The Effects of Various Soil Amendments

The Effects of Various Soil Amendments on the C:N Ratio

Introduction:

In this lab three amendments were added to a sandy loam soil, and left for twelve weeks and compared to a control with no soil amendment in order to see the effects of the amendments on the C:N ratio. These amendments were a Legume (a cover crop or crop residue), Cereal-Legume (a cereal cover crop), and Straw that is a crop residue. Organic amendments are added to the soil to improve the overall health of the soil. If the appropriate amendments are added it can result in an increase in microbial activity and therefore increase the decomposition rate, which can benefit the plants. As decomposition occurs organic residues are transformed into inorganic and simplified organic products. During this process carbon dioxide, water, energy and nutrients are released which is crucial because plants use released carbon dioxide for photosynthesis in order to create more plant material for microbial decomposition. Additional benefits of adding organic matter to soils include; increase water holding capacities in soils that have difficulty retaining its water (i.e. sandy soils), increase in formation of soil aggregates thus improving soil structure. As soil structure improves so does the permeability and infiltration rate of water through the soil, and thus improving the soil's ability to uptake and hold water. Better soil permeability also benefits the soil by preventing erosion in addition to an increase in the cation exchange capacity. Lastly one of the most important benefits is increased nutrient supply, with the addition of organic matter to soil Nitrogen and other minerals can be greatly increased and promote soil performance. The importance of minerals is shown in the process called mineralization whereby elements are converted from an organic form to an inorganic form, mainly by microbial decomposition. One of the most important factors influencing microbial activity is the soils C:N ratio which is defined as the carbon-organic nitrogen ratio of the mass of organic carbon to the mass of organic nitrogen in the soil, organic material, or microbial cells. Organic material with a high carbon to nitrogen ratio (C:N) such as straw, C:N ratio approximately 80:1, will have a slow decay rate because the material has an inadequate amount of nitrogen for the microbes which require it for decomposition. Microbes use the plant available nitrogen until they have expired resulting in less nitrogen for the plants to use. This leads to nitrogen immobilization by microbes that can produce a nitrogen deficiency in the soil, which can limit plant growth. Cover crops that have a low C:N ratio such as a legume (about 13:1) decay very rapidly in the soil. In conclusion, immobilization is the product of a higher C:N ratio thus immobilization should be reduced if a cover crop with a low C:N ratio such as a legume