Action: Retain crop residues
Biodiversity: One replicated study from Mexico found higher microbial biomass when crop residues were retained.
Erosion: One review found reduced water runoff, increased water storage and reduced soil erosion. One replicated site comparison from Canada found mixed effects on soil physical properties, including penetration resistance and the size of soil aggregates. One replicated study from the USA found that tillage can have mixed results on soil erosion when crop remains are removed.
Soil organic matter: Two randomized, replicated trials from Australia and China found higher soil organic carbon and nitrogen when residues were retained. One trial found this only when fertilizer was also applied.
Yield: Two randomized, replicated trials from Australia and China found higher yields when residues were retained. One trial found this only when residue retention was done combination with fertilizer application and no-tillage.
Soil types covered: clay, loam, sandy-loam, silt loam.
Crop residues are the remains of the crop after the valuable part has been harvested. For this intervention, residue retention is considered to be crop remains which are left in the field, rather than crop remains which are brought in from elsewhere and added to the soil (see ‘Amend the soil with fresh plant material or crop remains’). Soil aggregates are groups of soil particles held together by moist clay, organic matter (such as roots), organic compounds (from bacteria and fungi) or fungal hyphae (long, branching structures of a fungus). Some soil particles fit closely together, some do not, creating different-sized spaces. These spaces (or pores) within and between soil aggregates can store air and water, microbes, nutrients and organic matter. Large aggregations of particles retain the most nutrients. Soil penetration resistance is the soil’s ability to withstand penetration by water or roots. Often with low penetration resistance comes higher hydraulic conductivity, which is the ease with which a fluid (usually water) can move through pore spaces in the soil. A healthy soil has good hydraulic conductivity. Soil microbial biomass is the amount of tiny living organisms within a given area or amount of soil and is measured by levels of carbon or nitrogen in the soil.
Supporting evidence from individual studies
A review of 76 papers in 1991 (Unger et al. 1991) described a study (Russel, 1939) that found no water runoff under straw residue no tillage (0 mm) and highest runoff under disc tillage with no straw (60 mm). Another study (Greb, 1979) found higher water storage (157 mm) and wheat Triticum aestivum yield (2.16 Mg/ha) under stubble mulch with minimum tillage than under conventional tillage with dust mulch (a loose dry layer of soil) (102 mm, 1.07 Mg/ha respectively). Papendick (et al. 1990) found that the soil loss ratio (comparing loss from covered to loss from bare soil) decreased with increasing soil cover by crop residues (ratio of 0.8 at 10% cover,0.2 at 35% cover and 0 at 65% cover).
A replicated experiment in 1992 on silt loam at three sites in Illinois, USA (Bradford and Huang 1994) found decreased water infiltration rates and increased soil loss under both no-tillage (from >70 to 47.1 mm/h and 0.01-0.15 kg/m2/h) and tillage (from 64.1 to 37.2 mm/h and 0.1-0.6 kg/m2/h respectively) when crop remains were removed at site 1. Removing crop remains from a no-till system increased soil loss at site 2 from 0.01-0.13 kg/m2/h and site 3 from 0.01-0.16 kg/m2/h. The three sites were under corn Zea mays-soybean Glycine max rotations. Site 1 was under conventional tillage and treatments were: tilled vs. tilled with crop remains removed. Sites 2 and 3 had been under no-tillage for more than 15 years. Site 2 treatments were: no-tillage, no-tillage with crop remains removed, tillage residue replaced on the soil surface, and tillage residue removed. Site 3 treatments were: no-tillage, no-tillage with crop remains removed, tillage with residue removed, and tillage residue removed after three soil-drying days. Plots were 1 x 2 m and treatments were replicated six times at each site. Rainfall was simulated at an intensity of 70 mm/h on each plot for 90 minutes.
A replicated site comparison in 1984-1989 on loam soils in Alberta, Canada (Singh & Malhi 2006), found lower soil resistance (942 kPa) when residues were retained compared to removing residue (1,195 kPa) in no-tillage plots. Residue management had mixed effects on the proportion of larger soil aggregates within the soil and did not affect soil density or water infiltration. Treatments were replicated four times and included no-tillage (direct drilling) tillage with rototilling (to 10 cm depth), and two residue levels: straw removed and straw retained. Plots were 6 x 2.7 m. The crop rotation was barley Hordeum vulgare/rape Brassica napus. Soil density, penetration resistance, particle aggregation and water infiltration were measured.
A replicated experiment in 2005 on a sandy-loam in El Batán, Mexico (Govaerts et al. 2007) found greater soil microbial biomass when crop residues were retained (shown by 387 mg C/kg of microbial activity and 515 mg C/kg of microorganism growth), than when they were removed (319 mg C/kg and 384 mg C/kg, in both tillage treatments. Soil microbial biomass was higher in wheat Triticum aestivum compared to maize Zea mays. Zero and conventional tillage treatments were tested. Within tillage treatments were two residue treatments (retained or removed) and within these were plots of maize and wheat crops. Crop plots (continuous wheat, continuous maize, and rotated wheat and maize) were 7.5 x 22 m and fertilized at 120 kg N/ha. There were two replications of each treatment combination. Soil samples were collected to 15 cm depth from all plots. Total nitrogen and organic carbon were measured.
A randomized, replicated experiment from 1997-2002 on silty soil in China (Wang et al.2008) found 22% higher soil organic matter, 51 % higher total nitrogen and 97% more phosphorus under no-tillage with straw cover compared to conventional tillage with straw removed. Soil microbial carbon and nitrogen increased by 135% and 104% and wheat Triticum aestivum yield by 16% under no-tillage straw cover, compared to conventional tillage with straw removed. The effects of tillage and residue retention were not separated. Two treatments were compared in a wheat crop: no tillage with straw cover (standing stubble retained and all wheat straw was left as mulch cover (3.8 t/ha), and conventional tillage with straw removed (tillage to 15 cm depth twice, majority of straw removed (0.7 t/ha remaining)). Fertilizer, herbicide and insecticide application was the same for both treatments. There were three replicates of each treatment. Each plot was 9 x 78 m. Soils were sampled in 2007 up to 30 cm depth.
A randomized, replicated experiment from 1968 to 2008 on clay soil in Australia (Dalal et al. 2011) found higher soil organic carbon when crop residues were retained (20.5 Mg/ha) rather than burned (19.5 Mg/ha) in the topsoil. Crop residue retention only affected carbon levels when fertilizer was also applied (1.8 Mg C/ha more carbon with residues and a high fertilizer application rate, compared to no residue and no fertilizer). Nitrogen was 125 kg N/ha higher with retained residues than when burned and total soil nitrogen increased with fertilizer rate when residues were retained. Average grain yield was higher when crop residue was retained under no-tillage plus 90 kg N/ha/year (2.9 Mg/ha) compared to retaining residue under conventional tillage without fertilizer (2.3 Mg/ha). Wheat Triticum aestivum was the principle crop bar three years which were cropped with barley Hordeum vulgare. Treatments included: tillage (conventional tillage to 10 cm depth vs. no-tillage), crop residue management (burned or retained), and nitrogen fertilizer application (none, low (30 kg N/ha/year) or high (90 kg) application). Plots were 61.9 x 6.4 m and treatments were replicated four times. Soil was sampled in each plot at the end of the experiment to 1.5 m depth.
- Unger P.W., Stewart B.a., Parr J.F. & Singh R.P. (1991) Crop residue management and tillage methods for conserving soil and water in semi-arid regions. Soil and Tillage Research, 20, 219-240
- Bradford J.M. & Huang C. (1994) Interrill soil erosion as affected by tillage and residue cover. Soil & Tillage Research, 31, 353-361
- Singh B. & Malhi S.S. (2006) Response of soil physical properties to tillage and residue management on two soils in a cool temperate environment. Soil and Tillage Research, 85, 143-153
- Govaerts B., Mezzalama M., Unno Y., Sayre K.D., Luna-Guido M., Vanherck K., Dendooven L. & Deckers J. (2007) Influence of tillage, residue management, and crop rotation on soil microbial biomass and catabolic diversity. Applied Soil Ecology, 37, 18-30
- Wang Q., Bai Y., Gao H., He J., Chen H., Chesney R.C., Kuhn N.J. & Li H. (2008) Soil chemical properties and microbial biomass after 16 years of no-tillage farming on the Loess Plateau, China. Geoderma, 144, 502-508
- Dalal R.C., Allen D.E., Wang W.J., Reeves S. & Gibson I. (2011) Organic carbon and total nitrogen stocks in a Vertisol following 40 years of no-tillage, crop residue retention and nitrogen fertilisation. Soil and Tillage Research, 112, 133-139