Most people think of a cactus as a plant that does not require any water. Images of cacti littering the desert abound in media and reinforce this perception. In reality, cacti require plenty of water. However, they require less water than many other plants. In fact, watering a cactus often requires the gardener to strike a very difficult balance. Like other plants, cacti will die if they don't receive enough water. The problem is that cacti also quickly die if overwatered.
Plan a general watering schedule for your cactus. If you have a cactus growing outside, you might need to water the plant every few days. If the plant is inside, you might only need to water it once a month. The difference is that plants in pots retain moisture in the soil surrounding the root system of the cactus. Outside, the sun leaches moisture from the soil, it dissipates through more ground and other plants and weeds compete for it.
Test the cactus before you water it. Stick your finger into the soil at a depth of 1/2 inch or slightly deeper. If your finger is wet, the plant does not need to be watered. If your finger is dry, water the cactus.
Use non-softened water when you water a cactus. The salt used in the softening process can damage the plants. Use rain water if you have a system of gathering it.
Water the cactus until water runs out of the bottom of the pot, or until outdoor plants have been deeply watered. Shallow watering causes root tips to die off and weaken the plant. Less frequent, deep watering is more effective than constant surface watering.