Why do farmers burn their fields? Burning is one way to dispose of the straw left after harvest so fields can be made ready for seeding of the next crop.
Farmers and ranchers commonly use burning as a tool to invigorate vegetative production of pastures and rangeland. The practice of burning is not a new idea but started many generations ago with the burning of grasslands.
Burning is an inexpensive, labor efficient means of removing unwanted crop residues prior to tillage or seedbed preparation. If completed correctly, burning can be an effective management tool.
Farmers and ranchers burn stubble and excess vegetation for a variety of reasons, not just simply to remove straw stubble.
Burning cereal crop residues after harvest can somewhat reduce diseases where straw serves as a host to pathogens. Burning also results in changes in soil temperature, soil moisture, and nutrient availability.
Burning pastures and CRP grass/legume stands increases plant productivity by increasing the photosynthetic capability of plants. Burning grass pastures results in short-term increases in nitrogen mineralization, which results in a short burst of nutrient available for the plant.
Burning is also completed to control weeds and insects. For example, sagebrush can be nearly eradicated from rangelands when burned in the late fall when it is dry.
However, burning frequently has detrimental effects. Some of these effects are: (1) removal of the extra vegetative material that would add humus and nitrogen to the soil, (2) destruction of old vegetation in the soil which functions to increase water-holding capacity, and (3) injury to living vegetation, especially short grasses and shallow-rooted grasses like bluestem and the fescues, which may be killed by a single burning.
Recent research has shown that, although there are some short-term benefits to burning crop residues and grasslands, there are long-term detrimental effects to soil quality and overall cropland/grassland production.
It is important to understand how plants utilize nutrients from the soil and how burning affects available soil nutrients.
Long-term annual burning results in lower levels of soil organic matter and net Nitrogen (N) mineralization rates, but higher levels of plant productivity compared with no burning (Ojima 1987).
This indicates a change in N cycling and use. Microbial biomass is important to crop, pasture, and grass production in that a constant “pool” of microbes are needed to break down straw and stubble into nutrients that are useable by plants.
When there are very few microbes (a small pool), there is less total activity to break down straw and stubble and, thus, less nutrients available for the plant. This in turn relates to reduced production.
Fertilizer can be added to somewhat help increase the amount of available nutrients in these cases. However, research has shown that with the reduction of microbial biomass, more and more fertilizer is required to maintain production levels. Hence, the size and ratios of biologically active pools of Carbon (C) and Nitrogen (N) are important indicators of soil health and sustainable production.
Research by Ojima (1987), found that microbial biomass C and N were reduced by long-term annual burning, but were affected very little by short-term burning (1-2 years).
The study also showed that short-term burning created increased active N and N mineralization rates (increase in available N for the plant). However, long-term burning resulted in a decrease of soil organic matter and N mineralization rates. Research by Doran and Smith (1987), suggest that changes in nutrient cycling, and thus greater nutrient availability, are associated with changes in the organic matter “pools” in the soil.
Reduced -till systems receive inputs of straw, stubble, and residue from primary production, and the soil organic matter interacts with the primary production by providing nutrients. This relationship is regulated by factors such as moisture and temperature, and disturbances such as fire, tillage, or mowing, or grazing. Reduced tillage management systems such as no-till and minimum-till, that maintain large amounts of residues, increase the relative size of active pools of carbon and nitrogen which relates to greater production and better soil quality and overall soil health.
Research has shown that occasional burning of straw, stubble, and grass may provide the producer with an economical and effective management tool and in some cases increase small grain and grass production in the short-term.
However, the same research has shown that repeated, long-term burning of straw or grass (pastures) can have a more permanent negative effect on soil quality and overall soil health. Repeated burning can cause long-term reduction in yields. These long-term losses in yield cannot be offset by the addition of fertilizer.
Additionally, soils that are high in fertility may take several years to show the detrimental effects of burning.
However, research has furnished concrete evidence of the slow but sure consequences of repeated burning of grass or stubble to soil health.
Furthermore, what may look like a savings in fertilizer, pesticides for weed control, or insecticides for insect control, will eventually turn into increased long-term costs to maintain productivity due to continual loss of organic matter, organic nitrogen, organic carbon, and the size and quantity of microbial pools.