Every now and again, it's necessary to re-pot a plant. Most house plants that are kept inside year-round (without a summer sojourn out on a porch) need re-potting about every other year. If the plant has been heavily fertilized over a long period, the soil can build up an abundance of soluble salts. In some cases, the only solution is to change the soil. Other plants simply outgrow their old pots.

How can you can tell if a plant needs repotting? Yellowing and dropping of lower leaves, smaller or stunted new growth and roots emerging from the pot's drainage holes are sure signs. If a plant just isn't doing well and looks sick, moving it to a smaller pot with fresh, rich soil will often bring it back to life.

The new pot, regardless of whether it's larger or smaller, should be made of the same material as the original. But if the plant is unusually large and the extra weight of a clay pot is out of the question, then the practical approach is to re-pot in a plastic container. Don't increase or decrease a new pot by more than two sizes. When a pot is too large for the plant, the soil not being used by the plant's new roots can sour and infect the good soil.

In any case, it's good to sterilize all new pots, even if you've just bought them. Mix a mild bleach-water solution in a bucket [one part bleach to 10 parts water]; immerse the new pot in the bucket for half a minute. Allow plastic pots to dry thoroughly. Clay pots should be soaked in plain water for five minutes after emerging from the bleach bucket. They'll absorb the water you use to moisten the soil if you don't soak them well before adding the new plant.

The simplest soil to use is a sterilized potting mixture you can pick up at garden, hardware or grocery stores. Add to that some good compost if you have it (peat moss and cow manure if you don't) and perlite or quarry sand. For extra nutrition and insurance, get some steamed bone meal. An all-purpose soil mixture is about two parts sterilized soil, one part compost or peat, one part perlite or sand and a handful of bone meal added near the bottom of the pot. Peat moss, incidentally, should be moistened thoroughly before you add it to the mixture. The easiest way to get it good and damp is to add very hot water to it. If the pot's large, a layer of small stones or gravel at the bottom of the pot will aid drainage.

Spread out a newspaper on a table -- this is a messy operation. To remove the plant from the old pot, put your hand over the soil, grasping the base of the plant between your index and middle fingers. Tap the outside of the pot gently on the table, loosening the soil. The plant should come free easily with just the gentlest pulling upside down.

Set the plant upright. Get your new pot. If it's clay, put a piece of broken clay or a large stone over the drainage hole. If it's plastic, it will have several smaller drainage holes that don't need to be covered. Add pebbles if you wish, then add a layer of the soil mix, bringing it up and around the sides of the pot, as though you were making a pie shell. Add the handful of steamed bone meal, and then the plant, making sure it comes up to within a half inch of the rim. You don't want to change soil levels. Tap the bottom of the pot firmly on the table, giving it several good thumps. This will get rid of air pockets, which can cause rotting of roots.Keep adding soil and tapping the pot until all the spaces between the plant and the walls of the new pot are filled. Press the soil firmly with your hand.

When the plant is firmly in its new home, immerse the whole thing in water, just covering the soil but not touching the lower leaves, for about five minutes. When the soil has stopped bubbling, it's saturated and the pot can be removed and allowed to drain. In its new environment, the plant won't need feeding for a good long time. The new soil is rich in nutrients.

OTHER SOILS: Garden centers carry a soilless, artificial growing medium containing nutrients, for potting plants. However, if you use it, you'll have to keep the plant fed with chemical fertilizers suitable for that mix and plant. This may result in a buildup of salts that accompany commercial fertilizers. You'll have to flush out the plant with large amounts of water about once a month.

Instead of purchasing a potting mixture, you can start with a base of garden soil, or use garden soil half and half with a potting mix to stretch it. When using garden soil in place of a commercial potting mix, the aforementioned ratios still apply. You'll have to sterilize your garden soil, however. To do this, put it in a baking pan, cover it with aluminum foil and insert a candy or meat thermometer into the soil. Put the pan in you oven at the lowest setting and let the soil heat to 180-200 degrees (this takes about 1 1/2 hours) and hold it at that temperature for half an hour. Remove the pan and let the soil cool for 24 hours before using it. FOR HAPPY PLANTS

HOUSE-PLANT HOW-TOS: The U.S. Botanic Garden will be talking about indoor plants this weekend -- particularly jade, Swedish ivy, snake and dieffenbachia. The free, one-hour classes begin Friday and Saturday at noon and 2 at First Street and Independence Avenue NW. Call 225-7099.

PLANT LECTURES: The American Horticultural Society will sponsor a series of five 10 a.m. lectures, February 24, March 3, 10, 15 and 31. Subjects range from drawing plants to growing fruit at home.At the AHS River Farm in Mount Vernon. Cost for one is $12; the series for non-members is $50. Call 703/768-5700.