I've recently discovered that the base of my pineapple plant is very thin (see the attached pictures). It almost seems that if it grows bigger, it will just snap!
How can I help it not break under its own pressure?
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You soil is too wet. It is also too high up the plant. It should not be completely covering the pineapple top. It should look like it's a pineapple stuck in soil.
You want to use well draining potting mix. Something like peat and perlite. Or any indoor potting mix will do. Those a made to be light and fluffy. Your mix appears to be muddy. It does not need any other additives. Just a simple peat & perlite or Coir and perlite blend.
You want to keep the soil moist but not wet. I think you are giving your plant too much water. I can not see the size of your pot, but it does not need to be big. Does you plant have roots yet or are you just starting it. If you are just starting it, you are defintely giving too much water. If the plant does not have roots then it can not absorb water. If this is a large pot full of wet soil. The water has no where to go, but sit in the pot. You want to give the plant just enough water to keep the roots nice and moist.
I would invest in a bamboo skewer. You can insert it in the soil for 30 mins, then pull it out. Look at it and feel it. From that you will know how wet the soil it and how deep. It is very important in the beginning to keep the soil fairly dry to encourage the plant to make roots. The new root will be very fine and delicate. Too much water and they will rot. If this continues the plant will have to use it own reserve of water inside the pineapple top and your stem. These are place the plant will get water too survive if it does not have roots. Once it has some roots you want to keep that area moist. Never let it completely dry out, but also never soak it completely, not yet. Once you plant start to grow on top you can start to increase the amount of water. Giving it enough to get all the soil moist, but not wet.
Once the plant is fully rooted and growing. You start watering it in a regular manner, you want the soil to dry out before you water it again. Give it enough water to run out the bottom. Then let it dry out between watering.
If you are a person that tends to overwater. Mix more perlite into the soil you buy. Up to 50% Potting Mix t 50% Perlite. That is only if you are a person that tends to over water your plants. If not, you can use any indoor potting mix.
Use the bamboo skewer.
Pineapples are dry land epiphytes. This means that they use roots for anchoring, but get much of their moisture and nutrients from the air and what falls into the leaf bases which are like little cups. Probably your plant went through a dry period or some stress which constricted the base and it was too mature to recover. Not to worry, it will be very fibrous in there and quite possibly has lots of tensile strength and will not break as long as it does not rot. Maintain good growing conditions for the plant, which means dry at the base in consistently high humidity along with heat; likely not the same as personal living conditions.