Investigating the effects of soil amelioration on weed management in WA’s Wheatbelt
Project name
Increasing farming system profitability and longevity of benefits following soil amelioration
GRDC Code
DAW1901-006RTX
Effects of soil amelioration on weed management
With funding from the Grains Research and Development Corporation, the Department of Primary Industries and Regional Development (DPIRD) has an ongoing field trial investigating the impact of soil amelioration on sustainable weed management in no-till cropping systems in wet (Merredin) and dry (Wongan Hills) soil conditions.
Soil acidity is a widespread issue in Western Australia’s wheatbelt cropping zones often associated with intensive farming using high nitrogen fertilisers, and it affects both coarse and fine-textured soils.
Liming of soil is a practice used to reduce soil acidity.
When lime has been ineffectively applied to raise soil pH and subsurface acidity can develop.
This can result in aluminium (Al) and manganese (Mn) toxicity.
For example, aluminium will reach toxic levels when subsurface pH falls below 4.8, and this affects plant growth and overall crop productivity.
These subsoil issues often reduce grain yields, particularly in a season with a dry finish when crops rely on subsoil water reserves to complete grain fill.
Additionally, poor subsoil structure, high soil strength from sodicity and compaction, and restricted drainage worsen these challenges, affecting crop root growth and nutrient availability.
Soil amelioration practices using strategically timed, occasional tillage into conservation agriculture systems often incorporate lime via deep ripping, rotary spading, or mouldboard plough.
Implementing these practices can affect weed dynamics and herbicide effectiveness due to physical soil disturbance and changes in soil chemical properties and crop residue level.
Understanding the interactions
In recent years, there has been increasing reliance on pre-plant or pre-emergent herbicide with short to long soil residual activity to manage weed populations, especially with continued evolution of weed resistance to post-emergent herbicides.
Changes in soil characteristics following amelioration practices influence the sorption (fixation or capture of a gas or a vapor (sorbate) by a substance in condensed state (solid or liquid) and desorption (process where a previously adsorbed substance is released from a surface) of herbicides, modifying their availability in the soil solution.
This, in turn, plays a crucial role in determining the efficiency of weed control, potential crop injury, herbicide mobility, and persistence within the soil.
However, the relationship between these soil amelioration practices, and the efficacy of pre-emergent herbicides is, for the most part, unexplored in Western Australia.
The findings of this project will have benefits for both growers and agronomists, by increasing our understanding how soil amelioration techniques influence the efficacy of prominent, pre-emergent herbicides in the control of ryegrass, wild radish, and great brome.
Dr Arslan Peerzada said several crucial insights have been uncovered thus far.
“These findings highlight essential factors in maximining the benefits of soil amelioration, all of which have significant implications for crop management and weed control strategies,” he said.
Soil conditions during soil renovation
The research trial at Wongan Hills revealed that performing soil renovation, crop sowing, and herbicide application during dry soil conditions has proven to be detrimental to crop establishment and weed suppression.
Delayed crop emergence was observed in these conditions, followed by delayed herbicide activation.
This may have created a window for increased weed growth and reduced the efficacy of pre-emergent herbicides.
Growers should, therefore, exercise caution and consider more favorable soil conditions to maximise the benefits of soil renovation to ensure successful crop establishment and effective weed suppression.
Seeding depth in renovated soils
Maintaining the appropriate seeding depth contributes to successful crop establishment, ensuring rapid groundcover, maximises yield potential, and enhances the crop’s competitiveness with weeds.
This can be a challenge in ameliorated soil, primarily due to the uneven soil surface conditions following soil renovation.
Growers should be vigilant in regularly monitoring and adjusting seeding depths on uneven surfaces to ensure successful crop establishment to maximise yield potential in renovated soils.
Herbicide-related crop damage
Dr Arslan Peerzada further highlighted the potential consequences of variation in crop emergence, which could lead to specific herbicide-related crop damage due to change in herbicide availability from strategic deep tillage.
“At both trial sites, the most substantial crop injury to a specific herbicide occurred in spaded soils (57.5 per cent) followed by inverted soils (42.5 per cent) and deep-ripped soils (17.5 per cent) in comparison to no-till (5 per cent).
However, the crop visually recovered within two months, while harvest data will indicate if there was lasting damage to crop yield,” he said.
By adhering rigorously to herbicide labels for crucial guidance on soil compatibility and application guidelines, growers can effectively safeguard their crop against herbicide-related damage and enhance overall yield.
Caution in implement selection and precise execution
Renovating a paddock in dry conditions with mouldboard plough resulted in the formation of significant clods and researchers were faced with the challenges of adjusting implement settings for optimal soil inversion.
Therefore, it’s advisable to conduct a thorough assessment of the paddock’s soil type and condition before renovation.
This will help to facilitate informed decisions regarding the appropriate implement and its setting.
For instance, a mouldboard plough can effectively bury a significant portion of surface soil, ranging from 50 to 99%.
However, it's crucial to note that a poor inversion, often resulting from inappropriate implement selection or improper setting, may leave weed seeds inadequately buried while leaving topsoil at shallow depths.
This particular scenario has been linked to higher densities of wild radish following soil inversion in the trial site, which could impact the crop yield.
Therefore, a comprehensive understanding of soil conditions and careful implement management are integral elements for effective weed seed burial and optical crop outcomes.
Deep ripping seems to stimulate weed emergence
Deep ripping was found to stimulate weed emergence and promote growth of more weeds when compared to no-till conditions at both trial locations.
Dr Arslan Peerzada said “without effective management, these weeds can thrive in improved soil conditions and potentially outcompete crops.
This phenomenon underscores the importance of growers being prepared for additional weed burden when considering deep ripping as a soil amelioration practice.
Growers will require a more comprehensive weed management strategy in conjunction with deep ripping practices such as delayed sowing with use of non-selective herbicides to mitigate the potential weed challenges it may introduce,” he said.
Further study will be needed to fully evaluate the long-term effects of deep ripping on weed populations.
Herbicide effectiveness
Among the herbicide treatments tested in this study, the combination of multiple herbicides exhibited excellent control of annual ryegrass and great brome across all ameliorated treatments.
However, the combination proved less effective against wild radish under deep-ripping and no-till conditions at both trial locations.
This highlights the importance of tailoring herbicide strategies to specific weed types and renovation techniques.
No other tested pre-emergent herbicides subjected to testing consistently achieved satisfactory control across all amelioration treatments.
This underscores the complexity of weed management in ameliorated soils, where a single herbicide treatment alone struggles to suppress weed density below acceptable levels.
The key to success
Managing weeds in renovated soil solely using pre-emergent herbicides may prove challenging.
The diminished efficiency of most pre-emergent herbicides calls for a thoughtful and multifaceted approach to weed management while renovating soils.
Researchers advocate a balanced strategy that includes the integration of in-crop herbicides, or herbicide tolerant crops tailored to the specific weed problems in renovated soils as part of an integrated weed management strategy to ensure effective weed control throughout the crop growth.
For example, growers could use weed seed control techniques during harvest in the first years following renovation as a valuable complement to herbicide strategies.
Capturing and managing weed seeds in early years can significantly reduce weed seedbank in grower’s fields, fostering more sustainable weed management practices.
Contact
Dr Arslan Peerzada
DPIRD Research Scientist
E: arslan.peerzada@dpird.wa.gov.au
P: (08) 9699 2133