Plants that live in pots have to survive and grows with relatively little soil. The soil, therefore, must meet their needs as completely as possible. It must give physical support to the plant and provide air, water and nutrients to the root system. The soil should be porous enough to allow air and water to penetrate. It must drain well and hold moisture at the same time. Both air and water must be in the soil. A good potting soil is only half solid. The other half is open air space so oxygen will be availble to the roots.

Good topsoil or garden soil is becoming increasingly hard for indoor gardeners to obtain. Most of us can't just go out to the back yard and dig up some good rich soil for indoor plants.

Furthermore, soil from the garden, by itself, is rarely suitable for use in a pot because the soil structure deteriorates through frequent watering and rapid breakdown of organic matter and is not renewed by association with surrounding organisms found outdoors. Aside from this, the garden soil is teeming with potentially harmful organisms and weed seeds and therefore needs to be sterilized for indoor use.

However, many indoor gardeners like to use, and do use, garden soil for their plants when they can get it. With modern additives it can be made more nourishing and less subject to physical breakdown. If you want to use garden soil, it should be sterilized. To do this, spread a thin layer of most soil in a shallow pan, bake it in the oven at 180 degrees for half an hour (the odor may be objectionable) and then let it coll overnight.

Additional organic matter in the form of peat most or commercial humus will be needed for most plants. Unless the soil is very sandy, its porosity will need improvement. This can be accomplished by adding sand, perlite or vermiculite. The sand should be clean, coarse, builder's sand. Perlite and vermiculite are non-organic additives which can be found at garden centers and variety stores.

The following mixture of garden soil and recommended additives will be a satisfactory potting soil for most houseplants. Combine one part sterilized garden soil, one part organic matter, such as peat moss or leaf mold, and one part coarse sand or perlite. (One part may be one cup, one pail or only other convenient measure). For each gallon of the mix, add 1 tablespoon ground limestone, 2 teaspoon 5-10-5 fertilizer and 1 tablespoon bone meal. Variations of the mix may be required for special plants.

Indoor gardeners are coming to rely more and more on prepared potting mixes sold in packages at garden centers, variety stores, supermarkets and by mail order. If you have only a few plants, it is simplest to use one of these prepared mixes. You may want to experiment with different brands, such as Black Magic, Pro-Mix, En-Vee or Jungle Growth, to find the one best suited for your plants.Special mixes can be had for African violets, cacti and terrariums.

Almost all of these special mixes are composed of sterile ingredients. Those with a coarse texture and a high percentage of organic matter are best. I have found that in some cases it is advantageous to add perlite or coarse vermiculite to achieve a looser mix and better drainage. Inclusion of sand with vermiculite or perlite adds weight to the mix and gives roots a firmer hold and tall plants better physical support.

You can make your own soilless mix from these basic ingredients: peat moss, perlite, coarse, clean builder's sand and vermiculite. One standard combination is referred to as the 1-1-1 mix. It is one part shredded peat moss, one part sand and one part perlite or coarse vermiculite.A mix for cacti and succulents consists of one part peat moss, two parts perlite, two parts vermiculite and one part coarse sand.

With use of these types of mixes, two considerations are critical: feeding and watering.

Prepackaged soilless mixes contain sufficient nutrients to sustain a plant for two to four weeks. After the nutrients have been used up, care is needed in feeding the plant because the ingredients in the mix are not a substitute for food. It is usually practical to use one-quarter strength of a water-soluble houseplant fertilizer every time you water. Remember that you should not fertilizer more when the soil is dry.

No pottng soil will work if you don't water sensibly. The soilless mixes drain rapidly, so when using them you must learn to recognize signals that tell you if the plant is getting the correct amount of moisture. As the mix dries out, the color changes from dark to light. The weight of the potted plant also is an indication of whether the mix has dried.

Potting soils of whatever quality are not substitutes for attention to a plant's requirements for light, humidity, water and suitable temperature.

Joyce Taylor, Springfield, Va.: What suggestions do you have for getting rid of white flies? They are on our "cherry" plant and poinsettia, especially.

A: White flies quickly infest palnts and sometimes are out of control before you are aware of their presence because they favor the undersides of the foliage.

Many dedicated indoor gardeners will throw out an infected plant rather than risk the spread of white flies.

Try mixing 4 teaspoons of any dish-washing detergent in a gallon of water. Shake well and place in a sprayer that shoots a fine mist. Cover undersides and top sides of leaves. Several application every few days may be necessary to catch newly hatched flies.

You can make the detergent treatment more effective by adding Black Leaf 40 or a weak tea made from cigar or cigarette butts. Do not use tobacco on Jerusalem cherry.

Experience has shown that white flies are attracted to the color yellow. Commercial growers are having luck using yellow plastic, coated with a sticky substance, to attract and eliminate these pests.

A botanical insecticide containing pyrethrumrotenone, such as Red Arrow Insect Spray, will subdue these pests.

If you select a chemical spray, be sure it is listed as being for use against white flies. For your own safety, follow the directions given on the container for use of such sprays.