In a 12-year research study comparing continuous cropping with various rotations, soil organic matter increased in the rotation that included a four-year perennial pasture grass plus legume phase. When the pasture phase was followed by continuous wheat soil organic carbon gradually declined to the original level by the sixth grain crop.
Research was conducted at Warra, southern Queensland, on a vertosol soil (clay with shrink-swell properties), cultivated for cropping since 1935 and had typically lost 70 percent of its original organic carbon (2.23 to 0.68 percent). Total nitrogen declined from 0.203 to 0.06 percent in the 0–0.1m layer. Depleted nitrogen had resulted in reduced yields, low proteins and low economic returns.
Professor Ram Dalal, University of Queensland School of Agriculture and Food Sciences (also a recent awardee of an AM) led the study that was recently published in the CSIRO journal Soil Research. The research report is titled “Farming systems’ productivity and soil organic carbon stocks following fertilisers, no-tillage or legumes on a fertility-depleted soil in a semi-arid subtropical region”.
Rotations assessed over the 12-year study, other than a four-year grass/legume pasture followed by wheat, included a two-year rotation of lucerne followed by wheat, annual medic–wheat, chickpea–wheat; and continuous wheat. Conventional tillage compared to no-tillage, and nitrogen rates in cereal of 0, 25 and 75kg/ha for each cereal crop were tested. All treatments received a basal rate of 10kg/ha phosphorus fortifed with copper and zinc annually.
Zero till cropping systems can reduce soil aggregate disruption and combined with the inclusion of pasture in rotations can lead to increased soil organic material inputs. Soil organic carbon stocks increased under the four-year grass/legume pasture in the 0–0.1m depth only, then decreased steadily following the cropping phase.
A rotation of four-year pasture followed by wheat cropping for four to six years may maintain initial soil organic C stock, but a shorter cropping phase is required to increase soil organic carbon and nitrogen in the long term. There were no consistent effects of the short-term lucerne–wheat, medic–wheat, chickpea–wheat rotations, or tillage and nitrogen fertiliser application on soil organic C stocks over the 12-year period.
When all phases of the rotations for wheat production were compared, average yields were highest in the chickpea–wheat and the zero till wheat with 75kg/ha nitrogen treatments (around 2.5 t/ha average). Average yield was lowest in the conventional and zero till treatments with no nitrogen (1.9t/ha).
Wheat grain protein was highest in the grass/legume rotation (13.7 percent), although not signifcantly different from that in the conventional tillage with 75 kg nitrogen/ha (13.5), medics–wheat (13.3), and zero till with 75/kg/ha nitrogen (13) treatments. The lowest protein was in the no nitrogen continuous wheat treatments (9.2-9.5 percent).
Crop water use efficiency and gross margin were strongly correlated. Economic analysis of treatments suggested that restoring or maintaining soil nitrogen fertility, either through legume-based pastures, grain legume and/or nitrogen fertiliser, provides long-term positive economic return. The better yielding treatments had highest gross margins and best water use efficiencies. There was potential for some deep leaching of nitrate-nitrogen at 75kg/ha of nitrogen fertiliser. Also potential for some nitrate leeching is possible in wheat following grass/legume phase, as well as in the pulse wheat rotation.
- Bob Freebairn is an agricultural consultant based at Coonabarabran. Email firstname.lastname@example.org.