Q-You suggested [Weekend, March 30], mixing peat moss and a pound of ground limestone and a pound of 5-10-5 fertilizer per 10 square feet with the soil for a new garden. A University of Maryland specialist claims that this is a serious error, that three to four pounds of 5-10-5 per 100 square feet is adequate and that one pound per 100 square feet would be enough in most cases. Who's right?

A - The only way to know for sure how much lime and fertilizer to apply is to have the soil tested, something I have recommended several times recently in this column. Anything else is a guess.

If the soil in question is fairly good, the specialist would be correct in saying that my recommendation was too much; if it is poor soil, mostly clay or clay loam, my recommendation probably was not a bad guess.But even if the soil is fairly good, and "too much" lime and 5-10-5 is applied, there seems little chance of doing serious harm. Here's why:

Soil tests at the University of Maryland have shown that many soils in this area are too acid even for azaleas, which require an acid soil. Writing in Soil, the USDA 1957 yearbook of agriculture, W.H. Allaway, head of the soils and plants relationships section of the soils and Conservation Research Service at Beltsville, says it takes about 20 pounds of ground limestone per 100 square feet to raise the pH of clay loam from pH 4.5 to 6.5.

Peat is generally used as organic matter for soil improvement because it's readily available. Peat is high in carbon and releases too little nitrogen to supply the metabolic requirements of the soil organisms that decompose it, so a nitrogen deficiency occurs just when the plants need nitrogen most. The nitrogen will be used up by soil organisms or leached out of the root zone of the crop.

Excessive amounts of potash salts are sometimes toxic to germinatig seeds and young seedlings, but it leaches from the soil readily when a large amount is present in exchangeable or soluble form, according to W.L. Hill, fertilizer and agricultural lime section, Soil and Water Conservation Service at Beltsville, in the same yearbook.

Most of the total phosphorous supply in the soil is tied up in a form that is not avialable to the growing plant. Acid soils contain a large excess of iron and aluminum, Maurice Fried, soil scientist at the Agricultural Research Service at Beltsville, says in the same yearbook. These readily combine with water-soluble phosphates and convert them into less-soluble phosphates. The available soil phosphorous usually is only about 1 per cent of the total.

Phosphorous is fairly desirable as a starter fertilizer for seedlings set out in early spring to offset the effects of cold soil.

Q - My Christmas Poinsettia still looks good. What shall I do to keep it in good condition and have it bloom again next Christmas? A-Keep it watered until the weather gets warm outdoors, then cut it back to about four to six inches above the soil level, thus removing the old flowers and most of the side branches. Then repot it into a pot an inch or two larger, keep it outdoors in light shade, fertilize it every three or four weeks with a soluable fertilizer dissolved in water, and keep it well watered. The plant will be seriouslyy damaged if it goes too long without water. If the plant must be kept indoors, keep it near a window where it gets good light. The plant needs to go back indoors before cold weather. To bloom for Christmas, it needs uninterrupted darkness from sundown to sunup, October to December.if you have a question for Tom Stevenson, write to him at the weekend section The Washington Post, 1150 15TH ST.NW Washington, D.C. 20071.