by Kimi Ceridon
Soil, not to be confused with dirt, is the life blood of a garden. Dirt is dead and lifeless, but within soil, it is a complex, living ecosystem that keeps plants healthy. Nutrition for vegetables comes from the soil. Having healthy soil is important to an edible garden, but, if you are container gardening, it is difficult to maintain a healthy, thriving ecosystem from year to year.
The roots of plants absorb food, water, nutrients and minerals from soil. They spread into the soil in search of nutrition, forming a plant’s communication web and supply network. The plant above ground tells the root system what it needs and the roots below ground absorb it from the soil. All of the details of soil composition are far too complex to detail here, but there are important elements gardeners should understand.
Organic matter is the most important component in soil. Unfortunately, plants cannot digest organic material like food scraps and leaves without help from bacterial and fungal microorganisms. These organisms decompose or compost simple organic matter into readily available plant nutrition. They also aerate the soil, allowing roots to expand and water and oxygen to penetrate down to the roots. Together, organic matter and microorganisms are the heart of the nutrition in soil, providing not only the nitrogen, phosphorous, and potassium critical to plant health, but also other trace elements such as magnesium, calcium and iron.
Fertilizers are useful during the growing season and provide basic nutritional needs to a plant, but are not a complete replacement for healthy soil. Soluble nitrogen, phosphorous, and potassium (the numbers on fertilizer bags indicating the percentage available for plant absorption) are only the foundation of plant nutrition. Vitamins and minerals are an important component of soil composition and contribute to the nutrition of fruits and vegetables.
A container garden has little access to the outside world. As such, it is impossible to maintain container garden soil health indefinitely without adding nutrition. During the growing season, side dressings of compost and fertilizers help supplement plant nutrition. However, when starting fresh plants and seedlings in the spring, it is a great opportunity to replenish soil.
With new containers, it is easy to get plants off to a good start. Potting soil is available by the bag at garden stores, but rather than simply using a potting soil alone, mix one part compost to every four parts soil. Soil and compost are less expensive in bulk and indoor gardeners can save money by sharing the cost with neighbors and friends. It may also be less expensive to make a soil mix from scratch, such as the one recommended in All New Square Foot Gardening by Mel Bartholomew. For best results, an edible plant garden should get organic soil. Annually refresh containers full of soil from previous years by removing the old soil and blending it with compost. For containers with living plants, top off with a compost-soil mix or gently remove the plant from the container, loosen the roots and add a compost-soil mix to the bottom.
Once the soil is ready to go into containers, it can provide plants with a healthy base of nutrition that smells fresh and earthy. Do not overfill containers or pack the soil as roots need space to expand and support larger, bushier plants. Seedlings should have their first two real leaves before transplanting. Place filled containers in a sunny spot and keep the soil slightly moist.
As a complex ecosystem, understanding the many benefits of healthy, thriving soils is not only for gardeners and farmers. Soil nutrition is relevant for all types of food professionals. To learn about the wonderful ecosystem of soil, I recommend the following two books by Jeff Lowenfels:
- Teaming with Microbes: The Organic Gardener’s Guide to the Soil Food Web
- Teaming with Nutrients: The Organic Gardener’s Guide to Optimizing Nutrition
After 15 years in sustainable product design, Kimi Ceridon shifted focus from consumer products to food systems. She is active with NOFA/Mass Boston Ferments, Waltham Fields Community Farms and has led workshops on chicken keeping, backyard homesteading, fermentation, and brewing. Follow her at noreturnticket.kceridon.com.