Climate Resilient Soils Network

In Australia, we have huge opportunities to improve food security, environmental security and climate security. The potential to use agricultural vegetation and soils as a carbon sink is in the order of gigatons, and agriculture has the opportunity to become a major part of the solution.

The purpose of the Climate Resilience Soils Network is to support farmers to manage their soils in the face of climate change, water scarcity and soil degradation. Increasing soil organic carbon has well-documented benefits that include increased water efficiency and on-farm productivity, plus contribution to sequestration and reduction of agricultural greenhouse gas emissions. The potential to utilise agricultural vegetation and soils as a carbon sink is in the order of gigatons.

A genuine partnership between land managers and scientists, SoilCQuest aims to bring together farmers’ knowledge with scientific research to develop more sustainable methods of farming. The Climate Resilience Soils Network encourages collaboration to build knowledge, collate the evidence to support successes and improvements, share information and promote the broader use of climate-resilient agriculture techniques.

Grain Intercropping Grower Guide

As part of this project we created a Grower Guide in collaboration with Joel Williams from Integrated Soils.
Download the printable PDF of the Grain Intercropping Guide  with a financial analysis of a Canola and Arrowleaf clover Intercrop from Central West NSW.

Read on for the full text & deeper dive version with more grower insights and the scientific studies referenced in the guide.

Intercropping can be implemented in a range of different ways, but core to the practice is growing two or more crops together in the same field at the same time, with at least some part of their life cycles overlapping. Most of the time the crops are sown and harvested together, unless their planting and harvesting dates are staggered which is known as relay intercropping. 

Intercropping differs from companion cropping whereby two or more plants are also grown together but ultimately only one main cash crop is taken through to harvest, with the companion being terminated at some point during the season.

Despite the greater agronomic complexity that comes with managing intercrops, the practice has been gaining significant traction in recent years as growers in many parts of the world discover the many faceted benefits these systems can bring. Australian research trials have shown yield benefits from intercropping, with less land required to grow the same amount of grain as monocultures. 1

Economics & Risk Management

Although overyielding is observed with intercropping systems, savings on input costs are more commonly the main driver of increased profitability. Intercrops (particularly with legumes) demand less N and P fertilisers; while lower disease, insect and weed pressure in diversified cropping systems can lead to significant savings on often expensive plant protection products. Even when tonnage yield is not increased, intercropping with higher- value specialty grains can also improve profitability. 

Economic analysis by the Grains Research & Development Corporation in Oat/Lupin intercrop trials showed gross return was $191/ha greater in the intercrop. Economic advantage will be greatest when the two crops have similar prices and large amounts of over-yielding. Intercrops can help to reduce the economic risk from annual price variability, as well as yield risk. 

The diversity provided through intercropping also helps reduce risk against adverse events such as frost or drought and can also help even out in-field variability in soil type or topography. For example, farmers in other parts of the world have observed one of the intercrops might thrive in hotter years or ‘high spots’ while the other thrives in cooler years or ‘low spots’ of the field. Equally, one of the intercrops might fare worse after a frost or hailstorm leaving the other to fill the void and hence improve the per field yield as compared to a sole crop which – when susceptible – would suffer much greater losses. 

Intercropping for resilience and healthy soil & livestock

Wood Family Case study
Canola & Arrowleaf Clover Intercropping, Central West NSW

The Wood Family (Grant and Carmen, Luke and Belinda, Alex and Kate) at Manildra in Central West NSW have 1,740Ha of cropping & grazing, with wheat, barley, oats, canola, chickpeas & sorghum and Angus cows & Merino 1st cross ewes. 

Luke’s main motivation for intercropping is to build resilience into their farming system, by improving ground cover and drought resistance whilst reducing the impact of volatile input prices and supply chain disruptions on their business. Improved soil health is also a key motivation, with Luke observing more friable soil with increased porosity, aggregation, biology and darker colour.  

Luke also feels that the diversity of forage with an intercrop improves livestock health, with less issues associated with grazing solely canola. Additionally, the lower C:N ratio of the clover residues improves overall digestibility.

Luke’s expectation is not to grow two full yielding crops in one paddock in one year, describing intercropping as “Not 1+1= 2, but more like 0.5 + 0.5 = 1.3.”

“Intercropping adds another dynamic to the paddock- you have to think about things more- it’s like having two kids instead of one. You get double the joy when it works. It makes it more interesting than growing a mono. It’s more challenging than harvesting a straight canola crop. But it didn’t turn us off it. We were struggling to find a profitable legume in our system. So If we can grow a legume like this, where it doesn’t affect our canola and we still get the nitrogen benefits and the rotational benefits of having the legume in the system, we’ll keep doing it.” Luke Wood.

Building Soil Carbon

Intercropping can increase soil carbon by increasing the biomass of both plants, with the two species working in a complementary way to use resources at different times and from different parts of the rhizosphere (roots and soil).  

Legumes provide the greatest potential to increase soil carbon from intercropping. Legume based systems can store 30% higher soil organic carbon (SOC) when compared to other species; this is because legumes fix more nitrogen (N) which in turn contributes to carbon (C) sequestration 2. This is achieved primarily via two interconnected pathways:

Firstly,  because biological fixation of N leads to increased plant growth, this assimilates more CO2 from the atmosphere into plant biomass as C through the process of photosynthesis. When crop residues are then broken down, some C returns to the atmosphere as CO2 and some C is stored in the soil.

Secondly, legumes have biomass and root exudates with a lower Carbon to Nitrogen ratio (C:N) which are more efficiently used by microorganisms, forming more microbial biomass and respiring less CO2. This microbial biomass is then converted to more stable forms of soil carbon, mainly mineral associated organic matter.  In this way, by starting with a lower C:N ratio such as in legumes, more mineral associated organic matter can be formed from the same amount of biomass, and this more stable form of carbon stays in the soil for longer. 

Including legumes in intercrops can lead to a reduction in the use of synthetic N in subsequent crops due to nitrogen banking. Excessive synthetic nitrogen has been shown to be potentially destructive to existing soil carbon stocks. 3, 4

Non-legume intercrops also support soil organic carbon (SOC) formation. Research has shown that total root biomass in intercrops was on average 23% greater than the average root biomass in sole crops. 5  The increased aboveground plant biomass and the belowground plant biomass, including the increased microbial biomass C within the rhizosphere, can lead to increased C storage in soil. The increase in biomass increases soil organic matter (SOM) which increases soil aggregation. Soil aggregates are where carbon can be protected from microbial degradation, and increasing this stabilisation of carbon is essential to achieve C sequestration.

Intercrop systems that combine a tap rooting plant with a fibrous rooting plant can also build soil C. This results in a greater amount of root biomass, and importantly a higher diversity of exudates 6 is left for microbial processing and SOM formation.

Wood Family Case study
Canola & Arrowleaf Clover Intercropping, Central West NSW

In Luke’s opinion, he is seeing clear benefits to soil health from intercropping – with the intercropped paddocks having greater porosity and aggregation leading to a more friable soil. He also observed more life and biological activity in the soil along with a slightly darker soil colour when compared to mono canola crop paddocks.

Weed Suppression

There is no doubt that herbicide management can often be more complex within intercropping systems however there are opportunities to exploit complementary interactions between the plant partners leading to weed suppression. Intercrops can compete for resources (sunlight, nutrients) and release suppressive chemicals (allelopathy) both of which can reduce weed biomass.

Wood Family Case study
Canola & Arrowleaf Clover Intercropping, Central West NSW

Reduced competition from weeds is a clear benefit for Luke: “I don’t think we even saw a ryegrass plant in the paddock, whereas in previous years certain areas in that paddock had underperformed due to ryegrass numbers”. 

It’s unknown whether the ryegrass suppression was due to shading by the dense intercrop canopy or an allelopathic effect from the arrowleaf clover – both possible mechanisms observed in intercropping literature. “I am convinced that the two years of arrowleaf (first year as a cover crop, second year in the intercrop) has changed the soil physiology so that it does not signal the ryegrass to germinate”says Luke. With rye grass’s ability to grow in compacted soils with poor structure, the improvements in soil porosity and aggregation observed by Luke could be one part of the answer.

During this summer fallow there was also less barnyard grass, blackgrass, hairy panic, sow thistle and prickly lettuce. 

More nitrogen and phosphorus 

– Sharing and banking N

The ability of  legumes to fix nitrogen and bank residual soil nitrogen for subsequent crops in the rotation is widely understood. However, it has also been demonstrated that legumes can also share nitrogen with other plants in real time. They do this by secreting nitrogenous compounds from their roots which are scavenged by plants growing near them, such as an intercrop.

In addition to the N-rich root exudates from legumes, organic nitrogen can also be directly transferred between intercrops via mycorrhizal fungi growing symbiotically with intercrop roots. 

Research has shown that root-root proximity is a key driver of these nitrogen sharing processes, so it’s important to plant the intercrop close enough to the main crop and/or choose an intercrop variety that has more lateral roots that reach out into the soil.  

-Unlocking soil P for N fixation

Acquiring atmospheric nitrogen requires considerable amounts of phosphorus to support this energy-intensive process. This means legumes demand more P than most plant species and have adapted multiple strategies to scavenge soil reserves of this essential nutrient that is critical for N fixation. A diverse array of highly specialised root exudates are able to unlock both inorganic and organic forms of soil P – which upon being liberated – are taken up by both the legume and non-legume companion. 

In the same way that N can be banked in crop residues for the next season, P and many other nutrients are also banked in crop residues, with legumes accumulating relatively high amounts of P. Importantly, this residue banked P is stored in an organic form which – as compared to inorganic P – is much less likely to fix onto soil mineral surfaces or lock up with other soil nutrients such as Al and Fe. 

Due to being located below ground in the microbially active soil, the nutrients banked in root litter typically liberate faster than those in the shoot litter. Therefore plants with larger root systems have greater potential for nutrient release the following year, while shoot residues will typically release in subsequent years.

Things to consider

-Sowing

Choosing suitable varieties is as important as deciding which plant species to partner – varieties must have similar sowing and ripening dates. Researchers internationally have observed the variability between varieties and acknowledged the need to breed varieties specifically for intercropping systems. Should equipment allow, planting in alternate rows provides the opportunity to tailor the seeding depth for each plant species, however many farmers have success planting all in one row with a ‘middle ground’ seed placement or sometimes even making two separate passes with the drill.

-Harvesting and Grading

Once again, variety choice is key to ensure synchronised maturities and optimal harvestability with minimal losses. Anecdotally, many farmers have found that intercrops often align their ripening times which may be due to ethylene gas released by the earlier species inducing ripening in the latter. 

Intercropping can at times make harvesting easier with crops prone to lodging or fine seeded species like flax or camelina. Seed separating is a key barrier to the adoption of intercropping; however the wider system benefits and economic gains from overyielding or input reduction savings can compensate for this additional expense. Typically farmers start small by renting, borrowing or investing in simple kit such as sieves/ screens, gravity tables or rotary drums.

Wood Family Case study
Canola & Arrowleaf Clover Intercropping, Central West NSW

Luke uses a disc seeder and this limits the pre-emergents he can use, so he chose a pre-emergent that could be used for both canola and clover. With in-crop herbicides Luke found that the two crop species limited him a little bit. He could still use grass weed herbicides, but he was very limited with broadleaf weed control.

Luke used a regular seed grader, but if he wanted it exceptionally clean he would use a specialist grader. Luke decided to just clean it enough to sell the canola and then use the arrowleaf clover on the farm.

Because Luke wanted to harvest both intercrops, he chose canola & clover because they have similar ripening times. The clover ripened slower than the canola however and this delay of harvest time led to some canola losses due to shedding, which were overcome somewhat by the additional clover yield.  

Due to the very wet La Nina season in 2021, the arrowleaf clover was particularly dominant and grew taller than the canola. This presented some challenges for harvest with more lodging in the intercop canola than the mono canola. 

Deeper Dive- Grower Insights

Lessons learned- Luke Woods

• “If we weren’t planning on harvesting the arrowleaf seed, we could have grazed the canola harder as it will recover much better than the clover and this would have controlled the arrowleaf”. 
• “We could have controlled the clover with herbicide to get more of the yield, but this would have significantly reduced the yield of clover and germ quality on the arrowleaf”.
• “But this allowed us to maximise our arrowleaf seed, rather than maximising our canola yield”.

Motivations

Build Resilience
The primary motivation to experiment with intercropping was to build resilience in the farming system. Luke felt that intercropping could improve ground cover and drought resistance while he was also seeking resilience against volatile input prices and difficulties with supply.

Soil and Livestock Health
Luke is hoping that intercropping can improve his soil health with a particular focus on soil carbon and water holding capacity. He also felt there would be advantages for livestock health and performance through grazing a better quality forage.

Benefits

Ground Cover

• Luke’s previous experience on his farm is that crops always do well wherever he has good ground cover.
• The post-harvest residue of the intercrop was far superior to that of sole crop canola leading to greater quantity and quality of ground cover.
• The intercrop plot formed a more dense mat of cover that persisted well over the summer.
• This leads to greater suppression of summer weeds and potential to build soil carbon and water holding capacity over the long term (which will be followed up over the coming years).

Weed Suppression

• Weed suppression was observed both during the cropping cycle as well as post-harvest during summer fallow.
• The canola:clover intercrop induced significant suppression on annual ryegrass. It is unknown whether the ryegrass suppression was due to shading by the dense intercrop canopy or an allelopathic effect from the arrowleaf clover – both possible mechanisms observed in other intercropping literature. Luke said “I don’t think we even saw a ryegrass plant in the paddock whereas in previous years certain areas in that paddock had underperformed due to ryegrass numbers”.
• Luke also observed less weeds during the summer fallow, again, likely due to the dense mat of canola and clover residues. Barnyard grass, blackgrass, hairy panic, sow thistle and prickly lettuce are common trouble weeds that were suppressed during the summer fallow.

Livestock Health

• The diversity of crop residues provides better quality forage for livestock as compared to sole crop canola residue. The lower C:N ratio of the clover residues improves digestibility for the animal.
• Canola: clover intercrop provides multifunctional feeds for livestock – grain only, grazing only or a combination of both. This multifunctional forage provides a valuable risk mitigation strategy depending on seasonal variables.

Soil Health

• Changes to soil health are usually observed over many years and being a rather subjective concept, it can be hard to quantify. However, in Luke’s opinion, he is seeing benefits to soil health – in particular, in his intercropped paddock he noted greater porosity and aggregation leading to a more friable soil. He also felt there was more life and biological activity in the soil along with a slightly darker soil colour when compared to sole crop canola.

Economics

• Despite sole crop canola yields typically being higher when compared with intercropped plots, the arrowleaf clover is a much higher value grain fetching almost double the price of canola seed. This helped to financially compensate for the lower tonnage harvested from the intercrop plots.

Other Comments

Nitrogen Inputs

• No fertiliser reductions were observed however no lowered N rate was experimented with for this trial – inputs were managed the same for both treatments. We can safely assume there would be greater residual N in the intercrop paddock from the decay of the clover residue. Opportunities to reduce N inputs the following season is likely. 
• Evidence from intercropping studies highlight that legumes are able to share nitrogen with non-legume companions in real time, that current season. Legumes share nitrogen via the release of amino acids as root exudates which the non-legume can scavenge, however, root intermingling is key for this process so row spacing, planting arrangement and variety choice is important. When both crops associate with mycorrhizal fungi, an additional direct pathway to share amino acids through the fungal hyphae is also possible.

Herbicide Inputs

• As both treatments were managed the same in terms of all inputs, no economic gain was observed during the growing season. However, the saving on herbicide and labour costs of spraying summer weeds should be considered.

Challenges

Harvestability

• Being a relatively wet season, the arrowleaf clover was particularly dominant and grew taller than the canola. This presented some challenges for harvest with more lodging evident vs sole canola. 
• The arrowleaf clover also ripened slower than the canola and consequently, harvest time was delayed leading to some canola losses due to shedding. 
• These challenges would likely be less of an issue in a more typical rainfall year as normally legumes are the plants more prone to lodging, not causing the lodging. Experience from other parts of the world highlights a beneficial role of intercropping in preventing lodging of legume based intercrops.

Disease

• A slightly higher disease pressure was observed in the intercrop paddock due to the lanky, high biomass of the clover leading to a more humid and disease enhancing canopy. 
• Disease thresholds were minimal enough to not warrant spraying however and Luke’s feeling was that the slightly lower yield of the intercrop was primarily driven from the harvestability issues and not disease associated losses.
• Evidence from intercropping studies highlights that more often than not, disease pressure is reduced with intercrops primarily due to disease host dilution (less plant population of each species per hectare) and the physical barrier created between the companion and cash crop when planted in alternate rows.

References

1 Fletcher, A. J. K. (2020, February 26). The potential role of companion and intercropping systems in Australian grain farming. Should we be considering them? Grains Research and Development Corporation.

2 Kumar, S., Meena, R.S., Lal, R., Yadav, G.S., Mitran, T., Meena, B.L., Dotaniya, M.L. and EL-Sabagh, A. (2018). Role of legumes in soil carbon sequestration. Legumes for soil health and sustainable management (pp. 109-138). Springer, Singapore.

3 Khan, S.A., Mulvaney, R.L., Ellsworth, T.R. and Boast, C.W. (2007). The myth of nitrogen fertilization for soil carbon sequestration. Journal of Environmental Quality, 36(6), pp.1821-1832.

4 Ordonez, R.A., Castellano, M.J., Gerasimos N. Danalatos, G.N., Wright, E.E., Hatfield, J.L., Burras, L. and Archontoulis, S.V. (2021). Insufficient and excessive N fertilizer input reduces maize root mass across soil types. Field Crops Research, 267, 108142.

5 Cong, W.F., Hoffland, E., Li, L., Six, J., Sun, J.H., Bao, X.G., Zhang, F.S. and Van Der Werf, W., 2015. Intercropping enhances soil carbon and nitrogen. Global change biology, 21(4), pp.1715-1726.

6 Steinauer, K., Chatzinotas, A. and Eisenhauer, N., 2016. Root exudate cocktails: the link between plant diversity and soil microorganisms? Ecology and Evolution, 6(20), pp. 7387-7396.

This project is supported by The Ian Potter Foundation and the Department of Agriculture, Forestry and Fisheries through funding from the Australian Government’s National Landcare Program.