plant being forced to pay a tax in the form of reduced vitality, due to prolong periods of soil saturation. Sometimes, though, that's not a course we can immediately steer, which makes controlling how often we water a very important factor. In many cases, we can judge whether or not a planting needs watering by hefting the pot. This is especially true if the pot is made from light material, like plastic, but doesn't work (as) well when the pot is made from heavier material, like clay, or when the size/weight of the pot precludes grabbing it with one hand to judge its weight and gauge the need for water. Fingers stuck an inch or two into the soil work ok for shallow pots, but not for deep pots. Deep pots might have 3 or more inches of soil that feels totally dry, while the lower several inches of the soil is 100% saturated. Obviously, the lack of oxygen in the root zone situation can wreak havoc with root health and cause the loss of a very notable measure of your plant's potential. Inexpensive watering meters don't even measure moisture levels, they measure electrical conductivity. Clean the tip and insert it into a cup of distilled water and witness the fact it reads 'DRY'. One of the most reliable methods of checking a planting's need for water is using a 'tell'. You can use a bamboo skewer in a pinch, but a wooden dowel rod of about 5/16” (75-85mm) would work better. They usually come 48” (120cm) long and can usually be cut in half and serve as a pair. Sharpen all 4 ends in a pencil sharpener and slightly blunt the tip so it's about the diameter of the head on a straight pin. Push the wooden tell deep into the soil. Don't worry, it won't harm the root system. If the plant is quite root-bound, you might need to try several places until you find one where you can push it all the way to the pot's bottom. Leave it a few seconds, then withdraw it and inspect the tip for moisture. For most plantings, withhold water until the tell comes out dry or nearly so. If you see signs of wilting, adjust the interval between waterings so drought stress isn't a recurring issue. You might find this helpful. Al

I've had this happen time to time. It just seems that that leaf didn't develop properly. Eventually it'll either grow to normal size or turn brown and die. The latter is more common. Don't be to alarmed your plant has healthy new leaves so that's good.

water a plant when it needs water .. insert your finger and FIND OUT ... or use a teaspoon to gently dig around and find out about water at root depth .. weight of the pot is also fairly indicative of how much dead weight water is in it ... i usually dont water until i feel a significant lessening of said weight ... or the plant actually wilts a bit.. indicating more water is needed ... [of course you huge pot may not be easy to lift ... ] random schedules and amounts dont really help ... until you figure out the schedule for your house ... how cold does it get near that door at night.. in your drafty old house??? you probably should find a good local nurseryman for advice.. rather than a florist ... your plant shows no signs of underwatering .. thats not how i read the affected leaves ... my only worry is overwatering and rotting the roots off .. and im not sure you are even doing that ... the best grower i ever met.. actually left his plants to slightly wilt ... before he watered again .. if you did that .. then you would have the beginnings of making a schedule for your house.. in that location... with all those windows ... you dont mention how long ago it was brought in .. i bet some of the leaf damage is related to that move.. or moving it from grower.. to seller ... minor leaf damage can show up a long time after the event that caused it ... meaning.. there is no current problem to fix .... frankly.. i think you plant looks great .. i would snip off the ugly leaves. perfect watering.. and try not to kill it with too much love ... i would also plan on moving it outside in summer.. in full bright shade ... large houseplants usually really enjoy such .. and can store a lot of energy.. for the winter .. but make sure its secure in that wind wont blow it over.. etc ... and of course.. that depends where you are relax.. it looks great .. welcome to the forums ... ken