Building Your Soil

Climate-Conscious Gardening

Join Master Gardeners in the Arlington/Alexandria unit of Virginia Cooperative Extension in a series of monthly articles in 2021 as we explore the topic of climate change and practical actions individuals can take in their home landscapes in response. 

By Elaine Mills, Extension Master Gardener

In February, we discussed garden practices that could directly help to mitigate the effects of climate change. This month, we consider a set of adaptive strategies that gardeners can take to improve their soil.

According to Dr. Sara Via, Climate Extension Specialist at the University of Maryland, maintaining the health of our soil is the primary strategy for climate resilience. As the second largest ecosystem after the world’s oceans, soil not only provides physical support of our plants, but also is crucial in the cycling of water and nutrients. In addition, it provides habitat for a multitude of living things, including beneficial microbes that develop special relationships with plants.

The Soil Ecosystem

Conceptual diagram of a macroaggregate (from Jastrow and Miller)

In the soil ecosystem, plants share up to 40% of the sugar they produce from photosynthesis with various bacteria and fungi in exchange for access to water and nutrients, as well as protection from predators, diseases, and various environmental stresses. Nitrogen-fixing bacteria, for example, will convert critical nitrogen from the air, making it available to plants for proper root and top growth. Other free-living bacteria will coat plant roots, protecting them from diseases and drought stress. Ninety percent of plants will develop beneficial connections with fungi (referred to as mycorrhizae) that greatly increase root area, thus facilitating the uptake of water and nutrients, such as phosphorus. The branching filaments of these fungi also link plants—even those of different species—with chemical signaling in underground connections. Fungi that live in plant roots increase their tolerance to insect pests, disease, salt, and heat. Other soil fungi act as decomposers, breaking down dead organic material, and making it available to the soil biome.

The distribution of solids and pore space in an ideal soil.
North Carolina Extension Gardener Handbook – Soils and Plant Nutrients

The actions of plants, along with these microorganisms and animals, play a role in forming soil structure, which consists of mineral particles of various sizes referred to as sand, silt, and clay. The physical pressure of plant roots penetrating the soil helps to form aggregates of these particles. The plant’s root hairs increase soil cohesion by further binding soil particles together. The branching strands of fungi and sticky material produced by other microorganisms also chemically and physically bind these particles. Soil-dwelling animals, such as earthworms, mix soils and form burrows and air pores between the aggregates that allow water to either be held or to infiltrate. Ideal soils are half solids (minerals and organic matter) and 50% pore spaces filled with oxygen and water.

Restoring Healthy Soil

Good soil structure vs compacted soil
Building Soils for Better Crops

A challenge for many local homeowners is that their soil is compacted and degraded. The aggregates that would normally be surrounded by sizeable air pores have become flat and platy due to the effects of construction, foot traffic, and inappropriate tilling techniques. This can, in turn, limit the soil’s ability to absorb water, contributing to erosion when heavy rains simply run off into the sewer system. Other signs of compaction are pooling or puddling of water in low areas, bare spots where even weeds cannot grow, and difficulty in driving a trowel or shovel into the soil. Compaction also results in restricted plant growth and the shallow rooting of trees. Repetition of unsustainable practices leads to a downward spiral of poor soil health.

Spiral of poor soil health
“Gardening Sustainably in an Uncertain Climate”, David W. Wolfe, Cornell University

To build or restore healthy soil, it is best to mimic nature and to let the soil organisms resume their proper biological activity. This can be facilitated by adopting the recommended soil health principles of such organizations as the USDA’s Natural Resources Conservation Service and Cornell University’s Climate Smart Solutions Program.

Limit Disturbance

An initial best management practice is to limit disturbance of the soil. This means minimizing foot traffic and the use of equipment, especially when the soil is wet, as these reduce aeration and infiltration and crush the soil habitat. Such techniques as double-digging and rototilling are now discouraged as they break up the structure of soil aggregates, increasing erosion. These actions also result in the undesirable release of sequestered carbon dioxide into the atmosphere.

Deep root systems of prairie plants help build soil.
Based on illustration by Heidi Natura, 1995

A preferred method of breaking up compacted layers involves the use of plants with deep root systems. Native ornamental grasses incorporated into landscaping can reach as far as 10 or 12 feet. Tillage radishes utilized between rows in a vegetable garden can reduce compaction from foot traffic. A garden fork can be used to probe the soil but not to turn it over. The addition of organic matter will also improve soil structure, allowing it to hold water without poor drainage.

Limit Input

A second step in building soil is to limit the input of chemicals. As mentioned in the overview article in this series, the heavy use of fertilizers, fungicides, and herbicides comes with a high environmental cost. Their manufacture not only relies on fossil fuels as raw materials, but also results in the emission of greenhouses gases. In addition, their application adversely affects the soil biome.

Jim McGlone of the Virginia Department of Forestry points out that “you cannot feed plants. They are green and make their own food.” He says, therefore, that it shouldn’t be necessary to amend soil. In fact, the nitrogen salt in synthetic fertilizers can actually pollute ground water and dry out soil organisms. Too much added phosphorus inhibits the mycorrhizae, and excess nitrogen inhibits the nitrogen-fixers and leads to the emission of nitrous oxide.

Cutaway model of a domestic compost bin
Photo by Bruce McAdam

Instead, he advocates the use of compost to support the soil biome. It has the macronutrients (nitrogen, phosphorus, and potassium) that plants require, and as it breaks down, it will release nitrogen automatically. Integrating legumes, such as beans, peas, and clovers as cover crops in vegetable gardens can also minimize dependency on supplemental nitrogen. If the use of any supplemental fertilizer is considered, it is advisable to conduct a soil test for available nutrients and to use the product judiciously, following directions to avoid over-fertilizing.

To learn more about making your own compost, refer to the recipe in the  “The Climate-Friendly Gardener ” by the Union of Concerned Scientists and watch the video on “The Art of Composting: How to Make Your Own Black Gold” in our Master Gardener Virtual Classroom.

Test soil for nutrients and organic matter content.
Photo © Elaine Mills

Keeping Soil Covered

A third consideration in soil health is keeping the ground covered at all times. This will minimize temperature variation and protect soil aggregates and organic matter from erosion by wind and water. Raindrops can hit the soil surface at more than 20 mph in heavy storm events, creating gullies and carrying soil away in the runoff. The aforementioned cover crops can help in this regard in a vegetable garden, and ground covers used as “green mulch” can protect the ground in open areas, in mixed beds, and under shrub borders.

Retain fallen leaves to protect soil.
Photo © Elaine Mills

In wooded areas, it is an excellent idea to retain leaf litter. This “brown compost” will be broken down by fungi, making nutrients available to other soil organisms. Leaves that fall on lawn areas can be shredded and used as a protective mulch in adjoining ornamental beds. Another reason to “leave the leaves” is that a number of beneficial insects, such as the larval stages of butterflies, moths, and fireflies overwinter in the litter. When leaves are put out to the curb, we are disposing of both a generation of animal life and a valuable resource that can be recycled right on our own properties.

For further details on building healthy soil and strategies for addressing issues with the soil types of our region (clay of the Piedmont, or sand and marine clay of the Coastal Plain), watch the two-part video “Build Healthy Soil and Manage Water in Your Yard” in our Master Gardener Virtual Classroom.


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