Crop Residue and High-Carbon Char: Potential Soil Conservation Tools
Soil is the single most important resource on which our agriculture depends. Proper soil management is necessary to sustain long-term agricultural productivity. Soil loss through erosion or run-off hurts agricultural production with depletion of organic matter and fertility. It also has environmental implications. Much progress has been made to reduce soil erosion per unit of corn and soybean production (Figure 1) in the US, although progress appears to have slowed since 2005 (GAP Report, 2017). Reduced and no-till practices have declined from 25% to 19% and 21% respectively since 2005, with a corresponding increase of conventional tillage to about 60% (Field to Market, 2016). This trend, combined with more highly erodible land from the Cropland Reserve Program coming back into production over the same time period, has contributed to the slow progress in soil conservation. Between 1980 and 2015, corn and soybean yields in the U.S. improved by 61% and 29%, respectively, with corresponding decline in soil erosion by 58% and 47%.
Soil erosion includes water- and wind-driven erosion. Soil conservation practices are tools to prevent soil loss and degradation and build soil structure and organic matter. Depending on site-specific conditions, these practices may include, but are not limited to:
- physical barriers such as terraces (including terrace segments, tile outlet terraces, etc.)
- crop rotation
- reduced tillage
- cover cropping
- standing crop residue for wind erosion
- ground cover by crop residue for water erosion control
- contour farming
- maintenance of high soil organic matter and soil aggregation
- adding soil amendments
In Nebraska, where there is a distinctive gradient in precipitation across the state, the same practice might not be effective everywhere. Nutrient and pest management are a few other factors that play a role in conserving soil.
Nebraska Management Options and Research for Soil Conservation
Below we focus on crop residue management and amendment for soil conservation.
Residue Management and Its Effects on Soil
The value of leaving crop residue as ground cover to reduce water and wind erosion and slow runoff is well known. However, crop residue is a valuable livestock forage in Nebraska and removing crop residue from no-till land can be important for high yield production. Soil organic matter is maintained by decomposition of plant biomass that remains in the field, especially by the root material but also above ground material. Our best estimate of the minimum amount of residue that is needed to maintain soil organic matter is 2 or 3 ton/acre/year. (Two tons of corn residue usually gives about 50% cover.) This value is just a starting point since as tillage increases (frequency, depth, degree of soil disturbance), more residue is needed to balance the increased decomposition rates that result. For example, in three counties across Nebraska, the average amount of crop residue needed to provide sufficient ground cover to limit soil loss to no more than 5 ton/acre/year was estimated using the USDA-NRCS water erosion estimator RUSLE2 (Figure 2). For more information on residue management, please refer to NebGuide G1846, Harvesting Crop Residues.
Soil loss through wind erosion is especially important in semi-arid western Nebraska and worsens during drought. Ground cover with crop residue can control or minimize wind erosion. Maintaining standing crop residue is important for reducing wind velocity at the soil surface and trapping soil particles, as well as for trapping snow in winter to provide for much needed soil water in semi-arid areas. Ground covers of 30% and 60% are estimated to reduce wind erosion by 70% and 90% percent, respectively (see NebGuide 1537, Wind Erosion and Its Control). The NRCS Wind Erosion Prediction System suggested that often no crop residues can be removed under rain-fed, tilled conditions in western Nebraska.
State of the Soil in Semi-Arid Western Nebraska
In western Nebraska, crops are often grown in fields that have been leveled for irrigation, intensively farmed, or have been affected by wind and water erosion, all of which can decrease soil organic matter (SOM). Lack of SOM is a significant indicator of a degraded soil. When grown on degraded soil, plants are prone to less vigorous foliar growth, chlorosis, poor root development, and poor emergence due to soil crusting. Furthermore, lighter colored soils low in organic matter warm up slower and have less potential to produce nutrients from mineralization. Many intensively cultivated soils in the Great Plains have lost 30% to 50% of the original SOM level.
Soil organic matter affects many soil physical, chemical, and biological processes and properties (UNL Extension G2283, Soil Management for Increased Soil Organic Matter). Increased SOM reduces compaction risks and improves soil structure, water holding capacity, cation exchange capacity, and microbial activity. Soil organic matter loss can be particularly negative in coarse-textured soils like many of those in western Nebraska. Restoring lost SOM is a high priority to enhance crop production in general.
Current Research with Soil Amendments to Improve Soil Properties
In 2017, a field research study was initiated in the Panhandle at a low-productive field under center-pivot to assess carbon-rich char (also known as cinder), biochar from pine trees, manure, and municipality waste products as potential amendment to restore the soil quality and increase dry bean production. Char is residue left after inefficient burning of coal. Biochar is charcoal produced by pyrolysis, or non-combustion burning without oxygen, during which C molecules are converted to more stable cyclic forms giving biochar a half-life in the soil that may exceed 100 years.
Aerial imagery early in the season showed encouraging evidence of these products, particularly char as a potential soil amendment (Figure 3). The first year dry bean yield was significantly greater with the highest char rate (60 ton/acre) compared to the no-char plots. Dry bean yield increased by 12-52% with char application with the highest increase in the highest rate of char. This year, spring was cool and wet and there were chlorosis issues. Char might have improved aeration and/or infiltration by improving soil physical properties and that could have led to some beneficial effects. It would require at least a few years of monitoring of this field to document any other significant benefits of any of these amendments with respect to agronomic productivity or soil properties.
The soil of the char trial was calcareous, sandy loam and had about 1% SOM. The trial was in an area under center pivot that had low productivity compared to the rest of the field, probably due to leveling of the field sometime in the past. We expect that the char will be most effective for soils that are low in SOM and degraded.
Soil is a finite resource in the sense that it is hard to replace. It is therefore, important for our agricultural production sustainability to conserve and preserve this invaluable resource. Considering land conditions, climatic specificity for the area, cropping system, and other important factors, there are various conservation practices available. Any effort toward conservation would immensely benefit soil and agricultural production on that land for many years to come.