WORK ON finely structured WA subsoils during the past five years has shown that their physical fertility is dependent on structural features developed under native vegetation.
These features include networks of cracks and channels created by numerous wet/dry cycles and plant root and soil faunal activity. Under an annual cropping-pasture regime, the rate at which these pores are infilled by detached soil material is probably likely to exceed tbe rate at which new structural pores are created.
The research was conducted under Graham Aylmore at the University of Westem Australia. Professor Aylmore and his group reported the following to the GRDC (which had supported tbe research).
- From visual observations of profile structure, it is apparent that tbe physical fertility of many fine-textured subsoils is dependent on structural features developed under native vegetation.
- Land clearing has resulted in substantial reduction in the structural stability of subsoils as well as surface soils.
- Crop roots grow preferentially in biopores and cracks, leaving a high proportion of the subsoil volume essentially devoid of roots.
- Under an annual cropping-pasture regime, the rate at which tbese pores are infilled by detached soil material is probably likely to exceed the rate at which new structural pores are created.
- The network of stable biopores and cracks is fragmented following deep tillage and this is likely to have long-term disadvantages with respect to root and water accessibility and long-term sustainability in the least structurally stable soils.
- Traffic is not necessarily implicated in the densification process, but any activity that reduces topsoil stability may increase the risk of this subsoil structural degradation.
Can this be turned around?
The researchers believe stabilisation of favourable subsoil structures would undoubtedly lead to "substantial and sustainable increases in crop production.
This seems most likely to be achieved by tbe incorporation of a combination of natural and artificial organic stabilising agents, in association with appropriate crop rotations" (see story below).
Fine-textured subsoils occupy in excess of 60 per cent of the south-west agricultural area of W A. The researchers reported that the majority are poorly structured and highly susceptible to further structural degradation. Deteriorating subsoil structure has been implicated in the declining productive capacity of fine-textured and duplex soils of the WA wheatbelt.
In the first six months of the project, 33 pits were excavated and examined in finetextured soils of the central and soutbern wheatbelt at sites including Nungarin, Merredin, Tammin, Corrigin, Katanning and Mindarabin. The scientists assessed visible (macrostructure) features and samples were collected for laboratory assessment of microstructure and structural stability.
Their overall findings, as outlined above, have significant implications for cropping and tillage methods.
"At all the sites sampled in the course of this project we found subsoils to be less structurally stable than their equivalent topsoils, indicating tbe potential for more severe structural degradation to occur in subsoils," the research team stated in their report.
"Laboratory examinations of tbe physico-chemical characteristics of soils from randomly selected sites at some 10 locations throughout the wheatbelt indicate that changes in these properties following land clearing have, in fact, resulted in substantial reduction in the structural stability of subsoils as well as surface soils.
"In particular, there has been a significant increase in the susceptibility of these soils to clay dispersion.
"Measurements indicate that subsoils have changed significantly since land clearing in terms of both structural form and structural stability. Under agricultural use, finetextured subsoils are denser, have a lower volume of biopores and are less stable than their uncleared counterparts. Crop roots grow preferentially in biopores and cracks, leaving a high proportion of tbe subsoil volume essentially devoid of roots.
"The short-term effect of subsoil tillage is to allow more uniform root exploration of the subsoil, which has resulted in improved crop performance in the year deep tillage is performed. However, the network of stable biopores and cracks is fragmented following deep tillage and this is likely to have long-term disadvantages with respect to root and water accessibility and long-term sustainability in tbe least structurally stable soils.
"In tbe duplex soils, structure is often most limiting at the top of the B horizon, particularly where the A horizon is shallow. Bulk density in the upper subsoil commonly exceeds 1.7 and may be greater than 2.0; a level of 1.6 is considered to present a significant barrier to crop roots.
"High mechanical impedance and low porosity restrict root growth to existing cracks and biopores. Therefore, the physical fertility of these soils is substantially dependent on maintaining an open network of cracks and pores."