So at this point, you have probably taken the time to research raised bed gardening and have found that it is the perfect solution for those with limited space or time, poor soil quality, or limited physical abilities. If you have planned out your garden space, purchased or built a garden bed gardening kit, now it is time to move on to the soil or dirt, and seed or plants. If you haven't thought much about the type of soil you are going to add to your beds, now is the time to think about one of the most important parts to your garden.
Raised bed gardening will only be successful if the plants you grow survive and thrive. And your plants will only be as nutritious as the soil in which you plant them. Not all soil is the same. In good soil, if the ground has been undisturbed (not tilled, dug, or moved), a thin layer of organic matter should cover the topsoil. Organic matter is a combination of plant and animal residue and leavings in various stages of decomposition. Organic matter is essential to good, quality soil, as it is full of nutrients that plants need, and aides in aeration and drainage of soils. So, if your soil is full of good nutrients, your food will be, and when you eat it, you will get the full benefit of those nutrients.
Under the organic soil layer is the topsoil. The topsoil holds the vast majority of nutrients, soil organisms, and roots. The deeper the topsoil, the better for plant life. The next layer, the subsoil, provides water storage and some nutrients. In raised bed gardening, the subsoil will most likely be missing, especially in beds that are greater than ten inches in depth. Due to this fact, and because the beds are above ground, they will tend to dry out more easily than traditional gardens. On the up-side, they will warm up more quickly in the spring, which is helpful in getting the seed and plants established for fall harvest.
When adding soil to raised garden beds, remember that the soil you add will likely be there for quite some time. Take care to purchase a good quality topsoil and then add generous quantities of organic matter, either on top of the topsoil, or mixed into the top layer. Organic matter is like fertilizer to the plants. However, unlike commercial fertilizers that release nutrients quickly and can cause burn, organic matter releases nutrients slowly. The best organic matter is aged compost due to its bacteria count and micronutrients. Organic compost is recommended because some commercial compost comes from municipal waste and may have high levels of heavy metals. Peat moss can be added, providing nutrients and fiber to soil. Worm castings keep the soil loose and add micronutrients.
For raised beds with deeper depths, say 24" or so, larger rocks in the base, followed by a layer of smaller rock, can be added to decrease the amount of topsoil needed. Rock will also aid in drainage of deeper beds. When purchasing a pre-made bed or kit, keep in mind that only good soil should be added, no rock, because they are usually only about ten inches deep. When purchasing topsoil, getting a soil test is highly recommended. The acidity and alkalinity of the soil has a profound affect on plants. Soil tests will show the pH or alkalinity/acidity of soil. Do-it-yourself kits can be purchased at hardware stores or by checking with a local or state college extension service. However, the best measure of soil quality is done by taking a soil sample into a testing service for analysis. The approximate, recommended ideal pH to maximize nutrient availability in garden soils is 6.3 to 6.8. Acidic soils can be treated with lime and soils that are too alkaline may be treated with sulfur. Be sure to check with your local extension service or master gardener for application rates and details.
Remember that great soil in raised bed gardening is absolutely essential for great vegetables and flowers. Taking the time to get your soil right is essential in preventing problems and giving you the nutritious, quality fruit and vegetables that you deserve.
