Soil structure degradation, often in the form of soil compaction, is widespread in cropping lands and is assumed to occur largely as a result of the tillage and traffic effects of agricultural machinery. Vertosols are very susceptible to soil degradation, but it has been widely believed that natural amelioration due to their unique shrink-swell behaviour can effectively break up compaction, and that degradation was associated with tillage. Although controlled traffic farming systems (CTFS) have been widely adopted in Australia, primarily to reduce soil compaction, the impact of removing traffic has not been studied in terms of its consequences for a degraded vertosol.
The aim of this study was to assess the change in soil environment following the implementation of controlled traffic-zero till farming after 100 years of degradation by conventional farming. It was anticipated that biological activity, coupled with the unique
properties of a vertosol, would regenerate the soil structure towards those found in non-agricultural vertosols.
The study was conducted on a structurally degraded vertosol, which had been cropped conventionally since 1945 at Gatton, in the Lockyer Valley, Queensland. Changes in soil structure were investigated after random field wheeling was replaced with a controlled traffic system (time zero) through four cropping seasons (22 months After Time Zero (ATZ)) of a cereal legume rotation.
Traffic was controlled by establishing permanent tracks on 3m wheel centres, in four, 80 by 12m blocks at time zero. Three tracks in each block were either wheeled once, twice or three times at planting and harvest to simulate the number of times a farmer might traverse the field. After each planting, two pits (5 x 12m) were excavated to three depths, 100, 200 and 300mm below planting depth in two of the blocks. Soil measurements and observations were
made on 27 sites in each pit, at three positions, track, adjacent to the track and bed (centre of the 2.5m cropping zone), for each of the three wheelings at each depth. The rate and depth of soil environment amelioration from biological activity and shrink swell processes during the previous seasons crop was assessed by changes in hydraulic conductivity, soil water retention characteristic, bulk density and cone penetrometer resistance. Soil profile observations and surface characterisations, plant root observations, insect populations and soil responses to excavations were used as qualitative references to changes in soil physical condition.
The soil profile at time zero was degraded relative to virgin soils. There was no indication of any recent biological activity below 200mm. There was no evidence of the characteristic slickensides found in well structured vertosols and the site was devoid of animal and insect activity.
soil profile and responses to excavation were clearly visible. At time zero soil was difficult to excavate and smeared readily. In contrast, 22 months ATZ soil crumbled and flowed into the excavator. Shrink swell processes from biological activity, formed micro-cracks and typical lenticular soil units of well-structured vertosols and replaced continuous blocky structures that were evident to depth at time zero.
Due to the removal of cyclic compaction (annual traffic) insect and animal activity increased from zero populations at time zero to an abundance 22 months ATZ, which was indicative of how much the habitat had changed, becoming more conducive to colonisation.
Hydraulic conductivity at time zero was very low across all sites in comparison to well structured vertosols. Soil moisture characteristics indicated that available water capacity was 10.5mm per l0cm of soil. Bulk density at 1.40g.cm-3 was typical for
a degraded vertosol and cone penetrometer resistance was >2000kPa (root growth reduction >50%) from 60 to below 400mm. Hydraulic conductivity in the wheel tracks changed little from that measured at time zero, however it increased in adjacent and bed positions each year. Hydraulic conductivity increased 4 fold at 100mm to 125mm.h-1, and 200 and 300mm increased by 50mm.h-1, 22 months ATZ. Bulk density decreased significantly to 1.25g.cm-3 at 100mm depths. The deeper zones also decreased to 1.30g.cm-3. Amelioration of the soil matrix caused changes in slope and intercept of soil moisture characteristic curves consequently changing soil moisture content at drained upper limit (DUL) and drained lower limit (DLL). The increased range between these two moisture contents increased AWC from 10.2 to 15.4mm per 10cm depth of soil. Cone penetrometer resistance contours from
<200-1000kPa now extended to >400mm depth, significantly improving root zone conditions. From this data it would seem that those soil characteristic properties we usually view as constants eg. AWC, DUL and DLL may need to be viewed differently for repaired soil.
This study has demonstrated the key role cyclic compaction plays in conventional production systems. Significant reductions in biological activity in the soil environment occur where random traffic is practiced, limiting the regenerative capacity of vertosols. Once wheel traffic is removed the soil response is positive, immediate and practically usefull. Seasonal improvements in soil structure accrue and expand down through the profile.
This study has confirmed that soil degradation can be reversed under a controlled traffic-zero till farm management system. However the rate of change is very dependent on the number of wetting and drying cycles as determined by
rainfall, plant water extraction, biological activity. Soil depth and cropping zone ageing processes since the last tillage/traffic event. Based on parameters measured, to naturally ameliorate the soil from a degraded state to a condition that is halfway toward a non-degraded vertosol could take 5-9 years.
Significant gains in production can be made by CT/ZT, by expansion of the unimpeded root zone, through increased water percolation, air filled porosity, pore size distribution and available water capacity. Due to reduction in soil density and strength, it follows that weed control, seed bed preparation and planting should require less power, reducing capital costs and or improving timeliness of operations, and could have significant implications for soil opening devices and engagement tools. The improvement in soil structure, increased productivity and lower environmental impact is based on the integration of four elements, controlled traffic, zero till,
increased organic matter and soil inspection. The approach leads to a more naturally self-sustaining farm management system than conventional cropping practices.