Sustainable Agriculture – Background Information

The region’s agriculture sector faces risks from introduced pests, diseases and weeds. Diseases and pests can damage agricultural and horticultural production and affect trade in international markets. As well as preventing new animal and plant pests, and diseases and weeds from arriving, biosecurity aims to control those already present.

The Department of Primary Industries and Regional Development (DPIRD) works with primary industries to safeguard our agricultural resources from biological threats and to maintain our export opportunities. The Biosecurity, Agriculture and Management Act 2007 (BAM Act), is legislation for managing risks of animal and plant pests and diseases entering, emerging, establishing or spreading in Western Australia, to protect our economy, environment and the community. The BAM Act is designed to facilitate cooperation between government agencies including DPIRD and stakeholders including landholders.

Crop and livestock farms in the Northern Agricultural Region will be adversely affected by climate change (Kingwell & Payne 2015). Climate change will give rise to an increased number of adverse seasonal conditions and result in poorer production and reduced profitability over time. Projected increases in extreme events such as droughts and floods could trigger increases in insect outbreaks and weed prevalence as the climate becomes more inhospitable for native vegetation and the competitive advantage of weeds increases. Broadacre crop and pasture production may also decline in drier, warmer northern and eastern areas. Livestock welfare risks may increase if higher temperatures reduce the availability of feed and increase heat stress prevalence. Higher temperatures can also affect livestock productivity by reducing reproductive rates, growth rates and milk production (DPIRD 2020). While water erosion and salinisation are likely to reduce due to declining rainfall, wind erosion may increase in regions where declining rainfall limits groundcover.

Successfully adapting to climate change requires activities to be undertaken across various land tenures and industries, by a broad cross-section of the community. Adaptation strategies relevant to the NAR include focusing on reducing risk and maintaining profitability in the long term, rather than on maximising yields in the short term, and increasing soil water-holding capabilities by increasing soil carbon content and applying appropriate soil amelioration treatments (DPIRD 2015h, 2015i, 2015j, 2015k).

Dryland salinity is a significant issue causing land degradation in the NAR. It is most often caused by changes to the water balance after vegetation has been cleared (DPIRD 2019).  A recent assessment of groundwater trends in Western Australia (DPIRD 2013) showed that about half of the region has a moderate to high salinity risk. Lands on the Dandaragan Plateau and the East Binnu Sandplain are at greatest risk of experiencing an increase in dryland salinity. This is mainly due to increasing groundwater levels in these regions and medium to long term time frames for these areas to reach equilibrium. The coastal areas of the NAR have a low risk of expanding dryland salinity, and the remainder of the region has a moderate risk.

Managing dryland salinity can provide many benefits, including increases in whole-farm productivity, reduced on-farm and off-farm degradation, and protection of landscape and community values. It can take a long time to recover saline land for agricultural production, so patience and commitment are critical. Strategies to manage dryland salinity include revegetating cleared areas with native vegetation, planting salt-tolerant species, excluding grazing and using engineering options such as drains (DPIRD 2020).

Most of the soils of NAR are low in nutrients as they are ancient and highly weathered. They have low levels of organic matter (frequently less than 1%) and are inherently low in all nutrients, with phosphorus, nitrogen and trace elements such as copper and zinc being of most significance historically. Broader problems are now being experienced as a result of continual removal of potassium and sulphur, the use of fertilisers which are low in these nutrients and nutrient losses from leaching and surface water runoff.

DPIRD (2020) recommends that farmers undertake regular monitoring of soil and plant nutrient levels and carefully manage fertiliser applications. Increasing soil carbon is widely regarded as beneficial to soil function and fertility, and has been associated with increased agricultural productivity. Increasing biomass production and retaining crop and pasture residues has the potential to slowly increase soil organic carbon. Organic amendments such as manure and compost can also increase soil organic carbon levels (DPIRD 2020).

Soil structure decline is a phenomenon caused mainly by excessive or poorly timed cultivation, stock movement on wet soils, loss of organic matter through stubble burning and compaction by vehicles (AgTrialsWA 2020). Soils with poor structure typically exhibit symptoms of ‘crusting’ and ‘hard-setting’ on the surface. Degraded soils are characterised by reduced infiltration, increased runoff and increased compaction.

In the NAR, a third of all soils are highly susceptible to soil compaction. The north and east of the region are more susceptible to soil compaction than the south and, particularly, the south coast (DPIRD 2013).  Soil structure can be improved by reducing tillage frequency and timing cultivation when soils are not excessively wet or dry, retaining stubble, reducing vehicle compaction, removing livestock before overgrazing occurs and fencing off fragile soils near waterways (AgTrialsWA 2020; Moore 2001). Retention or build-up of soil organic matter plays an important role in maintaining soil structure (DPIRD 2013).

Soil acidity is a widespread problem and major constraint to agricultural production in the NAR. The majority of agricultural soils in the NAR are naturally acidic and are acidifying further. The annual use of agricultural lime is 40% of the estimated amount required to treat existing acidity and on-going agricultural soil acidification (DPIRD 2013). The soils in the NAR currently have poor to very poor soil acidity, with measured soil pH levels well below target pH (Soil Quality 2013). Soil acidity is particularly poor in the Swan Coastal Plain, along the south coast of the region.

Managing soil acidity is both achievable and profitable. Addition of lime is required to maintain soil pH at optimum levels and, in time, sufficient surface application of lime will treat subsurface acidity (DPIRD 2019). The inefficient use of nitrogen fertiliser is a major contributor to accelerated soil acidification in the region (DPIRD 2018). As a complementary strategy to liming, inputs of nitrogen fertiliser can be managed to reduce nitrogen leaching (DPIRD 2018). The impact of soil acidity can also be reduced by choosing crop and pasture species or varieties tolerant of low soil pH.  

Waterlogging occurs when there is too much water in the root zone, which decreases the oxygen available to growing plants. Waterlogging can be a major constraint to plant growth and production and, under certain conditions, will cause plant death. Waterlogging is most significant in low-lying areas that receive more than 400 mm of annual rainfall. Conditions are accelerated when a clay subsoil lies close to the surface. There are only a few areas in the NAR at high risk of waterlogging (DPIRD 2020), primarily in Gingin, Moora and parts of Coorow.

The best options for managing waterlogging depends on the severity of the problem, position in the landscape and land use (type of crops or pastures). Management can include a combination of actions to adapt to waterlogged conditions, reduce waterlogging and reduce water flows in the affected area (DPIRD 2020).

Water erosion is usually associated with intense rainfall events combined with unprotected land where there is insufficient drainage or cover to absorb or channel the water appropriately. Inappropriate tillage practice, poor ground surface cover, steep slopes and a lack of surface water flow-control structures can leave land unprotected.  The risk of water erosion has diminished across the region as winter rainfall has declined and farmers have increased stubble retention and adopted reduced tillage practices (DPIRD 2013). Water erosion risk is mainly low across the region, although there is a moderate risk of water erosion in the Nangetty area near Mingenew and the Moresby Ranges near Geraldton.

Localised water erosion occurs throughout the region, particularly in conventionally-cultivated hilly areas and near water courses. Critical times for water erosion risk are at the break of season and during intense summer thunderstorms when land cover is likely to be lowest. Water erosion can be avoided by establishing a minimum of 70% intact and anchored ground cover (DPIRD 2013). Farming systems that maximise crop and pasture biomass and provide continuous ground cover, reduce stocking rates or remove livestock from areas likely to erode and avoid soil disturbance will reduce water erosion risk (DPIRD 2020).

Non-wetting or water repellence is widespread on soils with low clay content or high organic matter levels. The extent and severity of water repellence appears to be increasing due to an increase in cropping frequency and the use of some dry sowing techniques along with drier autumns (DPIRD 2020). It is caused by an accumulation of waxy organic matter in the soil surface and results in uneven wetting of the soil profile and poor emergence of crops. Water repellence also increases the risk of wind and water erosion, off-site nutrient transport and possibly soil acidification through increased nitrate leaching.

Soils in the West Midlands and Swan Coastal Plain are very susceptible to water repellence due to a high proportion of sands over clay or gravel (DPIRD 2013). Over 40% of the soils susceptible to water repellence in the NAR are found in the West Midlands, due to the predominance of sandy duplex and deep sand profiles. Managing soil water repellence involves a combination of short term mitigation, long-term amelioration and avoidance (DPIRD 2020). Mitigation options include avoiding dry sowing and using a higher seeding rate. Amelioration options include claying (DPIRD 2020) and soil inversion (DPIRD 2020) and avoidance involves sowing perennial species permanently on water repellent soils. While they can be challenging to establish, water repellence has little impact on perennial plants once they are established.

With frequent high winds, seasonal wind erosion threatens most unprotected land in the NAR.  Minor wind erosion occurs every year in the agricultural areas, and extensive wind erosion occurs in years when strong winds, poor groundcover and loose soil coincide over large areas. Severe wind erosion is more likely in the second or third year of a run of dry seasons (DPIRD 2020). Wind erosion involves the detachment, transportation and re-deposition of soil by wind. Productivity is affected by the removal of fertile topsoils and through sand-blasting crops. Wind erosion can also degrade infrastructure and contribute to air pollution. 

Almost half of all the soils in the region have a moderate risk of being affected by wind erosion, and 10% of these soils are classified as having a very high risk (DPIRD 2013). Risk factors include the type of soil, degree of disturbance (trampled or cultivated), amount of ground cover (> 50% ground cover significantly reduces risk) and landscape position. Climate change may expose whole landscapes to increased wind erosion risk as rainfall continues to decrease. Wind erosion can be reduced by establishing a 50 to 70% stable ground cover, including living and dead vegetation and gravel (DPIRD 2020).