Vegetables need full sunlight as well as favorable amounts of moisture and fertilizer. They cannot compete with the foliage of trees for sunlight nor with tree roots for moisture or nutrients.

The ideal soil for them is fertile, deep, well-drained and high in organic matter. Heavy clay soils are slow to dry out and difficult to cultivate and work properly. Extremely sandy soils may lack organic matter and may dry out too rapidly between water applications. The best soil is between these two extremes.

The deeper the soil is stirred, the greater is its capacity for holding air and moisture. The soil should be spaded or plowed over 8- or 9-inches deep, provided the subsoil is not turned up.

When vegetable plant roots occupy the soil at a depth of 30 inches and utilize the moisture in it, they become far more productive, according to Dr. Fred P. Miller, University of Maryland agronomist and soil-and-water resource specialist. It could make a big difference during prolonged dry weather, he says.

The problem is that the soil is too acid at lower depths and lacks the fertility to support plant root growth. Liming and fertilizing could take care of that.

Many Maryland soils have plenty of moisture even below 30 inches, Miller says. But for most agronomic crops this depth is seldom utilized to a significant degree. Thus, crops can literally die of thirst while sitting within easy reach of plenty of water.

One of the best things for gardeners to do is double-spade, that is, dig the soil two-spade depths instead of one.

Next to soil preparation at planting time, the second most important feature of successful gardening is mulching, according to Carl Orndorff, retired Maryland nurseryman.

Organic materials so important to soil preparation are equally important for mulching, he says. Cultural advantages are more than gardeners realize.

Here are 10 major benefits of mulches:

Reduces baking and crusting of soils.

Allows better water entry into soils and prevents puddling by heavy rains.

Reduces wind and water erosion.

Stabilizes soil moisture by increasing water holding capacity and reducing evaporation.

Reduces extremes in soil temperature by insulating against high temperatures in summer and cold in winter. Also reduces heaving from freezing and thawing.

Promotes week control.

Organic mulch supports the microorganisms that break down crude foods into usable nutrients.

Stabilizes the pH of the soil, especially in those soils previously low in organic matter.

Aids soils in the holding of nutrients in forms available to plants.

Foods made available by mulching may reduce demands for supplemental fertilizing.

Here are Orndorff's suggestions of mulching materials for annual and herbaceous perennial gardens:

1. Favored choice, effective and usually available: shredded oak bark, medium-size grade; shredded pine bark, medium-size grade; pine needles; garden compost.

2. Good second choice, usually available: mixed bark and wood chips, composting preferred; shredded leaves or mixed leaves and garden wastes, composted preferred; wood chips or sawdust composted, same material except for particle size.

3. Good second choice, usually poor availability and/or high cost; cane fiber; rice hulls, peanut hulls, buckwheat hulls.

4. Poor choices: grain straw salt hay, a fire hazard, poor aesthetic value; coco hulls, corn cobs, mildews, crusts; peat, excellent soil conditioner, poor mulch; wood chips containing high percentages of tulip tree, sweet gum, and/or sycamore, possible manganese toxicity.

Inorganic materials, often used for various reasons: sized natural stone or gravel; sized crushed stone used for water erosion problems or for some special aesthetic effects; sheet plastic, not practical except in some annual gardens.

Mulches of neutral dark earth colors with medium to small practical size normally have greater aesthetic values. Flowers and foliage show favorably against dark backgrounds. Stone or gravel may be used to create special effects in color and/or texture.

A fact to be stressed forcefully, says Orndorff, is the depth of a mulch. The theory that if a little is good, more is better, cannot be applied here. Normally 1 to 2 inches in depth is best. Types of plants, types of mulch, and local soil conditions may justify some variations in depth of mulch. Some succulent plants including bearded iris may best be left unmulched.

Landon School, 6101 Wilson Lane, Bethesda, will hold its annual Azalea Festival, Friday, Saturday and Sunday, May 1, 2, 3, which benefits the school's scholarship fund. There will be tours of the garden, plants of azaleas, herbs and wild flowers will be on sale, admission is free. Tom Stevenson will be there Sunday 1 to 3 p.m. to answer garden questions.