Growing plants in a raised bed garden is not that much different than growing plants directly in the ground. There are many advantages to a raised bed garden and it can be a great tool for hobby growers. One aspect of raised bed gardening that is crucial to understand is the bed’s soil. The soil of a raised bed garden is the moisture and nutrient source for the plants that will be grown there. Just like agriculturists need to replenish the soil or rotate crops in order to keep the soil healthy, a raised bed grower must maintain a high level of soil health. After building a raised bed garden, many gardeners have it filled with top soil or some sort of garden soil blend. The original soil will most likely contain enough nutrition for the first season or two and the grower probably won’t need to add much to the soil. However, after some of the nutrients have been depleted, it will be important for the soil in the raised bed garden to be revitalized. Some growers want to build their own organic soils with a healthy balance of nutrients and aeration. There are many different substances a gardener can add to his or her raised bed garden and each will affect the soil a little bit differently. A closer look at some of the common soil ingredients available will give gardeners a good idea of the purpose to each and how to better use these ingredients in their gardens.
Compost is a general term referring to aged (broken down) organic substances. Compost can be made from several different sources but most compost used in gardening is derived from plant material or manure. Compost is one of the best soil additives for a raised bed garden. Compost not only contains vital nutrition for the plants but also a wide variety of beneficial microorganisms. These microorganisms are the key to maintaining a healthy soil which, in turn, leads to healthy plants. If a gardener has raised beds and doesn’t already have a compost bin, he or she should seriously consider building or buying one. Many kitchen scraps along with yard clippings and debris can be composted and then used to revitalize a raised bed’s soil.
Another great way to revitalize a raised bed garden is to add worm castings. Worm castings are a good source of nutrition and, like compost, supply a good amount of beneficial microorganisms. Many gardeners consider worm castings to be one of the best, if not the best, soil additive available. Worm castings are sold by the percentage of casting so always be sure to purchase the highest percentage available. For example, a bag of “worm castings” may only contain 50% castings. This means that the rest of the product contains some sort of filler (typically peat); the higher the percentage of worm castings in the product, the better. Although not carried by every nursery or indoor gardening retailer, there are 100% casting products available. The only downside to top dressing worm castings onto a raised bed is that worm castings tend to clump and harden the top layer of soil. Mixing the castings into the soil instead of top dressing will make a world of difference when amending a soil with castings.
Coco coir is not a revitalizing soil amendment. However, coco coir has some amazing moisture properties that can be very beneficial when building a soil for a raised bed garden. If the soil within the raised bed garden is becoming hard and compact, one solution is to amend the soil with coco coir. Coco coir has the capability of holding a lot of air and moisture at the same time. In other words, coco coir works as a soil aerator and also has the ability to hold moisture for longer periods of time. A common mistake made by growers introducing coco coir into the soil composition is using too much. Too much coco coir in a soil dilutes available nutrients and can lead to problems with certain elements in the soil, such as potassium. A small percentage of coco coir in the mix, roughly 5%, is all a grower needs to gain all of its advantages in a raised bed garden.
Some hobby gardeners like to test the raised bed soil with a soil test kit to see exactly what the soil needs. A basic soil test kit is inexpensive and is a great way for growers to get a little hands on experience with soil science. Most basic test kits include a test for nitrogen (N), phosphorus (P), potassium (K), and potential hydrogen (pH). With the results of the soil test, a gardener can add specific individual ingredients to the soil in order to increase the amount of a particular element. After conducting a soil test, a grower can choose from various additives to determine which will work the best in his or her soil.
High Nitrogen Bat Guano is a fast-releasing nitrogen source. High nitrogen bat guano is not only packed with nitrogen but also adds micronutrients and beneficial microbes to the soil. Due to the multiple benefits of bat guano, many gardeners are making high nitrogen bat guano their choice for a fast-releasing nitrogen source over other ingredients on the market.
Blood Meal is one of the oldest fertilizers and is still commonly used today. This is because of its remarkably fast-releasing nitrogen. Blood meal can be a very “rich” additive and works great for plants that are heavy nitrogen feeders.
Fish Meal is another ancient fertilizer source and a good source of fast-releasing nitrogen. Fish meal has been shown to enhance the microbial life in the soil and many fish meal sources also contain a significant amount of phosphorus (which can be beneficial to early root growth).
Soybean Meal is a slow-releasing nitrogen source. Soybean meal is great for leafy greens or plants that require a continuous release of nitrogen.
Feather Meal is a very slow-releasing nitrogen source. Feather meal is best used for plants that may require high amounts of nitrogen over a three to four month period.
High Phosphorus Bat Guano is a fast-releasing source of phosphorus. Many types of high phosphorus bat guano also contain significant amounts of calcium.
Bone Meal is an excellent fast-releasing source of phosphorus. Bone meal is also used to help promote strong root development. Like high phosphorus bat guano, bone meal is a great source of calcium.
Fish Bone Meal is equivalent to bone meal except it is derived exclusively from fish bones. Fish bone meal is a great source of phosphorus and calcium.
Rock Phosphate is a slow-releasing phosphorus source that can come in either solid (powder) or liquid form. The powder form is slower to release and will give the soil a continuous supply of phosphorus. The powder form is best used on plants that need a slow and steady release of phosphorus.
Langbeinite is a naturally occurring mineral and is water soluble. Langbeinite is not only a good source of potassium but also sulfur and magnesium. Langbeinite is a fast-releasing form of potassium.
Kelp Meal is another source of readily available potassium. Kelp meal also contains a variety of micronutrients and plant hormones. Kelp meal is one of the oldest and most effective organic fertilizer sources.
Greensand is a very slow-releasing potassium source. Most gardeners use greensand for its ability to improve the condition of a compacted soil rather than a potassium booster.
Dolomite Lime is a great pH buffer that ensures a soil’s pH will not turn too acidic. Dolomite lime has a neutral pH so growers can use it generously and not worry about causing the soil to turn alkaline. Dolomite lime is also a good source of calcium and magnesium.
Oyster Shell is a great calcium source and also helps to balance a soil’s pH. Oyster shell will raise an acidic soil’s pH.
Sphagnum Peat can be added to soil which has a pH that is too high. Sphagnum peat will make a soil more acidic.
Many companies that offer individual organic soil additives also offer premixed combination packages. They may be labeled “all-purpose” or “fruiting and flowering”. These packages contain a mix of individual organic ingredients that serve a specific or general purpose. An “all-purpose” fertilizer will generally contain a balanced amount of N-P-K along with trace elements and micronutrients. A “fruiting or flowering” blend may have little to no nitrogen and the proper ratio of phosphorus and potassium to help promote fruiting or flowering. Growers who are revitalizing their raised bed gardens can use these combination packages to do so. In many cases, these combination packages offer a more complete nutrient package and can be purchased for less than the cost of buying each individual ingredient. In the long run, combination packages save the gardener time and money; two very valuable things to a hobby grower.
It is important to remember that each year may be different when revitalizing a raised bed garden. The particular growing season, the plants that were grown, and the previous year’s additives will all play roles in what needs to be done to keep a soil healthy. Whether the soil needs revitalizing or the soil for a raised bed is being built from scratch, a heightened understanding about some of the commonly used soil additives will help a gardener optimize growing conditions. Just as soil in a field needs to be revitalized and amended, the soil in a raised bed garden should be “built-up” for each year’s growing season. Hobbyist gardeners who take the time to maintain the health of the soil in their raised bed gardens will be rewarded in bountiful harvests year after year.
Eric Hopper resides in Michigan’s beautiful Upper Peninsula where he enjoys gardening and pursuing sustainability. He is a Garden & Greenhouse contributing editor and may be contacted at Ehop@GardenAndGreenhouse.net.