When tomatoes and beans are through for the season, plant a cover crop of annual rye to prevent erosion during the winter. Then in spring, plow the cover crop under: It'll improve almost any soil, but particularly those that are mostly clay or sand.

An almost unbelivable amount of soil can be lost during winter by splash erosion and surface runoff. Splash erosion happens when raindrops hit the soil. Each drop, falling as a tiny projectile, breaks small particles from the soil mass. Then, during heavy rainfall, surface runoff of water removes the particles. A cover crop prevents this loss.

Heavy clay soils dry out slowly and are difficult to cultivate and work properly. Extremely sandy soils dry out fast between applications of water and fertilizer leeches away rapidly. The addition of organic matter, such as rye, improves the soil. You turn the rye under long before it reaches maturity; yet even when mature, the tops and roots provide organic material.

Grassy plants, which include rye, are the best soil-conditioners known. They produce an extensive system of small roots which thoroughly penetrate the soil.

Organic-matter content increases the amount of water that can be held in the soil for use by plants. Good drainage also is important, because too much water will crowd air out of the soil.

Without air, plant roots cannot grow or take up nutrients or water. With air in the soil, the carbon dioxide produced by soil bacteria and by the respiration of plant roots can be diffused into the atmosphere.

If water moves freely through the soil, air can fill the empty pore spaces. But when water moves too fast, the result is a droughty soil.

The most desirable condition is a soil in which air and water are about evenly divided. Addition of organic matter, such as that provided by the cover crop, helps establish and maintain such a condition.

You'll get a lot more organic matter if you can let the cover crop grow for a longer time. If the garden is large enough, it can be divided into two equal parts. One part can be usd for vegetables and the other for growing a soil-improvement crop which will be followed later in the season by vegetables for fall harvesting. Reverse the two sections the following year. Q. This spring we planted horseradish roots for the first time. When should the roots be dug? Should they be dried out or stored before using? Should a piece of each root be saved and replanted for next year's crop? Can you give a recipe? A. Horseradish roots should not be dug until late fall. Until then, they are hardly fit for use. Dig only enough for immediate use and dig others as needed through the winter. They should be used immediately, for pungency is soon lost. For another crop, plant cuttings taken in the spring.

Here's a recipe: Grate one cup horseradish root in a blender with 1/4 cup of vinegar and add other herbs, including dill and mustard. Q. Can you tell me how to bring my Christmas cactus into bloom? It's a beautiful plant, but no flowers . A. Force the plant to rest now by giving it only half as much water as it's been getting. About a month from now, resume regular watering. Flower buds develop during the resting period if the plant has night temperatures of 50 to 55 degrees. Flower buds will develop also with night temperatures of 60 to 65, if the plant gets 12 to 13 hours of uninterrupted darkness. At 70 degrees, there will be no flowers regardless of length of night. Q. Can herbs be grown indoors? If so, which kinds do best ? A.Parsley, sage, English thyme, caraway, sweet marjoram, summer savory and sweet basil, among others, can be grown indoors on a sunny windowsill. Q. The pine tree we planted in front of our house has become too tall. When can we cut it back and how far without ruining it ? A. Most pine trees naturally grow 40- to 60-feet tall. They can be kept small by pruning each year. Once they're large, it's too late to reduce their size appreciably without killing them or spoiling their ornamental value. The time to prune them is in the spring: Half the new growth can be removed. If all new growth is removed each year, the branches will be bare after old needles fall, and the tree will die. Q. Can rhubarb be forced into growth indoors in winter? We love rhubarb pies . A. rhubarb can be grown indoors, preferably in a greenhouse, but first it needs two weeks outdoors at 10 degrees. Then dig the roots, plant them in a container large enough for at least three inches of soil, work the soil well between, under and around the roots and water. Rhubarb tolerates poor light indoors. A night temperature of 60 is best. Don't let the soil dry out.