By Trent Roberts, Soil Fertility Specialist and Jarrod Hardke, Rice Extension Agronomist
Benefits of Residue Burning
Crop residue burning is an inexpensive and effective method to remove excessive residue to facilitate timely planting and to control pests and weeds. Mechanical residue management, such as disking and tilling, to achieve soil incorporation of residues is another common practice. However, mechanical management requires significant fuel and labor costs as several additional equipment passes are required to be made across the field. Winter flooding is also used to manage crop residues, almost exclusively in rice fields. This practice is effective but also requires additional equipment passes to roll or press the residue to the soil surface, and irrigation costs can be substantial to maintain adequate flood depth for decomposition.
For such a common and accepted practice, the benefits of residue burning are relatively few. The majority of producers who observe a crop rotation such as rice followed by soybeans typically gain minor cost savings but perhaps gain a great deal of time savings as fields can be burned much faster than they can be mechanically managed. Where residue can be burned and more land can be prepared sooner, the earlier the following crop can be planted the following year. For many Arkansas commodities, it has been documented that earlier planting dates lead to increased crop yields – particularly for corn and rice.
Those in areas where crop rotations are more difficult and continuous rice production is more common have a sound basis for burning crop residues – rice residue can be allelopathic to rice. Therefore, removal of the residue provides a direct benefit to establishing a viable crop the following season. In addition, in many of these areas the land has been expensively formed to little or no slope – in these situations any mechanical land management risks increasing erosion and affecting the soil preparation. Where the residue is burned but the remaining root stubble and unburned residue is left undisturbed, erosion is still managed so long as the soil surface is left intact throughout the winter. In addition, burning may provide a reduction in residue borne pathogens.
Drawbacks of Residue Burning
There are several negative effects of residue burning including 1) loss of plant essential nutrients from the field, 2) loss of organic carbon (C) which leads to decreases in soil organic matter, and 3) loss of plant residues on the soil surface to protect against soil erosion. The effects of residue burning can include both short-term and long-term cumulative effects which can result in a net negative effect over time. In addition to the net negative effects of residue burning the loss of nutrients due to residue burning can be directly quantified and a monetary loss calculated based on current nutrient values.
1. Loss of plant essential nutrients from the field: The influence of crop residue burning is different for each plant essential element, but they can generally be combined based on the relative effects. The nutrients most affected by residue burning are nitrogen (N) and (S) which can result in as much as 100% of the N and S contained in the crop residue being completely lost from the field as a gaseous form. Other plant essential elements such as phosphorus (P), potassium (K), and zinc (Zn) are not lost to the atmosphere, but will become concentrated in the ash and can be lost via wind or in runoff during or after burning. As much as 35-45% of the P, K, and Zn contained in the crop residue can be lost from the field via particulate matter losses (ash). Long-term cumulative effects of residue burning can lead to a net loss of nutrients from the field over time which can result in a decrease in soil test levels and the need to replace these nutrients to ensure proper plant growth and maximum yield potential.
Table 1. Nutrient content of crop residues commonly produced in Arkansas.
Table 2. Monetary value of nutrients contained in residues of common Arkansas† crops.
A high yielding rice or corn crop can easily produce 5 to 6 tons of “straw or stubble” (dry weight) per acre; however, not all of the residue will be completely burned and nutrients lost. For example, a field that produces 200 bu/acre of corn or rice with an assumed harvest index of 0.5 has estimated residue yields of 9,000 lb rice straw (4.5 tons) and 11,200 lb corn stover (5.6 tons) per acre. The potential nutrient losses via residue burning from a well-burned 200 bu/acre rice or corn crop is $25-80/acre depending on the completeness of the burn. A field that produces 80 bu/acre of wheat with an assumed harvest index of 0.5 has an estimated residue yield of 4,500 lb wheat straw (2.25 tons) per acre. The potential nutrient losses via residue burning from a well-burned 80 bu/acre wheat crop is $10-30/acre depending on the completeness of the burn.
As evidenced in the examples above, nutrient loss via residue burning can be significant and the cumulative effects over time can result in a significant cost to the producer to replace the nutrients lost via residue burning.
2. Soil organic matter is an important component of soils and has a direct influence on nutrient holding capacity, soil structure, and water holding capacity. Carbon (C) is the primary component of soil organic matter and the return of crop residues to the soil is the primary source of C and soil organic matter in agricultural production systems. When crop residues are burned, as much as 90-100% of the C contained in the residue is lost to the atmosphere. Burning of crop residues will eventually reduce soil organic matter content and the soil’s ability to supply N and S to future crops, hold plant available water, improve soil structure, and store or retain nutrients like P and K. These factors are all important to long-term soil health and productivity, but are difficult to place a price tag on. Increasing soil organic matter through the return of crop residues to the soil rather than burning can result in more infiltration of water and an associated increase in water use efficiency. It is estimated that for every 1% increase in soil organic matter the soil is able to hold an additional 20,000 gallons of water per 6 inch soil depth. Increases in soil organic matter will also improve soil structure and reduce crusting leading to better plant emergence and reducing the need for replants.
3. The primary impacts to surface water quality in Arkansas are sedimentation and turbidity, which are the direct result of erosional soil losses. When crop residues are allowed to remain on the soil surface they provide a barrier of protection from rainfall which results in a decrease in erosion rates and soil loss from agricultural fields. Burning of crop residues results in a majority of the soil surface being exposed during some of the wettest periods of the year (winter) which results in the potential for large amounts of soil erosion. It is estimated that the average soil erosion losses from agricultural fields in the Delta region of Arkansas can exceed 3 tons of soil per acre per year. Soil loss via erosion may not be that evident, but the loss of topsoil results in nutrient loss as well and can lead to decreases in crop productivity over time. Leaving crop residues on the soil surface during the winter rather than burning or incorporating the residues could result in a significant decrease in erosion losses from agricultural fields and an overall increase in surface water quality across the state.