Subsoil manuring – can the successful agronomic and economic impacts be extended to moderate and low rainfall zones?

Take home messages

• Subsoil manuring involves the incorporation of high rates of organic manure (up to 20 t/ha) into the upper layers of dense sodic-clay subsoils.
• The practice improves the physical fertility (structure) and the plant available water capacity of the clay subsoil (bucket size), and these changes appear to be long lasting.
• Subsoil manuring is expensive but the large, continuing increases in grain yield make the practice profitable, with payback occurring within 1-2 crops.
• It is unlikely that subsoil manuring can be extended into drier regions unless the costs can be reduced and the reliance on animal manures removed. More research and innovation is required to improve the practice.

Introduction

Subsoil manuring is a practice that developed following research undertaken to ameliorate dense, sodic-clay subsoils in the high rainfall zone (HRZ) of south west Victoria. The practice involves the incorporation of high rates of high-N organic material such as poultry litter (up to 20 t/ha fresh weight, with 20% moisture). The amendment is placed in rip-lines up to one metre apart, into the upper layers of the clay subsoil. There is increasing interest in subsoil manuring as it results in large yield increases in crop yields. The high costs of the investment in soil management are generally repaid by the 2^nd crop.

In this paper we will briefly summarise the key findings from the subsoil manuring research that has taken place over the last 10 years in the high rainfall zone of Victoria. We will then consider whether the benefits from subsoil manuring can be extended into the drier cropping zones of southern Australia.

The key findings from subsoil manuring research in the high rainfall zone (HRZ)

Adding high rates of organic amendment will aggregate sodic-clay in incubation studies

Ten years ago we were asked to undertake research to try and overcome the constraints posed by dense clay subsoils for crops in the high rainfall zone of south west Victoria. We considered a range of different approaches and found that there was frequent mention in the scientific literature about the structural benefits from adding organic matter to sodic clay soils. In addition, there were the classic ‘graveyard plots’ set up by Robin Graham from the University of Adelaide, where the deep incorporation of lawn clippings with trace elements had a long lasting effect on crop growth. This led to the research question as to whether the incorporation of organic amendment into sodic-clay subsoil would aggregate the clay (i.e. form the clay into larger soil aggregates)?This would provide pore space in the clay and allow root entry. So an incubation experiment was carried out where different rates of ground lucerne shoots were mixed with moistened sodic clay to see whether the clay would form into aggregates. Physical properties of the clay were measured after four weeks (Table 1).

Table 1. Effects of increasing amounts of finely-ground legume shoots of Medicago sativa, on the physical properties of sieved (< 2mm) sodic-clay subsoil after 4 weeks of incubation. (Source: Gill et al. unpublished data).

The rate of added amendment had a marked impact on changes in the physical properties of the sodic clay (Table 1). Adding 1% (w/w) had no effect, 2% had some effect, whereas increasing the rate of addition to 20% (w/w) increased both the macro-porosity and the hydraulic conductivity of the clay by more than 2½ times, and resulted in a four-fold increase in the mean size of water stable aggregates. Similar results were also occurring in other incubation experiments in our lab (Clark et al. 2009). The key message from these incubation studies was that if organic amendments were to be used to aggregate sodic-clay subsoil, then high rates of addition would be required.

Based on the incubation study, we estimated that the rapid aggregation of the sodic-clay subsoil required around 20 t/ha of a high quality amendment (>2.5% N) in concentrated riplines. Consequently, some 20 tonnes of lucerne pellets were incorporated in rip-lines 80 cm apart, at a depth of 30-40 cm into the top of the clay B horizon of a Sodosol cropping soil at Ballan in south west Victoria. The field was then sown to a long-season winter wheat crop in late May of 2005, and harvested in early January 2006. Soil samples were collected at the depth of 30-40 cm between the rip-lines and well away from the amendment. A total of 326 mm of rain fell during the growing season.

High rates of organic amendment, and crop roots, rapidly aggregate sodic-clay subsoils in the field

Very high wheat yields were recorded on the plots where the lucerne pellets had been incorporated in subsoil, some six to seven months earlier. The dry-land yields were in excess of 12 t/ha (Gill et al. 2008). There were also marked changes in the physical properties of the undisturbed clay subsoil between the rip-lines, which was sampled after the wheat harvest. The more than two-fold increase in macro-porosity, and the 50-fold increase in the saturated hydraulic conductivity were both highly correlated with grain yield and root length density measured in the soil cores (Gill et al. 2009).

Some four years later in September 2009, we dug a series of soil pits to see what had happened to the amended subsoil. The earlier incorporation of the lucerne pellets in the sodic-clay subsoil had changed the dense ‘sticky’ clay into a well-aggregated friable soil, with topsoil consistency (Figure 1). These rather amazing changes in the subsoil, resulting from the deep incorporation of high rates of organic amendment into the subsoil, occurred quite rapidly with one crop, and appeared to be long lasting having endured for at least four crops.

Figure 1. Changes in the appearance of the subsoil at 30-40 cm depth, sampled four years after subsoil manuring was carried out. Untreated subsoil on the left, treated on the right.

Wheat was again grown at the site in the following year in 2006, which was a drought year when only 178 mm of rain fell during the growing season. The organic amendment resulted in a 55% yield increase in 2006. Similarly the organic amendment resulted in a 61% yield increase in canola grain, with the 3^rd consecutive crop grown after the subsoil treatment. We noted that the grain yield increases in both years were associated with increased water extraction from the deep 40-80 cm subsoil layers late crop growth.

Subsoils, modified by subsoil manuring, result in large, consistent, and continuing increases in crop yields

In 2007 our field trials found that poultry litter (the floor material used in poultry sheds for broiler birds) was found to be equal in effectiveness to the legume pellets, and considerably cheaper. So we named the practice ‘subsoil manuring’ and continued with a new series of GRDC-funded field trials in 2009. The results were somewhat unexpected. Large, consistent grain yield responses occurred year-after-year, so long as we were able to successfully establish a commercial crop over the subsoil-manured plots (Table 2). There were 11 site x season comparisons made between commercial cereal crops and adjacent subsoil-manured crops from 2005-2012 in the higher rainfall cropping zone of south west and south east Victoria (mean annual rainfall > 500 mm). The average commercial wheat yield of 5.8 t/ha increased to 9.4 t/ha on subsoil-manured land. This average yield increase of 3.6 t/ha represented an average increase of 63% averaged over the 11 comparisons.

The rainfall pattern during the grain-filling phase had a direct effect on the magnitude of the crop response. For seasons with ‘dry finishes’ to the crop, with limited rainfall in the September- November spring months, the differences between commercial and subsoil-manured crops were considerably larger than in wet springs. For example in the dry spring of 2012, the average wheat yield increase at three sites was 4.3 t/ha, compared to the increase of 2.4 t/ha of wheat at two sites in 2011, when minimal water deficit stress occurred during grain filling.

Table 2. Summary of crop yields for commercial and subsoil-manured cereal crops, at sites across Victoria, from 2005-2012.

¹ Subsoil manured plots received 20 t/ha (fresh weight) of an N-rich organic amendment (less than 20% moisture content) which was incorporated in rip-lines, 80 cm apart, at a depth of 30-40 cm in the subsoil.

Now the question was why did these yield increases occur with subsoil manuring, and why were they so large in 2012 in the year of the dry finish?

Increases in crop yields with subsoil manuring are generally associated with increases in the use of subsoil water

The results suggest that subsoil manuring ‘opens up’ the clay subsoil, enabling the crop roots to proliferate in the clay and use the previously-unavailable deep subsoil water, late in the growing season. The evidence for this increased use of deep subsoil water is provided in Table 3.  The extraction occurred after crop flowering, which is a crucial stage of crop development, and explains why the large yields occurred on subsoil manured plots in 2012. In practically every case where large grain yield responses occurred in 2012, there was also a significant increase in the volume of soil water that was extracted from the 50-100 cm deep subsoil layer (Table 3). 

Table 3. Increase in crop yields for subsoil-manured crops (over and above the commercial crops), and the loss in subsoil water (mm) in the 50-100 cm subsoil for commercial and subsoil-manured crops, at sites across Victoria in 2012.

¹Subsoil manured (SSM) plots received 20 t/ha (fresh weight) of an N-rich organic amendment (less than 20% moisture content) which was incorporated in rip-lines, 80 cm apart, at a depth of 30-40 cm in the subsoil.

Subsoil manuring increases bucket size (plant available water storage capacity) in treated Sodosol soils

The increased availability of subsoil water with subsoil manuring is highlighted by the root zone’s ability to hold plant-available water, which is known as the ‘plant available water capacity’ (PAWC). This is also known as “bucket size”. This was measured at one site in north east Victoria in 2012, by measuring the differences between (i) the profile water content at the drained upper limit (the maximum water content in the profile following wetting with drip irrigation, and then allowing the soil to drain freely for several days) and (ii) the crop lower limit, when the profile had been dried out by the maturing crop beneath a plastic rain shelter. The subsoil-manured soil profile held 240 mm of water in the top metre, compared to 158 mm for the untreated soil profile. There were minimal differences in water content in the top 40 cm. However the subsoil-manured profile held an extra 78 mm of plant-available soil water in the 40-100 cm subsoil layer, compared to the untreated profile (Table 4).

Table 4.  The plant available water capacity (PAWC) in shallower (0-40) and deeper (40-100 cm) soil layers, in control and subsoil-manured plots at Stewarton (NE Victoria) in January 2013.

Given the greater efficiency with which deep subsoil water is used by the crop, compared to the water in the topsoil layers (Passioura 1976; Kirkegaard et al. 2007), then access to additional deep subsoil water can explain the large increases in crop yields with subsoil manuring, in years with a dry finish to the crop. The commercial crop would be encountering water-deficit stress during grain-filling, whereas the subsoil-manured crop had continuing access to soil available soil water in the subsoil.

Increasing the bucket size of the Sodosol soil is a very beneficial outcome from the subsoil intervention. However these changes require significant and costly human intervention. The issue is whether these interventions are feasible and economic. At the end of the GRDC-funded project in 2012, we therefore undertook a detailed economic analysis by carefully costing the inputs and outputs, and hence the costs and returns that were associated with the sequences of four crops grown at Penshurst and Derrinallum between 2009 and 2012.

An economic analysis of 4 years of field results indicate that subsoil manuring is expensive but profitable

We employed a partial budgeting approach to compare the extra costs and the extra returns for the subsoil-manured crops, and these were compared to the nil-intervention commercial crops, which grew side-by-side in replicated small plots in the paddocks. The per ha estimated costs associated with the 20 t/ha subsoil manuring intervention in 2009, at the Penshurst and Derrinallum sites, were higher than we had previously estimated (Table 5). 

Table 5. Costs for subsoil manuring (at 20t/ha) at the Penshurst and Derrinallum sites in 2009.

In fact the costs of purchasing and delivering poultry litter to the implement that incorporated the litter into the subsoil were disconcertingly high. They amounted to around 2/3rd of the total cost of the subsoil manuring intervention. The estimated freight costs alone for transporting the litter 261 km from Bendigo to Penshurst amounted to $440/ha. The fact that the Derrinallum site was 61 km closer to Bendigo meant that the incorporation cost declined by around $100/ha. Then there was the estimated cost that would be required to screen the litter (to make it flow through the implement), and then load the screened litter into the implement, and these amounted to $150/ha.  

The key finding from this analysis is that the payback period for the investment was surprisingly short (Table 6). In fact the large yield increases in the wheat crop at the Derrinallum site in 2009 (98% yield increase), and the high quality of the wheat from the subsoil-manured plots, meant that the investment was repaid in the first year. At Penshurst, the yield response to subsoil manuring was lower for the 2009 wheat crop, and this resulted in the payback occurring in the second year. Spring rainfall in 2010 was excessive at Derrinallum (a decile 9 year), and this led to the failure of the canola crop in this second year. Less rain fell at Penshurst compared to Derrinallum in the spring of 2010, and this allowed a small canola crop to survive, but only on the subsoil-manured land. Note that fertiliser savings for three years were factored into the analysis.

Table 6. The yield increases, and extra costs and benefits resulting from the subsoil manuring at 20t/ha at the Penshurst and Derrinallum sites in 2009.

Given the prices received for crops between 2009 and 2012, the magnitude of the crop yield increases with subsoil manuring, and their continuation over time, then it is not surprising that the practice was found to be highly profitable. Investing in subsoil manuring in 2009 meant that these farmers were very much better off in terms of financial and economic criteria.  At Penshurst we estimated that the average annual increase in wealth (Table 7), above the conventional way of using the land and capital, would be $546 per ha.  The amount of this annuity was less at Derrinallum due to the canola failure in the very wet spring in 2010.

Table 7. The financial results from subsoil manuring with 20 t manure/ha at the Penshurst and Derrinallum in 2009, based on the extra costs and returns from the 4 successive crops between 2009 and 2012.

¹NPV (Net Present Value) is the total addition to wealth per ha (in 2009 $s) over four years, from subsoil manuring in 2009, over and above other uses of capital that would earn 8% p.a. ²Annuity is the extra annual addition to wealth per ha (in 2009 $s) from subsoil manuring in 2009, over and above other uses of capital that would earn 8% p.a. ³MIRR (Modified Internal Rate for Return) is the percentage annual return over the four years on the extra capital that was invested in subsoil manuring in 2009. This could only be calculated if there was a negative benefit in year 1.

These results show that subsoil manuring has potential to become a worthwhile farming practice in the HRZ where all of this research has been conducted.  Given that there are large areas of cropping lands with sodic clay subsoils in the drier areas of southern Australia, then the question that then arises is whether subsoil manuring might also be profitable in lower rainfall areas.

Can benefits from subsoil manuring be extended into the lower rainfall zones?

The major benefits from subsoil manuring relate to improvements in the ‘rainfall productivity’ for the crop, resulting from the increase in the plant available water capacity of the soil profile (known as ‘bucket size’). There needs to be a clay-subsoil (at least 20% we think) and the clay in the upper B horizon of the subsoil needs to become aggregated to “make the deep bucket”. Then there needs to be efficient capture of enough rain into the profile to hopefully “fill the deep bucket”. So the success of extending subsoil manuring into lower rainfall zones will be a function of (i) whether the deep bucket can be created in the clay subsoil, and (ii) the amount of rainfall that can be captured and stored in the deep bucket for use by the following crop.

We do not know the answers. Research is currently underway in Victoria to provide some of them.  Consequently, this paper we will look at feasible arguments both for and against the proposition that subsoil manuring will be profitable in the lower rainfall zones.

Scenario A:  No, the benefits cannot be extended into lower rainfall zones

Lack of rainfall:  Our research suggests that the aggregation the clay subsoil requires the presence of the organic amendment in the rip-line, plus the presence of proliferating roots in and around the amendment. It is possible that a short growing season in drier regions might curtail root growth around the amendment, and the microbial activity that is associated with amendment decomposition and root growth. This would reduce the effectiveness of the aggregation. We think that you will need at least one big rainfall year in either the 1^st or 2^nd season after subsoil manuring to ensure that you get the subsoil transformation. Alternatively a normal four to five month growing season may do the job? Current research should provide the answer.

Even if the aggregation is achieved, then there is still the question of having enough rain to wet up the subsoil. The size of the financial investment in subsoil manuring requires large increases in crop yields to be profitable. Large yield increases require extra water being made available to the crop. It follows then that the lower the rainfall, the less profitable subsoil manuring will become. It is likely that two years will be too short a period to expect payback for the subsoil manuring investment in the low to intermediate rainfall zones.

Interestingly, a modelling study by CSIRO (Lilley and Kirkegaard 2007) predicted that the subsoil would fail to wet up completely in 21% of years after a low-yielding wheat crop, at Ardlethan (mean annual rainfall of 484 mm) in central NSW, whereas this only occurred in 8% of years at the wetter site of Cootamundra (mean annual rainfall of 624 mm). This indicates that moving into drier country will mean that there will be more years when you cannot wet up the subsoil because there is insufficient rainfall.

Increased chemical constraints in the subsoil: The subsoils of Sodosol soils in the Victorian HRZ are mainly constrained by physical limitations caused by low macro-porosity and perhaps high soil strength in the clay subsoil. These physical constraints are then exacerbated by the high sodicity of the clay (high sodium content).  The subsoils are generally non-saline with a neutral to acidic pH, and do not have high boron concentrations.  This is not the case in the lower rainfall regions where additional chemical subsoil constraints caused by salinity, high alkalinity, and possible boron toxicity are more prevalent.  Where these chemical problems occur, they may constrain root proliferation and microbial activity in the B horizon, and this may reduce the effectiveness of subsoil manuring.

Remoteness from manure sources: The lower rainfall areas are more remote from metropolitan areas, and so low rainfall cropping farms are likely to be a long way from manure sources. This will make the subsoil manuring process, as it is currently practiced, more expensive. Given the reduced profitability with less rainfall, then more expensive manure would further damage the profitability of the practice in the low rainfall regions.

There may be local sources of manure.  In the current research in Victoria, we have used locally available manures such as duck manure and other local composts at some Wimmera and Mallee sites. Whether these will result in subsoil transformation will depend on the rainfall / subsoil moisture at times most conducive for microbial activity. Time will tell what will happen but it appears that the winter and spring rainfall has been a limiting factor in 2014.

Scenario B: Yes, the benefits of subsoil manuring might be able to be extended into the lower rainfall zones

Chemical constraints in the subsoil will not prevent the aggregation of the clay subsoil: It may be that the biological responses to subsoil manuring (manure breakdown, root proliferation, enhanced microbial activity), will still occur in the upper part of the clay B horizon, despite the existence of chemical constraints. This would indicate that microbes that are tolerant to the constraints do exist down in the subsoil or are introduced with the amendment.  This will mean that the transformation of the physical properties of the clay subsoil will still occur. Crops with greater tolerance to these chemical constraints might be grown following the subsoil treatment. Further research is required to determine the extent to which these chemical constraints will limit the aggregation of the clay subsoil. 

Changes to subsoil manuring will make it cheaper and more farmer-friendly: It is likely that further research over the next five to ten years will improve the practice of subsoil manuring.  Hopefully, this research will remove the reliance on animal manures.  On-farm sources of plant biomass, such as crop residues enriched with fertiliser nutrients, might be able to be used as inputs to aggregate the clay subsoil.  If this happens, then the cost of the practice is likely to be reduced. Consequently, there will be a reduced need for the large increases in grain yields in order to make subsoil manuring profitable.  The lower grain yield responses from subsoil manuring in the lower rainfall regions might still be profitable.

Our research to date indicates that cereal crop stubbles have a very high carbon/nitrogen ratio, and so they would need to be enriched with nutrients. The use of on-farm windrow composting with additional nutrient enrichment may be another option to use the stubbles as effective subsoil amendments. The issue is whether such amendments are cheaper than the animal manures, or whether they can be used in lesser amounts (than 20 t/ha).

Improvements in fallow efficiency:  Subsoil manuring success in the HRZ is all about ‘harvesting rainfall in the subsoil’.  Practices such as rain-drop splash, are already enhancing rainfall infiltration.  Combining these with subsoil manuring to aggregate the upper layers of the clay B horizon will assist rain to rapidly infiltrate into the soil profile. 

It is worth noting that in the drought year in 2006 at Ballan, subsoil manuring resulted in an increase in wheat yield from 3.6 to 5.6 t/ha (Gill et al. 2007), despite only 178 mm of rain falling between May and November compared with the normal May-November rainfall of around 325 mm. The reason was that subsoil manuring enabled 150 mm of the 200 mm that fell as summer rainfall in 2006, to be stored in the profile at sowing in May 2006.  This meant that the fallow efficiency was 75%, which was more than twice the fallow efficiency of 35%, which occurred on untreated land.

The point is that if subsoil manuring could work in the drought of 2006 at Ballan, then perhaps it can work in drier regions in average rainfall years.

Conclusions

Subsoil manuring is a new farm practice that increases ‘bucket size’ in the profile of cropping soils with dense sodic-clay subsoils. The practice has resulted in large, consistent and continuing crop yield increases at sites across the Victorian HRZ.  The increases in grain yield from two crops will generally pay for the subsoil intervention. Some HRZ farmers are now beginning to invest in the practice.

It is unlikely that the practice can be extended into drier regions in southern Australia unless there are refinements made to the practice. First there needs to be less reliance on animal manures and this will mean that alternative subsoil amendments need to be used. Processed crop stubbles seem the logical choice here, but will they work and transform the subsoil, and will they be cheaper than imported manures? The second point is that it may be possible for lesser amounts of these processed stubble amendments to be used than the 20t/ha of poultry litter used in the HRZ, to still deliver the subsoil transformation.  Preliminary data would suggest that this is possible.

What is clear is that more research and development work is required. This needs to be a collaborative effort with switched-on, innovative farmers.

Acknowledgements

We are indebted to GRDC for providing funds for the ULA0008 research project. We would also like to thank the land owners/managers of the different farms where the trial sites were located. In this respect we thank Brent Hermann at Penshurst, Peter Yunghanns and John Sheahan at Derrinallum, George Burdette at Wickliffe, Jaikirat Gill at Stewarton and Dassanayake from Melbourne University at Dookie.

References

Clark GJ, Sale PWG, Tang C (2007) Organic amendments initiate the formation and stabilisation of macroaggregates in a high clay sodic soil. Australian Journal of Soil Research 47, 770–780.

Gill JS, Sale PWG, Tang C (2008) Amelioration of dense sodic subsoil using organic amendments increases wheat yield more than gypsum in a high rainfall zone of southern Australia. Field Crops Research 107, 265-275.

Gill JS, Sale PWG, Peries RR, Tang C (2009) Changes in physical properties and crop root growth in dense sodic subsoil following incorporation of organic amendments. Field Crops Research 114, 137-146

Kirkegaard JA, Lilley JM, Howe GN, Graham JM (2007) Impact of subsoil water use on wheat yield. Australian Journal of Agricultural Research 58, 303–315.

Lilley JM, Kirkegaard JA (2007). Seasonal variation in the value of subsoil water to wheat: simulation studies in southern New South Wales. Australian Journal of Agricultural Research 58, 1115-1128.

Passioura JB (1976) Physiology of grain yield in wheat growing on stored water. Australian Journal of Plant Physiology 3, 559–565

Contact details

Peter Sale
AgriBio, Centre for the AgriBiosciences , Ring Road, Bundoora, Vic.  3083
(03) 9032 7460
p.sale@latrobe.edu.au