Organic gardening, which involves avoiding the use of synthetic fertilizers and chemicals, requires a long-term outlook with respect to soil preparation. In fact, the key to successful organic gardening is to feed the soil with organic matter, which feeds the plant, rather than to feed the plant with inorganic fertilizer as in conventional production. 

Most vegetables grow best in a fertile, well-drained but moist soil that is rich with organic matter (humus/compost). Constantly soggy or wet soil is not tolerated. 

Few gardeners are lucky enough to have ideal soil for growing vegetables. Overly sandy soil might not hold enough water and nutrients. Heavy clay soils are often too compact and lack good drainage. But, in most cases and with a little work, you can easily turn the average to poor soil you have into a good vegetable growing medium by amending the soil. 

Soil pH
Most vegetable plants grow best in a mildly acid to neutral soil that ranges from 6.0 to 7.0 on the pH scale. Many average garden soils fall between a pH range of 6.0 to 7.0, but it's still a good idea to test the soil. 

Testing Soil pH  Soil pH is a measurement of the alkalinity or acidity of soil and is measured on a scale of 1-14, with 7 as the neutral mark. Any measurement below 7 indicates acid soil conditions, and anything above 7 indicates alkaline. If you're unsure about the pH of your soil, or whether or not it is suitable for growing vegetables, it's a good idea to test the pH in the planting area. You can quickly test soil pH with an inexpensive soil pH tester probe. To raise the pH (make more alkaline) you can add pelletized limestone to the soil. To lower the pH (make more acid) you can apply Soil Sulfur, Aluminum Sulfate, or Chelated Iron. Adding organic compost to the soil or using compost as mulch can also help to increase acidity and maintain acid soil conditions.

Because it takes a long-term outlook to build a good soil, don't be disappointed if your results are less than ideal the first year or two. 

Site Preparation

Since you won't be using any chemicals in a garden, new garden sites should have all plant matter removed or turned under. Bermuda or other sod grasses, or other invasive plants, should be removed and discarded or added to the compost pile (if no synthetic fertilizers or chemicals were used). Soil should be turned under to expose roots and rhizomes to desiccation. In addition, soil solarization (discussed further below) can help control these hard-to-control grasses and weeds.

Soil Preparation

If you're lucky, you've got good soil that can be turned or tilled to the proper depth. But some soils may have hardpans, which are impervious layers several inches under the soil. These hardpans are often found on old farmland or new home sites where equipment has compacted the soil. In either case, these hardpans must be broken up. On clay soils this can be very difficult. If the soil cannot be broken up, consider making a "raised bed" garden. More on that below. 

Conventional Garden Soil Preparation

For the conventional vegetable garden, soils should be turned a total of 12 inches deep, or deeper. Tractor-mounted plows or a rototiller set to the deepest depth can be used to till the soil. Another method is to double dig the garden. This method is labor intensive but not too bad when you're doing a small garden. 

Tip:  Because most vegetable plants like a well-drained soil rich in organic matter, organic matter such as compost should be added during this deep-turning process. The amount of organic matter to add varies with the chosen material, the type of native soil that will be used as a base, and weather conditions. For most gardens, adding and mixing in 8 cubic feet of organic matter per 100 square feet of garden bed is recommended. If you do not make your own compost, most nursery and garden centers have compost available in bags or in bulk. 

Step 1
To double dig, start at one end of your garden using a round point shovel to dig a trench 6 to 8 inches deep and 12 or so inches wide. Place the soil on the opposite end outside the garden, as shown in the diagram above. Then use a spade or garden fork to loosen the soil 6 or more inches deep at the bottom of the trench. 

Step 2
Soil adjacent to the trench on the inside edge of the trench is moved to fill the existing trench, creating a new trench in its place. Again with a spade or garden fork, loosen soil in the bottom of this trench to a 6-inch depth. Continue in this fashion until the entire garden has been double dug. 

Step 3
The soil from the first trench can then be moved into the last trench. This method of garden preparation will leave a deep turned soil but is very labor intensive. 

Raised Bed Garden Soil Preparation

What is a "raised bed" garden? "Raised" means that the garden soil level is higher than the surrounding soil. "Bed" implies a size small enough to work without actually stepping on the bed. Raised bed gardens are perfect where space is limited and are a good idea for sites with clay soil or areas with poor drainage.. A bed should be no wider than 4 feet but can be as long as desired. The bed does not have to be enclosed or framed, however framing with lumber, blocks or other materials offers other opportunities.  

Raised beds yield up to four times more than the same amount of space planted in rows. That’s due not only to their loose, fertile soil but also to efficient spacing of plants - by using less space for paths, you have more room to grow plants. 

As with conventional gardening, organic matter in soil (such as mushroom compost, composted cow manure or homemade compost) is important in raised bed gardening for two reasons.

  1. As organic matter breaks down, it releases nutrients that crops can utilize.

  2. Organic matter improves the water- and nutrient-holding capacity of the soil.

How much organic matter?

In a framed raise bed, I usually use a 50/50 ratio of organic compost, such as composted manures, mushroom compost, or your own homemade organic compost, thoroughly mixed with native soil. Native soil is the soil in your yard. If the raised bed is a "mounded' bed without frames, I usually add 10 to 15 cubic feet of organic compost to native soil.  

Note: If you want to go all organic, make sure to use native soil from an area that has not been exposed to synthetic fertilizers or chemicals.