I think all are being over-watered. I'm pretty meticulous about how I make the gritty mix. Everything is screened to its appropriate size and prescreened ingredients (bark/grit) are screened to remove all dust ....... and my normal watering interval indoors under lights is 6 days for about 80 (or so) of 100 plants. The other 20 (or so) are in pots with soil volumes ranging from less than a teaspoon up to 1/2 cup and require special consideration.

Start using a 'tell' to 'tell' you when it's time to water and things will probably straighten out. It won't be over-night, but you should start to see a difference right away.

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 plenty of air (oxygen) in the soil to
drive root function. Many off-the-shelf soils hold too much water and
not enough air to support the kind of root health most growers would
like to see; and, a healthy root system is a prerequisite to a
healthy plant.

Watering in small sips leads
to avoid over-watering leads to a residual build-up of dissolved
solids (salts) in the soil from tapwater and fertilizer solutions,
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

Hi Al, thanks for the feedback. I’ve read a lot of your informative posts and have used the info presented to guide my growing experience, but I admit I’m guilty of not relying on a ‘tell’ to determine when to water. Given how fast draining the gritty mix is and based on what I’ve read regarding how difficult it is to over-water in the Gritty Mix, I didn’t think over-watering would likely be an issue since the mix would be more forgiving in that regard. I will try lengthening the watering interval and will report back.

I use the gritty mix almost exclusively because it's the most productive medium I've ever used, and the easiest to grow in/ keep plants healthy in, but it is important to make sure there isn't a disparate difference in the size of individual ingredients it's made from, and that all ingredients are screened. Example: if you mix equal volumes of play sand with BBs, the fraction of the mix roots will actually be growing in is sand - the BBs do nothing for aeration or drainage (maybe a little for drainage, but not for aeration) until the fraction of BBs is so large there is no longer enough sand to fill the air pores between BBs, that is the point where air porosity really starts to rise to truly beneficial levels - assuming the mix stays homogeneous.

What Turface product did you use? You want Turface MVP or Allsport, and it should be screened such that you keep/use what doesn't pass through aluminum insect screen. The bark should be 1/8-1/4 if fir bark, and 1/8-3/8 if pine bark. The grit should be 1/8-3/16.

Also, I shouldn't assume this is over-watering unless what your mix turned out to be varies significantly from what I described above. Light has a lot to do with how much fertilizer you provide. The rate at which you're fertilizing might be too high for the level of light your plants receive. Example - if you are fertilizing a plant at luxury levels in full outdoor sun, the simple act of bringing it indoors to a lower light environment can immediately cause fertilizer burn, even though no additional fertilizer was provided immediately before or after the change in light. So, the level I fertilize at with plants under light might be too high for a low-light environment.

So, if the 'tell' tells you you're watering before the soil has dried down enough, that's probably the issue. If you discover that watering isn't the issue, cut the strength of fertilizer solution in half or double the interval ..... and be sure you're flushing the soil when you water. That's one of the main attributes of the gritty mix - you should e able to flush the soil at will w/o having to pay an over-watering tax on the plant's vitality.

w/fir bark ^^^

w/pine bark ^^^

Al

Hi Al,

I realize how important particle size is, so I did take measures to carefully screen all of the ingredients. Based on what you suggested above, it looks like my grit is a bit on the smaller side.

Repti Bark - screened 1/4"-1/8"

Turface MVP - screened 1/12"-1/8"

Manna Pro Poultry Grit - screen 1/12"-1/8"

One note worth mentioning re the poultry grit, I noticed the package label indicated that it included probiotics. I wasn't sure if this could be detrimental to the mix or not so I thoroughly washed the grit prior to use.

I confirm I am flushing the soil each time prior to watering/fertilizing. I actually thought I could be under-fertilizing because the new growth on the Agapetes looked light green and the stem never harden. In fact, following this intuition I ended up adding CRF (Osmocote Plus) as insurance even though it doesn't have the same NPK ratio as FP. Since I'm using FP together with CRF, going forward I think I'll reduce the FP dose to no greater than 1/8 TSP/gal and see how that goes. Thanks again.

The soil you're using will definitely support enough of a PWT that I'd want to put some sort of work-around in place. You could push a wick up through the drain hole, which will allow quite a bit of additional water to drain after drainage has stopped. To see this work, push a toothpick up into the drain hole after the pot has stopped draining and watch the water start draining again. I use a strand from a 100% rayon mop head you can usually find at hardwares, Walmart, Amazon ....... .

Here's how I use them,

but you can push a 6" piece up into a drain hole so about 3" hangs down from the pot as it drains. The best place for a drain hole is through the pot bottom at the sidewall. That way you can gain considerably more advantage by tipping the pot after watering. You'd be doing B and E below, which will help a lot.

You're on the right track though. Next batch, use the next size up - MannaPro's poultry size grit. Or, use Gran-I-Grit in grower size or #2 Cherrystone if you can find those. I'd skip the CRF next time and go with the 9-3-6. It gives you more control and the ratios in it are already formulated to cover at least 95% of the plant's you'd be likely to grow. It's best to tie fertilizer intervals to the number of times you water instead of the calendar. If you fertilize every 3rd or 4th time you water, and you're flushing the soil, that would be about right. Since all plants shouldn't be on the same fertilizing schedule, keep track of how many times every planting is watered by dropping a marble in a pot or hanging a button on a paperclip clipped to the edge of the pot. When it's time to add the 3rd or 4th marble or other item, it's time to fertilize.

Al

Hi Al, I’m a bit confused on the recommended screening size. If our goal is to eliminate PWT, I understand why it’s necessary to screen grit to 1/8”-3/16” but why is it acceptable to screen Turface with insect screen which is smaller than 1/10”? Wouldn’t this also allow for some PWT? Would it be prudent to screen Turface to 1/8” as well?

Your point on fertilizer dose as a function of light is interesting. I didn’t realize fertilizer burn could occur so quickly. I had the understanding that fertilizer burn occurred as a result of salt level build up and this does not happen immediately. Now I’m beginning to wonder what light levels are considered for the FP dose recommendations.

" ..... but why is it acceptable to screen Turface with insect screen which is smaller than 1/10”? Wouldn’t this also allow for some PWT? Would it be prudent to screen Turface to 1/8” as well? I regularly make mention of the fact that there is a compromise re using Turface MVP screened over insect screen in the gritty mix. I usually 'lose' around 20% of the product when screening over=insect screen. If we increases the size to a #6 screen, which would allow approximately 1/10" particles to pass, you'd lose more than 60% of it to screening. They DO make Hort-Grade Turface, which would be a better option. I don't use it only because I haven't figured out how to buy it in large quantities.

Re the fertilizer dosage - I use about 2x the recommended production rate for 9-3-6 in the summer when temps allow, but only the maintenance rate in winter, even though I'm growing under lights.

"I didn’t realize fertilizer burn could occur so quickly. I had the understanding that fertilizer burn occurred as a result of salt level build up and this does not happen immediately." Fertilizer burn can occur in several ways - over-fertilizing (immediate), a gradual build-up associated with not flushing the soil at watering time, reducing photo load or temperature for plants recently fertilized at luxury levels.

"Now I’m beginning to wonder what light levels are considered for the FP dose recommendations." Don't over-think it. Start at lower levels and increase solution concentrations or shorten intervals if leaf color is appears chlorotic or leaves show deficiency symptoms. Don't try to micromanage by adding a little extra this or that because someone suggested it's better to do so - it usually ends up being a minus rather than a plus.

Al

Hi Al, Thanks for the clarification and heads up on the larger size Turface.

You mentioned your approach to fertilizing “when temps allow”. Can you expand on this? I assume you were referring to ambient temp and not soil temp though I am curious to the effects of both.

I don't fertilize during the dog days of summer (over 80-85*) because plants go into transpirational over-drive = the plant uses much more water = a greater volume of nutrients left behind in the plant when more water from the nutrient stream is used during evapotranspiration at the same time growth is slowing = a much greater susceptibility to fertilizer burn and the plant's reduced ability to move water into cells (osmosis).

A correlation: the best way to force back-budding (when temperatures are between 60-80*F) is to provide an ample supply of nutrients - especially N, plenty of the brightest light the plant will tolerate, and plenty of air movement. Both bright light and air movement force use of an extra measure of water, which, overall, delivers a greater amount of nutrients , which serve as the building blocks of growth.

Also, there are significantly more ammonium toxicity symptoms when plants are being fertilized with ammoniacal nitrogen or urea, (most are) and actual soil temperatures are above 80-85* - this is particularly true when soil compaction or waterlogged conditions are in play. High root and shoot temps force rapid use of sugar/carbohydrate during respiration. Sugar also happens to be required to chemically detoxify (cytoplasmic) ammonia. So, high root temperatures combined with increasing concentrations of ammonia inside the cell is dangerous to cell survival as soon as a temperature point is reached at which all the sugar in the root is consumed, with none left to prevent NH3 toxicity

Al

Very interesting, Al! So, now, what if I water and fertilize at night when it cools down? (Well, if it cools down that is.)

Or, is this same heat/fertilizer interaction going to occur the next day? Sorry if this is an obvious question. We are in a heat wave and I just fertilized last night ... uh oh!!!

Thanks!

It's the TDS (total dissolved solids) in the soil solution that determines how much is too much. Tomorrow, because some of the soil solution will have been used or will have evaporated, the TDS level will rise until the next time you water. Try to shade your pots and/or flush them thoroughly tomo, then wait to fertilize until you're sure the actual soil temp is below 85*. If pots are shaded, soil temps should be lower than ambient temps by 10-15* when it's above 80*, this, due to evaporative cooling.

Al

Thanks, Al - this is very helpful!! I think I will hunt out a thermometer just to be sure. I need some anyway.