Potting Soil and Houseplants
How often should one put new soil on potted plants or replace the soil altogether?
Let the plants and your growing goals dictate how to proceed. Stunted houseplants with undersized new growth and those where the water quickly passes through the pot are candidates for a move. A few days after watering is a good time to move the plants. The soil is moist enough to hold together but not too wet to handle. Slide the plant out of the pot. Loosen or cut any roots that are growing around in a circle. Then move the plant to a pot one size larger. Avoid the temptation to move it into an even larger container. The plant will spend its energy sending out roots instead of top growth plus the extra soil will hold moisture increasing the risk of root rot. You can always replace the soil that settles out the drainage holes. Spread a thin layer of potting mix on the surface to replace what washed out the drainage holes. A friend of mine reduces this problem by plugging the drainage hole with florist foam. This material allows water to drain but keeps the soil in the pot. Slow the growth of larger plants by allowing them to stay in the same container a bit longer. Flowering houseplants and cacti prefer to be a bit potbound. You even may consider root pruning larger plants that have reached the maximum size either you or the available containers can handle.