Renick Peries, Melissa Cann and Darryl Pearl
Department of Economic Development, Jobs, Transport and Resources
Peer Review: Abdur Rab, Research Scientist Soil Physics, DEDJTR
Subsoil manuring (SSM) is a practice with a potential to almost double the grain yields on dense and sometimes sodic clay soils in the high rainfall zone (HRZ). The practice involves the placement of large volumes (10-20 t-1) of nitrogen rich organic manures within the dense (soil bulk density of 1.4 gcm-3 or higher) clay matrix of subsoils. These soils have low macro porosity (around five per cent and are therefore poorly aerated and have a low plant available water capacity (PAWC). The practice, over time, leads to the breakdown of heavy structural units of clay into smaller aggregates. The process appears to be driven by soil microbial activity assisted by root-soil-microbial interactions. Work conducted in the Victorian HRZ (550-750 mm) since 2003 has generated the evidence that this practice has the potential to increase grain yield in this region from 42-96 per cent across sites and seasons. Beginning in 2014, this practice is being tested in Northern Victoria where soils and rainfall conditions are different to those in the Victorian HRZ but where certain dense subsoil properties and chemical constraints exist.
- These trials were conducted in ‘tough’ environments in ‘tough’ times. Below average rainfall at most sites impacted on crop growth and root proliferation.
- Inadequate rainfall to fill the bucket and to drive crop growth and root proliferation that will trigger soil change, appeared to result in limited success with SSM in the low to medium rainfall zones.
- Despite the above, changes were still observed at some sites in the physical parameters monitored.
- Timing of rainfall appeared to be important to trigger soil-root-microbial interactions: something that could not be expected in sub-optimal rainfall years.
- Marginal differences in seasonal crop water use was observed that also resulted in small differences in grain yield.
- Some sites showing exceptional results are worthy of being followed up for a few more years to better understand the process of subsoil change in the low to medium rainfall zone.
- Chemical constraints may have contributed to inconsistent soil physical change contrary to the expectation from SSM.