It's actually not that difficult to determine whether or not you have happy
Kratom plants or not; healthy leaves are broad, with a rich green color.
So, whenever the leaves don't look as lush as they do in the first image
below, I will walk you through the most common problems and how to fix them.
First, a HEALTHY KRATOM LEAF:

Notice how silky it looks, how the edges are all smooth, and there's a deep,
green color to the leaf. Also, this leaf is large and broad...another
sign of a happy Kratom plant.
The most common problem is UNDERWATERING your Kratom plants. What has
worked best for me, is to wait until the top 1" or so of the soil dries out
before watering again. And then, when I DO water, I place the plants
in the bathtub, and for each gallon pot size, I run 1/4 as much water
through the soil. So, in a 1 gallon pot, I water it with 1 quart of
water, letting it sit in the tub until the water has drained out of the
bottom.
An UNDERWATERED KRATOM LEAF:

Notice how it wrinkled at the TIP first...this is important. If the
plant is curling and drying out along the edges and not the very tip
instead, it's most-likely a sign of OVER-watering. But this poor leaf
wilted first, and then started to curl over several days, so you have plenty
of time to react.
Next, when using hydroponics or plain soil, often the Kratom leaf, which
needs a lot of nutrients, often doesn't get enough. This is easy to
tell because the Kratom leaf will turn pale, and start to take on a
yellowish appearance.
A NUTRIENT DEFICIENT LEAF:

Notice how it also has some browning around the edges. That is further
evidence that that leaf isn't taking up enough water or nutrients. One
of the EASIEST ways to fix this problem, is to make sure that you have the
proper pH for the water you're using.
Kratom plants like a pH of 5.5 to 6.5 best. And, most plants can't
absorb nutrients below 4.0 or above 7.0, so pH of your water is critical.
Nest is the opposite extreme; where the plants is getting TOO MANY
nutrients. This can happen from over-anxious growers who want to give
their plants LOTS of vitamins, but Kratom, when given too many nutrients,
reacts quickly and negatively.
An OVER-FED KRATOM LEAF:

Notice the large brown spots, commonly called "burn". Also, notice how
yellow and "rough" the leaf looks. Part of the white-ish look to the
leaf is the nutrients actually perspiring out of the plant, and building up
on the leaf itself. This is a VERY dangerous condition, and should be
remedied the moment you're sure of your diagnosis.
Another OVER-FED LEAF & NITROGEN-HEAVY:

Not only does this one show heavy burning around the edges from too heavy or
rich of a nutrient solution (too much fertilizer), the leaf is showing signs
of absorbing far too much nitrogen, which often tends to over-darken the
leaf.
It's an easy fix; though: Simply wash your leaves, your soil, or your
reservoir with fresh luke-warm water for about 5 minutes. Then, re-water or
re-fill with fresh water, and if using nutrients, use half as much to see
how the plant reacts.
If you have heavy damage from that looks similar to the immediate photo
above, then before using half the nutrients, run WATER-ONLY for a week, wash
your plant, growing medium, and reservoir again after the week is up, and
THEN add 1/4 to 1/2 as many nutrients as you did when the leaf burn
occurred.
~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
Since Kratom is such a hearty plant, it gives you plenty of time to react to
any number of unhappy moments it may experience as you get to know your
plant and its growth cycle. Never push it too hard, and know that it's
a fast-growing plant, so over time and with a little patience, you will have
more leaves than you know what to do with.
See different information in the How to
Grow Kratom article as well.
Also, see Kratom Plant Secrets
for great growing secrets for Kratom plants.