You cannot directly place kelp on soul sand. Kelp needs a water source block to be planted. Therefore, you'll need flowing water turning into a source block for it to grow.
Here's how you can use kelp and soul sand in a useful manner, specifically for creating a water elevator in Minecraft:
Creating a Soul Sand Water Elevator with Kelp
This method leverages the bubble column created by soul sand in water to create an elevator.
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Build the Shaft: Construct a vertical shaft of any desired height, at least 1x1 block in size. Make sure the shaft is completely enclosed to prevent water from flowing out.
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Place the Soul Sand: At the bottom of the shaft, place a block of soul sand. This is what will create the upward bubble column.
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Fill with Water: The next part is essential. Instead of just pouring water from the bottom, place the single water source at the TOP of your elevator and let it flow down to the bottom. This ensures every block in the shaft becomes a water source block, which is needed to make the elevator work.
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Plant Kelp: Once the water has filled the shaft, plant kelp at the bottom block (the one directly above the soul sand). Allow the kelp to grow to the top of the shaft. This converts all the flowing water blocks into water source blocks. This can now also work with a water bucket.
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Harvest the Kelp: Break all the kelp. This leaves you with a full column of water source blocks, and the soul sand at the bottom will now generate a bubble column that pushes entities upwards.
Now, any player or mob that enters the water column above the soul sand will be rapidly propelled upwards.
Why this Method Works
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Water Source Blocks: Kelp converts flowing water into source blocks as it grows. This is crucial for the soul sand to function correctly. Without source blocks, the soul sand will not generate a bubble column.
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Bubble Column: Soul sand, when placed under a column of water source blocks, generates bubbles that push entities upwards, creating the elevator effect.
