Answer: Yellowing on cacti is often a sign of stress due to underwatering. On the other hand, cacti don't like to have their roots sitting in wet soil! It's best to water slowly, deeply, and infrequently allowing the water to penetrate the entire root zone (make sure it's not just running down the sides of the pot and out the drainage hole). Cacti typically don't need fertilizer, but your plants sound as if they are really producing this summer. They may have used up available nutrients in the soil. You could apply a cacti formulation, or if you can't find one, use a fertilizer with a low first number, such as 1-6-7. Water before and after applying fertilizer to lessen the chance of root burn. Typically, cacti don't need alot of nitrogen (the first number). I hope this info helps!
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