Protection of the Soil against Rapid Changes in Acidity

Protection of the Soil against Rapid Changes in Acidity Acids and bases are released as minerals dissolve and organisms go about their normal functions of decomposing organic materials or fixing nitrogen. Acids or bases are excreted by the roots of plants, and acids form in the soil from the use of nitrogen fertilizers. It is best for plants if the soil acidity status, referred to as pH, does not swing too wildly during the season. The pH scale is a way of expressing the amount of free hydrogen (H+) in the soil water. More acidic conditions, with greater amounts of hydrogen, are indicated by lower numbers. A soil at pH 4 is very acid. Its solution is ten times more acid than a soil at pH 5. A soil at pH 7 is neutral—there is just as much base in the water as there is acid. Most crops do best when the soil is slightly acid and the pH is around 6 to 7. Essential nutrients are more available to plants in this pH range than when soils are either more acidic or more basic. Soil organic matter is able to slow down or buffer, changes in pH by taking free hydrogen out of solution as acids are produced or by giving off hydrogen as bases are produced. (For discussion about management of acidic soils, see chapter 20.) Stimulation of Root Development Microorganisms in soils produce numerous substances that stimulate plant growth. Humus itself has a directly beneficial effect on plants (figure 2.7). The reason for this stimulation has been found mainly to be due to making micronutrients more available to plants—causing roots to grow longer and have more branches, resulting in larger and healthier plants. In addition, many soil microorganisms produce a variety of root-stimulating substances that behave as plant hormones. Darkening of the Soil Organic matter tends to darken soils. You can easily see this in coarse-textured sandy soils containing light-colored minerals. Under well-drained conditions, a darker soil surface allows the soil to warm up a little faster in the spring. This provides a slight advantage for seed germination and the early stages of seedling development, which is often beneficial in cold regions. Protection against Harmful Chemicals Some naturally occurring chemicals in soils can harm plants. For example, aluminum is an important part of many soil minerals and, as such, poses no threat to plants. As soils become more acidic, especially at pH levels below 5.5, aluminum becomes soluble. Some soluble forms of aluminum, if present in the soil solution, are toxic to plant roots. However, in the presence of significant quantities of soil organic matter, the aluminum is bound tightly and will not do as much damage. Organic matter is the single most important soil property that reduces pesticide leaching. It holds tightly on to a number of pesticides. This prevents or reduces the leaching of these chemicals into groundwater and allows time for detoxification by microbes. Microorganisms can change the chemical structure of some pesticides, industrial oils, many petroleum products (gas and oils), and other potentially toxic chemicals, rendering them harmless. ORGANIC MATTER AND NATURAL CYCLES The Carbon Cycle Soil organic matter plays a significant part in a number of global cycles. People have become more interested in the carbon cycle because the buildup of carbon dioxide in the atmosphere is thought to cause global warming. Carbon dioxide is also released to the atmosphere when fuels, such as gas, oil, and wood, are burned. A simple version of the natural carbon cycle, showing the role of soil organic matter, is given in figure 2.8. Carbon dioxide is removed from the atmosphere by plants and used to make all the organic molecules necessary for life. Sunlight provides plants with the energy they need to carry out this process. Plants, as well as the animals feeding on plants, release carbon dioxide back into the atmosphere as they use organic molecules for energy. The largest amount of carbon present on the land is not in the living plants but in soil organic matter. 

That is rarely mentioned in discussions of the carbon cycle. More carbon is stored in soils than in all plants, all animals, and the atmosphere combined. Soil organic matter contains an estimated four times as much carbon as living plants. In fact, carbon stored in all the world’s soils is over three times the amount in the atmosphere. As soil organic matter is depleted, it becomes a source of carbon dioxide for the atmosphere. Also, when forests are cleared and burned, a large amount of carbon dioxide is released. A secondary, often larger, flush of carbon dioxide is emitted from soil from the rapid depletion of soil organic matter following the conversion of forests to agricultural practices. There is as much carbon in six inches of soil with 1% organic matter as there is in the atmosphere above a field. If organic matter decreases from 3% to 2%, the amount of carbon dioxide in the atmosphere could double. (Of course, wind and diffusion move the carbon dioxide to other parts of the globe.)