(adsbygoogle = window.adsbygoogle || []).push({}); What you need
-1 Cactus
-1 Block of Sand
-4 Block of None Falling Material ( Stone , Wood and, others )
-1 Bucket of Water / Water Block
-[Optional] Any type of shovel

As you can see i have all the needed equipments

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Start of by Placing the Wooden Block down

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After that place the Sand Block on top of the Wood

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And finally Place 2 blocks on top of the block of sand

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Next place a block on the side of the top block

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Then Destroy the 3 and 4th Block of the "Pole"

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Then dig around the the wooden block and create a flow for the water

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Finally Place the Cactus on the Block of sand and add the water behind the wooden block

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How it works
The Cactus will grow up 1 block but, because we have the wood placed next to the spot where the cactus will grown the block will break since cactus can grown next to other blocks

After a few minutes...

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Hmmm, wonder what I'm doing wrong then

It won't seem to work for me

this won't work as far as I know.
The sand has to be touching the water

Check out mine:

No it doesnt what will happen is he cactus block will grow up a block and will instantly be destroyed then it will fall into the water and follow the stream i've built a huge farm do this

oh, i thought sand needed to be touching water for it to grow, i've built one before

nope, works for me

nice by biggest 1 was with 500 cacti i think

why would you want to farm cactuses anyway they dont do anything