adapted from Missouri Prairie Journal
and published on the Internet
by the Kerr Center for Sustainable Agriculture
July/August 1997 Newsletter -- Vol. 23, No. 4.
adapted from Missouri Prairie Journal
Most cultivated soils of the Great Plains have been tilled for 50 to 100 years. During this time, prairie soils have undergone major changes. Degradation of prairie soil occurs through erosion, loss of organic matter, salinization, and acidification. Soil erosion after cultivation has two causes, wind and water. As a result, organic matter, primarily carbon (C) and nitrogen (N), declines affecting soil productivity. In undisturbed ecosystems, the amount of carbon entering and leaving the soil is in balance. Cultivation can reduce soil organic matter and organic N concentrations 25% to 60%, with corresponding decreases in nitrification rates and quantity of organic N nitrified. The type of cropping system also influences the rate of organic C and N loss.
Cultivation also changes the soil environment affecting the number and kinds of soil organisms. Clearing forested or grassland areas for cultivation drastically alters the soil environment. First, the quantity and quality of plant residues (food for soil organisms) is significantly reduced. Second, the number of species of higher plants is reduced. Monoculture or even common crop rotations provide a much narrower range of plant materials than forests and grasslands. Generally, agricultural practices such as monoculture and pesticide application also contribute to a general reduction of species diversity and total organism population. However, some agricultural practices positively effect soil microbiological activity. For example, the application of organic fertilizers such as manure or compost increases microbe activity.
The productivity and stability of soil as a medium for plant growth depends greatly on the balance between living and nonliving components. Energy from the sun and nutrients essential for growth are stored in crop plants and recycled through decomposition by micro-and macroorganisms in soil. The soil organic matter formed during this process serves as a continuous nutrient supply and a factor stabilizing the soil's physical environment. To maintain productivity, soluble nutrients removed from soil by plant growth must be replaced. In natural systems, the action of soil microbes and fauna are major determinants of efficient nutrient cycling and plant growth. Therefore, biological decomposition of plant residue is the largest source of nutrients.
Cultivation and organic amendments influence microbial activity in soil. A diverse population of microorganisms exists in the soil. For the major soil microbes, the C:N ratio is 5:1 for bacteria, 6:1 for actinomycetes, and 10:1 for fungi. For most organisms for every carbon molecule assimilated, two are "lost" as CO2. If the average C:N ratio for the total microbe population is 8:1, then soil with a C:N ratio of 24:1 would be ideal. At higher C:N ratios, nitrogen becomes limiting.
Jordan et al. (1995) studied microbial activity under different tillage and fertility practices. They measured soil microbial biomass carbon, direct counts of fungal and bacterial biomass, and soil enzymatic activity to determine microbial activity on two sites located in the same climatic region. One area was the uncultivated Tucker Prairie, and the other was Sanborn Field located on the University of Missouri campus at Columbia which has been under various cropping and management practices since 1888. Seven plots representing different long- and short-term cropping histories were investigated at Sanborn Field. Tucker Prairie is a virgin prairie site with native plant cover that includes big and little bluestem, prairie dropseed, and Indiangrass. Not surprisingly, all measured parameters on the undisturbed soil of Tucker Prairie exceeded the cultivated sites on Sanborn Field. Differences among the Sanborn Field plots were influenced by their cropping, fertility, and tillage history. No-till continuous corn under full-fertility treatment exhibited higher measures of microbial activity than conventional tillage continuous corn with or without full fertility. Continuous corn plots that received fertilizer had higher measures of microbial activity than nonfertilized continuous corn plots. Plots planted in timothy with or without manure had higher microbial carbon than other plots and were similar to levels on the Tucker Prairie site.
Prairie soils under cultivation were certainly different before being put into production. The very act of cultivation creates a drastic change in the soil environment causing shifts in microbial populations affecting the nutrient cycle. A lack of understanding of the prairie as a natural ecosystem and valuable resource when it was first put to the plow allowed for most of the prairie's destruction. We have only recently begun to understand the intricate relationships that formed over the eons during the development of these vast grasslands. A better understanding of prairie soils provides us with knowledge that we can apply towards preserving the prairie and sustaining land that was once prairie but is now in production. Such knowledge is also vital to the sustainability of our culture. As said last century by John James Ingalls, "grass is the forgiveness of nature. . .immortal. Its tenacious fibers hold the earth in its place." The prairie flower Indian paintbrush will warm our souls and lighten our spirits every spring with its bright and cheerful flowering.
-Jordan, D., R.J. Kremer, W.A. Bergfield, K.Y. Kim, and V.N. Cacnio. 1995. Evaluation of microbial methods as potential indicators of soil quality in historical agricultural fields. Biol. Fertil. Soil 19:297-302.