A school garden offers students the chance to observe a small patch of the natural world, its cycles and interdependent communities of organisms. Healthy soil, in particular, contains billions of organisms and is the foundation for a healthy and productive garden ecosystem. In this article, we review the building blocks of healthy soil and some of the various techniques that support its creation. Fortunately, many of these techniques are great activities for students and give them a deeper understanding of ecology and Earth’s natural processes. It is also important to note that regenerative, soil-building techniques are forgiving and flexible — you can use what you have available — and are usually free or low cost (e.g., straw, garden clippings, plant-based food waste). Using these techniques to build soil keeps plant waste out of landfills, sequesters carbon, and saves water.

BUILDING BLOCKS OF HEALTHY SOIL

Healthy soil consists of minerals, water, air and other gases, and organic matter. It also contains a vast community of organisms that break down nutrients in the soil so they can be taken up by plants. And they also break up the soil so that air, water, and plant roots can penetrate. These fungi, bacteria, and invertebrates are an essential component to rich and productive soil. In order to increase the amount of organic matter, a healthy diet for soil includes lots of plant materials. These can be added in a number of ways, including sheet mulching, lasagna (layer) gardening in a raised bed or in the ground, cover cropping and other green manures, hugelkultur, bioswales, straw bale gardening, and composting and vermicomposting.

Sheet Mulching. Sheet mulching is a soil-building technique that smothers unwanted plant material and breaks it down into organic matter and nutrients. Many gardeners use cardboard to cover a space with weeds (before they go to seed) or grass or any other plant material. They then add a layer of mulch such as manure or straw on top of the cardboard. Over time and with water, fungi, bacteria, and invertebrates like earthworms break the plant material and cardboard down into rich soil. This technique is particularly useful for mulching large areas, but it can also be used in a raised bed. This process does take time, however: six months to a year, depending on your planting goals. For more information, click here.

Lasagna Gardening. The term lasagna gardening refers to the method of building soil through the layering of materials. Rather than disturbing soil (tilling) and adding nutrients, lasagna gardening allows you to maintain soil structure while adding organic matter and nutrients over time. Sheet mulching is a form of lasagna gardening. You can also create a more intentional lasagna garden by creating a bed with alternating green (nitrogen) and brown (carbon) layers — from the bottom up — of food scraps, cardboard, manure, and straw. Add a thick layer of soil on top and plant. The soil level will sink down as the material decomposes. This system warms up quite a bit, so it is good to set it up in cold weather — not in the summer. If you have access to free bulk plant-based materials, this is a very cheap way to fill planters with growing medium. To learn more, click here.

Cover Crops and Green Manure. Cover crops are plants that improve soil fertility and structure by suppressing weeds and protecting soil from erosion. Green manure refers to the cover crops after they have been cut down and incorporated into the soil. These crops hold nutrients such as nitrogen and phosphorus and then release them into the soil once they are incorporated. Cover crops are commonly in the legume, mustard, and grass families. Legumes, in particular, add nitrogen to the soil while they are growing with help from a bacterium present in the soil (though some soil may need an infusion of this bacteria). Cover cropping and green manures are a great way to grow legumes such as fava or other beans. There are warm season and cold season crops to choose from. This soil-building technique is useful for fallow in-ground or raised beds or as part of your overall garden crop rotation. To learn more, click here.

Hugelkultur. Hugelkultur is a German word for “hill culture” or “mound cultivation.” The hugelkultur method of building a mounded bed over paved surfaces is one that works in the same way as a raised bed or a compost pile on a paved surface. It utilizes materials on hand that would otherwise go to landfill, sequesters carbon, builds biomass, and is another name for composting in place. Hugelkultur mounds, however, can be planted like a raised bed and add topography to an otherwise flat, paved surface. To learn more, click here.

The steps to building a “hugelkultur” mounded bed

• Start with a “moisture barrier” such as a black tarp or root barrier sheet to prevent water seepage and cracking. Then layer cardboard to absorb moisture and decompose over time. You can use wood pallets to raise the materials off the paved surface. Even with a barrier, it is important to note that paved surfaces will stain over time.

• Use wood logs as a border. Hardwoods such as alder, apple, aspen, birch, cottonwood, maple, oak, poplar, and willow work best as they last longer, but any wood on hand will do. The logs act as a sponge, holding water. They also decompose and add nutrients and structure to the soil over a long period of time. The larger the logs, the more water they absorb and the slower they decompose. Hay bales, pallets, or rocks can also serve as a border material.

• Add browns and greens. Within the mound or pile, add brown, carbon-rich material like dry leaves, twigs, branches, hay, or straw. Layer this with green, nitrogen-rich plant material like kitchen scraps, coffee grounds, cow or horse manure, lawn clippings, brewery hops, juice pulp, egg shells, and non-diseased tree or crop trimming.

• Add soil. Lastly, add 6 to 8 inches of soil on top of the mound and plant.

• Use the mound as a barrier and capture water. Place the mound toward the prevailing wind since all moisture, mist, dew, rain, and humidity will get absorbed and stored in the wooden logs and be released slowly for plant hydration and general decomposition. After the first year, hugel mounds require occasional additional watering.

Additional Techniques

Bioswales. Bioswales are a way of capturing and slowing stormwater runoff to sink or absorb it back into the ground. They are usually made of soil and shaped into elongated mounds or channels and planted with native plants. They also filter water and absorb contaminants when runoff is absorbed and filtered through plant roots. Bioswales and rain gardens can be used anywhere there is a need to divert or convey stormwater runoff and filter pollutants. Creating a bioswale around your garden area by cutting out and removing a border of asphalt and building it up into a bioswale may be a way to manage runoff on a paved schoolyard and add to the beauty of the garden. To learn more, click here.

Straw bale gardens. Bales of straw can be set on paved and unpaved surfaces alike and are an easy way to build a garden bed or border. If using straw bales as a garden bed, bales should be prepped with nitrogen-rich organic fertilizer and deep watering for two weeks before planting seedlings, seeds, and tubers directly in the straw. As plants grow, the straw decomposes and becomes compost for your plants. At the end of the season, straw and plant debris can be left to continue to compost and a fresh bale can be set on top for another planting. This type of gardening and soil-building technique is a great option for any garden but may be especially useful on paved surfaces when building more expensive raised beds is not possible. To learn more, click here.

Composting

One of the most basic ways to build soil health is to add compost to your beds and planting areas. Compost is organic matter that has been broken down by fungi, bacteria, and invertebrates and is full of microorganisms and nutrients. Though it looks like soil, you do not grow plants directly in compost as it will be too “hot” for growth. Plants need to grow in soil that has been amended by compost. Your school can create compost in a variety of ways, including with worms (vermicompost). It can be done on both paved and unpaved surfaces. Below are a few composting methods to consider.

A three-bin system consists of three open-air square bins built side by side in one row. They are often made of repurposed wood boards, cinder blocks, hay bales, pallets, or metal mesh squares. Generally, each bin is 3 feet by 3 feet by 3 feet, so that when one bin is stuffed you begin filling the next. They can be built on soil to get the decomposition going, or on a paved surface with a tarp or protective barrier to prevent seepage. This method of composting garden materials is the best way to create hot compost that breaks down rapidly and kills pathogens and weed seeds. 

There are also enclosed compost bins or tumblers, generally made of plastic or metal. These are great for pest prevention but can fill up quickly. Enclosed compost bins come in many shapes and sizes. Hugel mounds, as mentioned above, can be used for passive, cold decomposition over a long period of time depending on the amount of biomass in the mound. Underground systems, such as trench composting or lasagna gardening in a channel, can be helpful for dealing with large amounts of debris. 

Vermicomposting is a great way to compost food waste in the garden. Vermicomposting requires either a single layer/chamber bin or multiple layer/chamber bin. Red worms are best for these systems and can be purchased online, at a local garden center, or donated from a community member who already has a bin. Worms eat vegetarian food waste, such as vegetable and fruit scraps, as well as tea bags and coffee grounds. It is important to avoid meat and dairy products so as not to attract pests. In addition to food scraps, worms also need a bedding such as torn-up newspaper with plant-based inks. In a couple of weeks, your scraps will be transformed into “black gold” that can be scattered throughout your garden beds and along other plantings or can be used to make compost tea. Students love feeding the worms and this makes for a great “anytime activity” for when a lesson is finished. To learn more, read this book by Mary Appelhof or click here.

WHAT TECHNIQUE WHEN

If you have an existing in-ground planted area, use lasagna gardening, hugelkultur, bioswales, sheet mulching, or composting to build soil health, prevent plant material from going to landfill, conserve water, and sequester carbon.

If you have bare, depleted soil or a grassy area, use cover crops, sheet mulching, lasagna gardening, hugelkultur, bioswales, and compost. Each method will help improve the health of the soil and by extension what grows in it.

If you have paved surfaces, building a hugelkultur may be an attractive technique to create borders and vertical elements in the garden. Mobile enclosed compost bins and vermicompost bins work well in paved or small environments since they are relatively small and easy to manage.

KEY THINGS TO KEEP IN MIND

Remember to include soil building and composting in your planning. Consider what type of soil-building techniques may be right for your space and resources. If you choose a more active composting system such as a three-bin system or vermicompost, consider an appropriate location for students and volunteers. It is also important to create a system for summer maintenance or dormancy when school is not in session. Most grade levels can be involved in some way in the creation and maintenance of each of these techniques and systems. Their involvement as part of a lesson or as part of routine maintenance is important for building knowledge and know-how. Lastly, it is important to consider the source of your plant material, if coming from outside the garden. Free or low-cost materials should be checked for disease and allergens. Building healthy soil means growing healthy plants, which means growing healthy bodies. In the long term, an investment in soil health creates a thriving garden that supports a diverse set of organisms and habitats.