Green Planet: How Plants Keep the Earth Alive

Chapter Seven: Plants Create Soil

[127] chapter seven plants create soil The nation that destroys its soil destroys itself. —franklin delano roosevelt During the Dust Bowl of the late 1920s and 1930s, millions of tons of topsoil from the western great plains of the United States blew for thousands of miles in the air, leaving a withered and almost sterile substrate that would grow very little food or forage. In many midwestern towns, midday became as black as night, and some people even died from the inhalation of dust.This was at the end of a long drought, but the dust storms were not completely the fault of the drought. They were also the result of overgrazing and excessive plowing, which left the soil that could otherwise have survived a drought vulnerable to the relentless wind. In April 1935, during the height of the Dust Bowl, soil scientist Hugh Bennett met with the U.S. Senate Public Lands Committee. He wanted to convince them to create a government agency to combat soil erosion. When the clouds of dust blew over Washington D.C., he invited the senators to look out the window. They saw the nation’s farmlands, upon which both the present and the future of the country depended, blowing out to sea.The Senate established the Soil Conservation Service (now the Natural Resources Conservation Service) of the U.S. Department of Agriculture on April 27, 1935. This agency helps farmers to incorporate new techniques that will reduce the loss of topsoil to the forces of wind and rain. We all know that plants need soil. The purpose of this chapter is to convince you that soil needs plants. Soil is not dirt. Soil contains dirt, but it is a world teeming with organisms, on which the aboveground CH007.qxd 11/12/08 10:51 AM Page 127 world utterly depends. Plants grow out of the soil and feed on it. However, it was also plants that created the soil, and that continue to feed it. There was a time when the earth had no soil. Before plants began to live on land about 400 million years ago, the continents had plenty of dirt, but no soil.There was sand, there was silt, there was clay; but there was no humus, therefore there was nothing for soil organisms to eat. It was dead.Then terrestrial plants evolved, grew their roots into the dirt, and transformed it into the living material called soil. Plants can create soil in a devastated landscape. We desperately need for them to render us this service. A Tour of the Soil Soil is permeated by billions of tiny passageways that allow oxygen from the atmosphere to reach the plant roots and the soil organisms that need it. Carbon dioxide from the respiration of roots and soil organisms diffuses out of these galleries into the atmosphere. If you were extremely tiny, and standing on the surface of the soil, you could walk right into the soil through these tunnels. If you did so, you would see both inorganic and organic components (fig. 7.1). Inorganic Components After entering the soil, you would see tiny fragments of rock.The largest fragments are called sand, and the fragments smaller than sand are called silt. The smallest particles are clay. Your journey would be uncomfortable , because these clay particles would be sharp, like billions of tiny blades. Clay particles are not simply smaller than silt and sand; they have been chemically transformed by the weathering effects of water and oxygen . As a result of this, they have a negative electrical charge on their surfaces.The negative charges attract positively charged atoms and molecules . Many of these atoms and molecules are inorganic nutrients that plants require for their metabolism and growth. Potassium allows plants green planet [128] CH007.qxd 11/12/08 10:51 AM Page 128 to maintain the water balance of their cells, just as your body requires sodium. Calcium glues plant cells together. Magnesium is a component of chlorophyll. Ammonium is a form of nitrogen from which plants manufacture their proteins. These atoms and molecules are dissolved in the water that is in the soil. If you continued your walk into the soil, however, you would notice that the water clings to the soil particles, especially to the clay. There must be a force that is holding the water onto the soil particles, counteracting the force of gravity. This force...