Hurricane Floyd is yesterday's tropical storm, but some of the 4.57 inches of rain it left behind is still in our soil today. To make sure your soil is prepared for the next downpour, now is a good time to ensure that it'll keep the plants moist but your property well drained.

There's the deep kind of drainage, called percolation. This, in conjunction with moisture-holding, will determine if a tree, shrub or perennial is going to thrive. You also must consider surface drainage, where water might run toward the wall of the house and cause a wet basement. Check and repair these situations before doing anything else to the landscape.

* Percolation. Soil percolation is as important as sunshine for healthy plants. And you get only one go-round before plants go into the ground. You never get the opportunity again to reach a plant's root zone so easily.

Dig holes in several areas where you'll be planting. Dig them 12 inches deep by 18 wide. Fill the test holes with water and time how long it takes the water to drain. The water should drain from the holes within minutes, not hours. On some jobs, I've seen water standing the following day.

To improve drainage, rototill a two- to three-inch layer of compost into the top 10 to 12 inches over as wide an area as possible. This preparation can make a tremendous difference in plant health. Composted yard waste can be used. Composted organic material also is available at a garden center or by looking under "Mulches" in the Yellow Pages.

* Retaining moisture. Incorporating compost also is the key to holding moisture in the soil. A good planting medium should have 20 percent organic content. Then it'll hold moisture and have enough air space so the roots get oxygen. Unless you have a forest floor that has been undisturbed for many years, add composted organic material.

In college when we were testing soils, we would find only 1 percent to 2 percent organic material, even from Maryland and Virginia farmland. Do a rough check of the soil's organic content. If you have children, maybe they can do it for a school project:

1. Dig soil from several parts of the area you want to prepare. Take it from a depth of one to three inches. Collect a total of 1 1/2 to two cups.

2. Clean the soil of plant pieces and stones.

3. Dump the clean soil into a quart jar of water.

4. Shake the mixture and let stand overnight.

The way the soil settles will provide a profile. It shows three levels: sand at the bottom, silt or loam in the middle and clay on top. The organic content will show as a dark layer floating in the water and covering the clay. Measure the dark layer on top and you'll probably find, as I did, that virtually no organic material exists in soil that hasn't been amended with compost.

You also will get a very general indication of the soil's texture. Measure the thickness of each layer. If the sand is the thickest but there is some clay, it's sandy clay. If the top layer is widest, it's clay. And if the middle level of silt equals the other layers, it's called loam. Sand won't hold moisture well. Clay holds it too well and the plant can't use it. Adding compost makes moisture available to the plants.

* Surface drainage. The most serious problem caused on the surface is water in the basement. In almost every situation, the problem is caused by the lack of surface-water runoff. Check the downspouts and gutters to see if water puddles there and inspect for low spots where water might flow. The problem can be hidden by gardens or shrubs planted close to the side of the house. Correcting it can be simple and inexpensive.

Before considering solutions such as a sump pump, wall excavation, waterproof paint or sophisticated underground drainage systems, look behind the plantings if you have a leaky basement. You will usually find that it is caused by a low spot at the wall.

Alleviating the problem is simple if there's a place to channel the water. I recommend a downhill slope dropping three to six inches per 10-foot run. If a grade slopes down and away from the house and continues slightly downhill to the edge of the property, the basement stays dry and the area remains well drained and usable for plantings or picnics.

If you also will be mulching the beds for moisture retention, as well as weed and erosion control, I prefer organic, partially decomposed mulches that condition the soil as they decay. Use compost or aged, double-shredded hardwood bark. But, do not consider the mulch as part of the soil against the wall of the house. It won't channel water away the way a mantle of soil will. Use soil with less organic material for surface drainage at the wall.

Piling mulch on too thickly can keep air, moisture and nutrients from reaching the soil. It should be no more than two inches thick. And, never pile it against the trunks of trees and shrubs.

On a final note, the worst thing you can do to a garden is walk or work on it when it's wet. The soil compacts and clumps up; you'll lose all the improvements that have been made. If this happens, wait until the soil isn't muck and cultivate it into the loose friable structure that will again allow the roots to get air and moisture. Plants in healthy soil still need water, but they hold onto it and use it for longer periods than their poorly sited counterparts.

Lerner is president of Environmental Design in Capitol View Park, Md. His e-mail address is lernscap@erols.com