132 Southern Farming Systems | 2014 Growing Season Trial Results

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Subsoil manuring: is the expense and effort justified? Answering the frequently asked questionsCorinne Celestina1

1 Southern Farming Systems

SummaryThe proof of concept with subsoil manuring has been established but growers are not taking up the technology. SFS is working to commercialise the subsoil amelioration technique, trialling locally-available organic amendments and developing machinery suitable for broadacre use.

IntroductionCrop yields in the high rainfall zone (HRZ) of southern Australia are often limited by subsoil constraints that occur on the duplex soils common to the region. Duplex soils consist of a light surface soil overlying dense clay subsoil. These strong texture-contrast soils give rise to subsoil constraints including waterlogging, nutrient deficiencies and sodicity and are acknowledged to limit crop yields by restricting water movement and root growth.

Subsoil manuring is a practice that involves the deep incorporation of high rates of a fertile organic amendment into the subsoil layer in order to overcome these constraints to production. A decade of research on hostile clay soils in the southern high rainfall zone has verified that this technology is capable of significantly and semi-permanently improving soil chemical, physical and biological properties and increasing plant yields and biomass production. Trial results have shown that wheat yields can be improved by anywhere between 2-5t/ha and canola yields 0.5-2t/ha, particularly in years with a dry finish (Peries 2014).

The proof of concept with subsoil manuring has been established but despite this, only one grower has taken up the technology on a commercial scale. The cost of the operation, access to sufficient quantities of a suitable amendment and the lack of commercial machinery remain significant barriers to adoption. Furthermore, many growers still question whether the expense and effort of subsoil manuring is actually justified.

This paper aims to address the ‘frequently asked questions’ related to subsoil manuring and clarify the role Southern Farming Systems (SFS) is playing in commercialising the technology.

How does subsoil manuring work?Subsoil manuring improves the physical, chemical and biological fertility of the soil. It has both an immediate fertiliser effect and a longer-term soil conditioner effect and is sustained by positive feedback loops (e.g. Clark et al 2007; Gill et al 2008, 2009) such as:• Deep ripping shatters hard soil layers and breaks

up the soil without inversion;• Placement of the organic amendment helps

prevent re-compaction and helps reform soil structure;

• Organic substrate provides a source of microbes as well as stimulating microbial activity;

• Bacterial exudates and fungal hyphae improve the aggregation of clay particles;

• Increased root growth into subsoil layers further improves soil structure;

• Improved soil physical properties increase air and water movement and root proliferation;

• Increased capture, storage and availability of deep soil water late in the season especially in dry springs;

• Substrate also provides increased and sustained supply of micro- and macro- nutrients for plant growth and for alleviating sodicity.

• Increased bucket size (plant available water capacity) helps alleviate waterlogging in the short term.

Figure 1. The prototype Peries-Wightman subsoil manuring machine in action at Westmere in 2014.

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gWhy not just deep rip, or deep rip with gypsum?Management of hostile subsoils for improved crop production has commonly involved the use of deep ripping with or without gypsum. Deep ripping involves physically breaking up and loosening the soil to a depth of around 40 cm in order to disturb layers with high bulk density. To prevent re-compaction or a loss of soil structure due to slaking and dispersion when soil gets wet, a binding or flocculating agent such as gypsum or organic matter is needed to help reform and stabilise the structure of ripped soil (GRDC 2009).

In duplex and sodic clay soils, the effects of deep ripping tend to be highly variable and there is limited evidence of yield responses to deep ripping – with or without gypsum (Kirkegaard et al 2008; GRDC 2009). Deep ripping is less successful on heavy clay soils compared to sandy soils, and where the subsoil is naturally sodic and has high bulk density rather than simply compacted due to tillage and traffic (GRDC 2009). Therefore, on sodic soils prone to waterlogging, ripping is unlikely to have significant long-term beneficial effects unless the structure of the soil is simultaneously stabilised through amelioration with gypsum or organic matter (GRDC 2009).

Even then, amelioration of subsoil sodicity can be very expensive, uneconomical and of variable effectiveness (BCG 2008). Gypsum is only effective with high application rates and it has little residual effect on highly sodic soils – therefore it is not a long-term solution to sodicity (BCG 2008) compared to organic matter. Furthermore, higher rates of gypsum are required to ameliorate sodicity in the subsoil compared to the topsoil because sodicity tends to increase with depth (BCG 2008).

At a cost of roughly $80/t for gypsum delivered to the farm gate, plus $40/ha for deep ripping, the total cost for ameliorating a sodic clay subsoil with an exchangeable sodium percentage (ESP) higher than 10 would be in excess of $440/ha (NSW Agriculture 2000; GRDC 2009). The duration of response to gypsum is 2-3 years so the operation would need to be repeated frequently.

Why use poultry litter? Why not just use fertiliser on its own or something like sand?Trials testing deep ripping alone or with sand, inorganic fertiliser blends or gypsum found that there was no sustained benefit to crop yields or plant growth one to three years after treatment (e.g. Hamza and Anderson 2002; Armstrong et al 2007; McBeath et al 2010; Gill et al 2012). In comparison, deep ripping with organic amendments such as poultry litter, pelletised lucerne and Dynamic Lifter® has shown continued benefits at least six years after amelioration (Peries 2014).

These results suggest that for a permanent or semi-permanent change to soil physics, chemistry and biology, an organic amendment should be used. Subsoils are inherently low in organic matter. Increasing organic matter in the subsoil by deep placement of organic amendments can not only enhance chemical and biological fertility but also improve soil structure and bucket size (Clark et al 2007; BCG 2008). In comparison, using inert or inorganic amendments does not target all aspects of the soil; e.g. using something bulky like sand or gravel to hold the rip lines open can increase porosity but does nothing for soil fertility or biology.

There’s not enough poultry litter available for everyone. What other organic amendments can we use?It’s true that there is a finite supply of poultry litter available – in Victoria the annual supply is roughly a quarter of a million tonnes (RIRDC 2014), enough for subsoil manuring only 12,500 hectares of farm land! There are a range of other locally available organic amendments – from both on- and off-farm sources – that may be suitable for subsoil manuring, either on their own or as part of a blended product. These include municipal green waste, biosolids, compost, animal manures, crop stubbles and fresh legume fodder.

SFS currently have two trials underway at Inverleigh and Wingeel which aim to test these locally available waste products for their suitability and effectiveness when used as the substrate for subsoil manuring. Initial results from these trials will be available after harvest in 2015. The trials form one element of two larger commercialisation projects which aim to remove the barriers to adoption of subsoil manuring and enable industry to take it up on farm scale.

Why not just apply the organic amendment to the surface?Deep incorporation of the amendment allows the hostile clay layers to be targeted directly where the constraints occur (Adcock et al 2007). The actual ripping operation is also critical to the process of subsoil manuring as it acts to shatter and disturb the dense soil layers at depth (GRDC 2009) and creates an opening where the organic matter can be inserted.

In general, surface applied amendments – whether organic matter, gypsum or fertiliser – move down the soil profile very slowly, if at all (Armstrong et al 2007; BCG 2008). Incorporating the amendment at depth also prevents losses of nutrient to the atmosphere by volatilisation, which can occur when nutrient-rich products like un-composted poultry litter are surface applied (RIRDC n.d.).

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It’s very expensive. Is it economically viable?Subsoil manuring with 20 t/ha poultry litter is very expensive. However, economic analysis (Sale and Malcolm 2014) based on selected trial results has found that although it is expensive, costing roughly $1300/ha, subsoil manuring with 20 t/ha poultry chicken litter is highly profitable with marginal internal rates of return in the order of 160% and a payback period of 1-3 years. Even at a lower rate of 10t/ha and half the total costs, with subsequently lower increases in crop yields, the technology was found to be economically viable due to the large, consistent and ongoing yield increases (Sale and Malcolm 2014). Like other capital investments, the technology should be viewed as not just important for increasing yields but also for reducing risk.

The majority of the cost of subsoil manuring with 20 t/ha poultry litter was attributed to purchasing the amendment ($320/ha) as well as freight and handling costs ($500-600/ha) (Sale and Malcolm 2014). This represents a significant cost for using off-farm sources of organic amendment. The ability to use organic matter produced on farm and improve machinery efficiencies could eliminate much of the cost currently associated with subsoil manuring.

When will we have machinery for broadacre use?Access to commercial machinery suitable for subsoil manuring on a broadacre scale is perhaps the key barrier preventing the adoption of this technology. There are some prototype subsoil manuring machines available for growers and researchers to trial the technology, including the Peries-Wightman Machine, but these are only capable of ameliorating small areas.

As part of the two commercialisation projects that SFS is involved in, we have partnered with engineers from the University of Melbourne to finalise the design of a commercial machine by the end of 2015, for production in 2016. By the end of 2015 we will have design specifications of the machine as well as functional prototype(s) for demonstration and trial purposes.

Other groups are simultaneously working to develop their own machinery for commercial use but at this stage there is nothing available. SFS is keen to partner with any interested parties in order to bring this technology to commercial reality.

ReferencesAdcock D, McNeill AM, McDonald GK, Armstrong RD (2007) Subsoil constraints to crop production on neutral

and alkaline soils in south-eastern Australia: a review of current knowledge and management strategies. Australian Journal of Experimental Agriculture 47, 1245-1261.

Armstrong RD, Eagle C, Jarwal SD (2007) Application of composted pig bedding litter on a Vertosol and Sodosol soil. 2. Effect on soil chemical and physical fertility. Australian Journal of Experimental Agriculture 47, 1341-1350.

Birchip Cropping Group (‘BCG’) (2008) Identifying, understanding and managing hostile subsoils for cropping: A reference manual for neutral-alkaline soils of south-eastern Australia. The Profitable Soils Group.

Clark GJ, Dodgshun N, Sale PWG, Tang C (2007) Changes in chemical and biological properties of a sodic clay subsoil with addition of organic amendments. Soil Biology and Biochemistry 39, 2806-2817.

Gill Clark Sale Peries (2012) Deep placement of organic amendments in dense sodic subsoil increases summer fallow efficiency and the use of deep soil water by crops. Plant Soil 359, 57-69.

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

Figure 2. Profile of a duplex soil showing a lighter topsoil over a dense clay subsoil

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

Grains Research and Development Corporation (‘GRDC’) (2009). Deep ripping fact sheet. GRDC, Kingston, ACT.

Hamza MA, Anderson WK (2002) Improving soil physical fertility and crop yield on a clay soil in Western Australia. Australian Journal of Agricultural Research 53, 615-620.

Kirkegaard J, Angus J, Swan T, Peoples, N, Moroni, S (2008) Ripping yarns: 25 years of variable responses to ripping clay soils in south-eastern Australia. In: Global Issues, Paddock Action, M.Unkovich (ed), Proceedings 14th Australian Agronomy Conference (Adelaide, 21-25 September, 2008).

McBeath TM, Grant CD, Murray RS, Chittleborough DJ (2010) Effects of subsoil amendments on soil physical properties, crop response, and soil water quality in a dry year. Australian Journal of Soil Research 48, 140-149.

NSW Agriculture (2000) SOILpak for vegetable growers. NSW Agiculture, Orange, NSW.

Peries R (2014) Subsoil manuring: an innovative approach to addressing subsoil problems targeting higher water use efficiency in Southern Australia. Southern Farming Systems 2013 Growing Season Trial Results. Southern Farming Systems, Inverleigh, VIC.

Rural Industries Research and Development Corporation (‘RIRDC’) (no date) Chicken litter as a fertiliser for broadacre grain crops: a user’s guide. RIRDC, Kingston, ACT.

Rural Industries Research and Development Corporation (‘RIRDC’) (2014) Chicken litter: alternative fertiliser and ways to increase soil organic carbon. RIRDC, Kingston, ACT.

Sale P, Malcolm B (2014) The economics of subsoil manuring – the numbers are out. In ‘Proceedings of the 2014 Victorian GRDC Grains Research Update for Advisors. pp. 181-186. (ORM Communications, Bendigo, Vic)

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