Capturing the Most Basic Nutrient: Sunlight

by Wyatt Fraas, Center for Rural Affairs

Most discussions of nutrients for crop or grazing lands begin and end with the mineral elements, like nitrogen, phosphorous, and potassium. Livestock discussions also include vitamins and amino acids. Water is another critical nutrient for growth, reproduction and maintenance. But sunlight is the most basic nutrient of all. Plants depend on sunlight to photosynthesize their food. Livestock depend on plants, or animals that eat plants, for nourishment.

At the most basic level, farmers and ranchers turn sunlight energy into products to use or sell. Plants convert sunlight into food and fiber for animals and people. Farmers can sell or eat the plants, or they can harvest them to feed livestock. Allan Savory of the Center for Holistic Resource Management describes this process in the above graphic.

Unfortunately, solar energy is lost at each of these steps. About 6% of the energy that hits a plant can be turned into more plant material. Only about 10% of the plant's energy is available as food for plant-eaters. The rest is lost to decay, or is used by the plant. Only about 10% of the energy in those plant eaters is available to predators. The rest is lost as heat, or through body maintenance and decay. Only about 10% of the energy in the predators is available to humans and other critters who feed on meat-eating animals like fish, poultry, and bears. This energy forfeited at each conversion is truly lost - neither we nor other living organisms can make use of it. Fortunately, this energy flow is constantly replenished by sunlight.

How do we improve our collection of solar energy? Farmers and ranchers control the time when plants convert sunlight to plant material. A mix of 'warm' and 'cool' season grasses in a pasture, for example, will convert sunlight throughout the growing season. Young leaves are slightly more efficient at photosynthesis than older leaves.

Producers decide how much land area is covered by sunlight-trapping plants. Crop planting density directly affects the number of plants in a field that are converting sunlight energy into useful forms. Cover crops make use of otherwise bare ground between rows, during fallow periods, or before and after crop growing seasons.

Farmers and ranchers also control the volume of plant leaves that trap sunlight. Leaf area varies with the type of plant, as well as the planting density. Tall plants with broad leaves will intercept more sunlight than short plants with narrow leaves.

Tallgrass prairie provides a natural example of how time, area and volume interact to capture a maximum of sunlight energy. A wide variety of plants, including grasses, legumes, forbs and trees, grow from the spring thaw through the hot dry summer, until the fall freeze. Some of these plants remain green throughout the winter.

On the native prairie, patches of soil are sometimes bare of green leaves. Annual plants use these spaces between perennial plants in the spring and fall, while a canopy of taller plants' leaves covers those spots in the summer. Not too long ago, bison and other grazers periodically fed on and then avoided parts of the prairie. Modern management-intensive graziers mimic this pattern to maintain nutritious, fast-growing pastures.

Technology can enhance the land's ability to capture sunlight. Fertilization can make plants healthier, bigger, or more numerous. Pest controls can keep plants healthy, and irrigation can extend the growing season into dry periods.

But technology costs money, which can only be recovered through increased plant production. When wealth is generated from on-farm resources, it can be measured in "solar dollars." Plant production supported by outside resources, such as petroleum energy, is subsidized by non-renewable "mineral dollars." Mineral dollars have brought many material gains, but our dependence on mineral resources has resulted in air and water pollution, wildlife losses, and social inequities, among other problems.

Allan Savory proposes that generating wealth from sunlight is the smartest choice we can make: "...we can generate income from human creativity, labor, and constant sources of energy such as geothermal heat, wind, tides, falling water, and most of all the sun....

"A characteristic of wealth derived from this combination is that it tends to not damage our life support system or to endanger mankind.... A further characteristic is that it is the only form of wealth that can actually feed people." Quotes from Holistic Resource Management, Allan Savory, 1988, Island Press

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