Most people figure that gardening means putting plants in dirt and watering them faithfully. And that works, to some extent. But for a garden to really thrive, you need to put the plants in a good growing medium.

Not all soil is good. Sometimes it has too much sand or too much clay. Sometimes it's too acidic or lacking in nutrients. Regular watering helps, but plants need a medium they can really sink their roots into.

Fortunately, there's a huge variety of natural materials you can add to your soil to improve it. These materials, called soil amendments, include things such as grass clippings, sand or the droppings of a variety of creatures. Whatever you use, it should be mixed thoroughly into the soil to have the greatest value for the plants. If you leave it on top of the dirt, it's mulch.

First, find out what kind of soil you have. Soil-testing kits are available at garden centers, or you may contact the nearest office of the Cooperative Extension Service for help. (To find yours, go to www.csrees.usda.gov and click on "Local Extension Offices.")

Once you know what is in your soil, you can figure out what to add to make it better. The most common nutrients are generally referred to as N (nitrogen), P (phosphorus) and K (potassium). Other additions can help retain moisture, improve texture, give the roots more oxygen and improve drainage. Here are some common, and some not-so-common, soil enhancers and a bit about what they do.

Organic Additions

* Compost. Compost is what most people think of when they think of soil amendments. In recent years, a whole industry has sprung up to make composting easier.

The idea is that you pile up non-meat, uncooked kitchen scraps and yard waste. Over time, the pile breaks down into a rich, organic soil. (There are some new products called "compost accelerators" on the market to speed up the process.)

You may want a place to hide the unattractive heap while it does its work. Be careful what you add to the pile, making sure that garden or yard material is disease- and pest-free.

You can also make a simple compost of leaf mold. Corral material in a wire bin or plastic bags with a few holes poked in them, wet it down and leave it alone for a couple of years. This works as a soil amendment and as a top-dressing.

An even simpler compost is to keep grass mowed regularly and leave the clippings on the lawn. They will immediately begin adding organic matter to the soil.

* Peat moss, coir and pine bark. Peat is actually sphagnum moss from peat beds, generally in Canada. This material -- derived anaerobically, or without oxygen -- adds no organic material to the soil in the form of nutrients. It is used primarily to lighten soil and help retain water, but it is somewhat acidic, so it should not be used for plants that require alkaline soils -- flower beds and certain shrubs. One of the most common outdoor uses is rose gardens.

Coir, a byproduct of the coconut fiber industry, helps soil retain air and water without adding nutrients. So does finely ground pine bark.

* Improved soil. Some processes create a naturally rich soil. For instance, if you live in an area where mushrooms are grown -- southeastern Pennsylvania is a prime spot -- you can find "mushroom manure." Mushrooms are grown on a mixture of horse manure, straw, chipped corncobs and possibly other organic items that have been steam-pasteurized. After one crop of mushrooms, the growing medium is discarded or sold to garden centers. It's especially good for flower beds, and for beds that need "sweetening" with a slightly alkaline additive. It's not good for plants that like acidic soil, such as azaleas and rhododendrons.

* Worm castings. Created largely by worms called red wigglers, the castings are the result of worms eating their way through organic material (mostly food waste) and leaving behind a material that is rich in good microorganisms and high in nitrogen, phosphorus and potassium. The castings also appear to offer some protection against aphids, mealy bugs and scale.

* Animal manures. The excrement of domestic and agricultural herbivores such as cows, horses and pigs has been used for eons to improve soil quality, largely by farmers. In the home garden, it can improve soil for vegetables, annuals and perennials. The most common form is "well-rotted" cow manure, because there are a lot of cows and the material is readily available. It is low in nitrogen, and therefore less likely to "burn" plants, but it may have high levels of salt.

If you live in an area with a lot of horses, you may find horse manure, which is similar to cow manure but may contain seeds and other undigested bits, because horses are less efficient digesters than cows. Horse manure should be composted longer that cow manure.

In places with lots of chickens, you might find chicken guano. It's higher in nutrients than cow manure but can also burn plants, so it should be used sparingly. Some people swear by bat guano as a soil amendment; it has a higher concentration of nitrogen than other manures, so it should also be used sparingly.

In the past few years, a new source of herbivore manure has arisen from zoos. Marketed under names like ZooPoo and ZooDoo, this soil amendment can come from giraffes, zebras and gazelles, but usually elephants are the largest contributors. Exotic animal manure compares favorably with cow manure, and it's a clever recycling idea that is practiced by zoos around the world.

* Organic shellfish products. For those who like to cultivate a little local culture with their garden, there is Chesapeake Blue Organic Soil Enhancer. This nifty product combines crab discard (chum) with wood chips to provide the excellent qualities of fish as fertilizer without the odor usually associated with fish products. It works well in acidic or sandy soil and can be used for bulbs, flowers, vegetables, ornamental trees and shrubs, and in containers. It is said to prevent blossom end rot in tomatoes. Or you can try Neptune's Harvest Crab Shell, high in calcium and magnesium and especially good for bulbs. One online source for both is homeharvest.com/soilamendmentsseafood.htm.

Inorganic Additions

Inorganic soil amendments used to improve soil texture aren't usually added as nutrients. Minerals such as rock phosphate and lime are used in small quantities. Other minerals are used to improve soil texture, aeration and water-retention ability.

* Grits. Chicken grit (finely ground granite) and coarse sand will lighten heavy soil. Other inert elements such as perlite (volcanic material) and vermiculite (mica) help soil retain air and water.

* Soilless mixes. These prepared mediums can be used to amend garden soil or to substitute for soil in circumstances where it could become compacted or waterlogged, as in container gardening. They are usually some combination of sphagnum moss, coir, perlite, vermiculite, ground clay, ground pine bark, pelletized lime, rice hull ash and rock wool. Some mixes have fertilizer and wetting agents added.

Joel M. Lerner is president of Environmental Design in Capitol View Park, Md. E-mail or contact him through his Web site, www.gardenlerner.com

Whether kitchen scraps or grass clipping from the lawn heaped in a pile in a corner of the yard, the result is generally the same: a rich, organic soil. Just make sure the garden or yard material is free of disease and pests.