Bountiful organic gardens (grown without synthetic chemicals) start with nutrient-rich, organic soil. If your designated planting spot is blessed with loam—a moisture-retentive soil that contains a mixture of roughly 40 percent each sand and silt and 20 percent clay—you've got the foundation for a garden filled with vigorous plants.

But there's hope for other gardens, too. With a little effort and the right ingredients, you can turn sand, clay, or barren ground into rich soil that will sustain plants and create abundant harvests.

Soil Health is Key

If plants do poorly, there's a good chance it has something to do with the soil. Maybe it's lacking nutrients. (Find out with a simple soil test.) Or the soil could be compacted from being trampled by people, animals, or heavy equipment. Plants in compacted soil cannot develop good root systems, making plants more susceptible to drought, so the soil needs to be loosened and aerated.

That's why soil health is vital when growing an organic garden. Healthy soil should have good soil tilth (large pores for water movement and air infiltration), sufficient depth (deep enough to allow plants to grow to their genetic potential), proper levels of nutrients, good drainage, and large populations of beneficial organisms.

The most obvious clue that your garden soil is unhealthy is that plants grow poorly. If it's not clear which type of soil you have, send a sample to a state-certified soil-testing lab for help. But you can also investigate soil type yourself by examining its texture. Pick up a trowel's worth and put it in your hands. Does it feel gritty? Too much sand. Is it powdery? Too much silt. Is it harsh when dry and sticky when wet? Too much clay. The combination of these three types—and in which specific proportions—determines the texture of your garden soil. That texture affects drainage and controls the availability of nutrients.

You want organic garden soil that's dark, crumbly, and literally full of life. Fortunately, no matter what the texture may be, all soil can be improved over time by incorporating organic matter into it. Take sandy soils, for instance. They're made up of large soil particles, so water and nutrients run through gaps relatively quickly. Adding organic matter (typically compost) to sandy soil helps it retain both moisture and nutrients for plants to use.

Clay soils are just the opposite. They contain very small, densely packed particles that hold moisture but don't allow much air space for plant roots. Compost helps separate those tiny clay particles so water can drain more freely and plant roots can get needed oxygen.

Best Organic Garden Soil Amendments

For the quickest results when creating good organic garden soil, add compost, the decomposed remains of plant materials (similar to what Mother Nature deposits on forest floors). Compost keeps soil from baking and cracking during drought and encourages water to penetrate the ground rather than run off. Compost also introduces beneficial bacteria and other microscopic life forms that make soil more productive. Spread 3-4 inches of compost on a garden bed, then use a shovel or tiller to incorporate the rich organic material. You can buy compost in convenient bags or in bulk by the truckload. You can also make it yourself.

Add other forms of organic matter to the garden to feed earthworms, which in turn aerate and fertilize. Try coffee grounds, banana peels, eggshells, shredded leaves, grass clippings, wood ashes (in small amounts), and sawdust in your compost. Bagged amendments also enrich your soil, such as peat moss, composted cow manure, cotton seed hulls, or potting mix (it contains perlite, a lightweight potting additive that aids aeration and water retention).

If you have heavy clay soil, you may be tempted to add sand in addition to compost, thinking the former will improve drainage. But the sand can bond with the clay to form a virtual concrete. Using the all-natural mineral gypsum (aka calcium sulfate) is better. Gypsum makes fine clay particles lump into larger particles, creating air pockets that aid drainage and allow oxygen to reach plant roots. This mineral is usually pelletized to make it easier to apply to a garden surface with a lawn spreader or hand-held spreader. Water the gypsum into the garden soil with a lawn sprinkler or hose.

Use Compost to Improve Garden Soil

A soil profile shows how soil is layered. The thin top layer is dark brown to black and consists of decomposed and semi-decomposed organic matter. Below that is the topsoil, which is typically deep brown and is the layer of soil richest in organic material. Below that is the subsoil, a clay-like base with minerals but few nutrients.

Where to Buy Organic Garden Soil

Buy organic garden soil at Lowe's, Home Depot, Target, Walmart, Ace Hardware, your local nursery-and-garden centers, and online through sources such as GrowOrganic.com and PlanetNatural.com. When buying a commercial product (instead of mixing your own), look for soil marked OMRI Listed, which means the Organic Materials Review Institute has evaluated it.

Organic Soil for Vegetable Gardens

What's the best organic soil mix for vegetable gardens? It varies. Determine the soil pH before adding any amendments to your vegetable garden. (Either take soil samples to the local cooperative extension service for testing or use a commercial soil test kit.) Fall is best for testing. Most vegetables grow best in soil with a slightly acidic pH between 6.0 and 7.0 because most nutrients are available in that range.

Adding crushed limestone helps make acidic soil (below 7.0) more alkaline. Adding peat moss helps make alkaline soil (above 7.0) more acidic. Check with your state's Extension Service for more information on the best soil amendments for organic vegetable gardens.

If you don't want to amend the soil, buy bagged organic soil for vegetable gardens at Lowe's, Home Depot, and other retailers. For example, Kellogg Garden Organics All Natural Garden Soil for Flowers and Vegetables is designed to mix 50:50 with your yard soil. In addition, Nature's Care Organic Garden Soil (from Miracle-Gro) contains coir (coconut fiber), which also aids in moisture control.

Soil for Organic Raised-Bed Gardening

Raised garden beds offer many benefits to the organic gardener (as long as they aren't built with pressure-treated lumber or railroad ties that leach chemicals into the soil). With a raised bed, you can maintain near-perfect organic soil and planting conditions, kiss drainage issues goodbye, and tend to plants with less stress on your body. And you'll never have to worry about crushing your plants or compacting your perfect loamy soil by stepping on it.

So how should you fill a raised bed with the loose, rich, organic garden soil that plants love? The easiest solution is to buy bagged garden soil specifically mixed for raised beds, such as Nature's Care Organic Raised Bed Soil amended with alfalfa meal, bone meal, earthworm castings, and kelp meal.

Another option: Build the soil yourself, customizing the mix to suit whichever crop(s) you plan to grow. Some organic gardeners use 50 percent topsoil, 30 percent high-quality organic compost, and 20 percent organic materials, such as shredded leaves, mineralized soil (check with a landscaping supply company), worm castings, ground bark, wood-based fire ash, and completely composted cow or chicken manure. Mix all ingredients on a tarp before adding to the bed's box to prevent creating pockets of a single ingredient. Top dress the surface with 1 to 2 inches of compost, then a couple of inches of mulch. Amend your soil at least once a year with organic nutrients to make up for what the plants take out.

More Healthy Soil Practices

Planting Cover Crops Improves Poor Soil

If you plan to make wholesale changes to a large garden plot, your best bet is to sow a cover crop to revive soils with little organic matter. A cover crop is a quick-growing annual, such as buckwheat, rye, barley, or clover. It's also called green manure because these plants are turned over (or plowed back into the soil) when they're still green and immature; that's when they're highest in nitrogen. Keep in mind that cover crops are relatively low-maintenance, but they still need to be watered during times of drought.

Try this cover crop rotation to improve poor soil in a vegetable garden. Sow a thick crop of buckwheat (a warm-season cover crop) in the spring or summer, then turn it over when it reaches 8-10 inches tall. Wait two weeks and sow another crop of buckwheat, again plowing it under when it's 8-10 inches tall. Let the ground lay fallow (left unsown to increase fertility) until midfall, then finish by sowing winter rye. Plow that crop under in spring. Then wait two to three weeks before planting vegetables because decomposing plant matter ties up valuable nitrogen.

Double-Digging Loosens Hardpan

Double-digging is the labor-intensive practice of temporarily removing topsoil in order to loosen hardpan—a subsoil so compacted that it's nearly impenetrable by plant roots.

Start by excavating a row at the end of a garden bed. (If you have to remove sod, set it aside for now. Then flip it over, grass-side down, and place it on the bottom of the trench after you're done digging.) Dig about a foot deep, saving the excavated soil on a tarp. Loosen the newly revealed hardpan at the bottom of the hole by sticking a spading fork deeply into it and twisting it around. Don't turn the soil over. Add an inch of compost, then refill the trench with soil you remove from the next row.

Continue excavating row after row until you reach the end of the bed. Use the soil you placed on the tarp to fill the last trench. Level the soil, then top the bed with a couple of inches of compost. Once you double-dig a garden bed, don't walk on it, or you'll compact the soil all over again.