Subsoil manuring comes of age in HRZ
Date: 06 Jan 2015
Raised beds helped to revolutionise cropping in the high rainfall zone (HRZ) of southern Australia by reducing the risk of waterlogging.
However, the next step in not only mitigating waterlogging but breaking up the tightly-packed clay subsoil and improving the soil moisture-holding capacity in the HRZ could be achieved through subsoil manuring.
For close to a decade, trials across the Victorian HRZ have shown that wheat yields can be improved by anywhere between 2 tonnes a hectare to 5t/ha and canola yields 1-2t/ha, particularly in years with a dry finish to the growing season.
In the early 2000s, staff at the Department of Environment and Primary Industries (DEPI) Victoria – including project officer productive soils Dr Renick Peries – observed crops established on raised beds producing huge amounts of biomass which was not translating into grain yield.
The raised beds had improved the rooting depth of plants, but in the hostile clay subsoils which are common in the Victorian HRZ, the crops were not accessing the large amounts of water held within the heavy clay matrix that was not available to them.
“There was a lack of connection between the topsoil and the subsoil, which got me thinking about ways to make that connection,” Dr Peries said.
Influenced by the idea of some farmers in the HRZ spreading chicken manure on their crops at rates of 5t/ha, Dr Peries began experimenting with the idea of digging trenches on some of the raised beds and burying chicken manure in them.
“I thought that if I can get this organic matter and microbes to the roots of the crop it might provide a self-sustaining system of soil structure change,” he said. “Unfortunately the 5t/ha wasn’t an accurate figure and at the time we went through a period of drought – the millennium drought – where we didn’t see a lot of improvement in the soils.”
But fast-forward to the present day and that initial idea of getting organic matter and microbes down to the roots of the plant has proved to be fruitful for the HRZ.
Research funded by the Grains Research and Development Corporation (GRDC) and carried out by DEPI, La Trobe University and Southern Farming Systems has shown that even at the high rates of between 10-20t/ha, subsoil manuring is economically feasible.
There are also a handful of prototype and commercial machines – “subsoilers” – doing the rounds on farms in southern Victoria’s for early adopting growers to trial subsoil manuring on their own farms.
Economics and profitability
Dr Peries said costs of about $1200-1500/ha to subsoil manure might sound prohibitive, but cost-benefit analyses had shown that those expenses could be recouped in less than three years through extra yields.
An economic analysis of subsoil manuring compiled by Associate Professor Dr Peter Sale, La Trobe University, and Associate Professor Bill Malcolm, University of Melbourne, concluded that subsoil manuring at 20t/ha or at 10t/ha was profitable and financially feasible.
Using data from farms at Penshurst and Derrinallum, both in south-west Victoria, the analysis found farmers who invested in subsoil manuring in 2009 were better off financially, comparing land that was subsoil manured versus land conventionally growing crops (tables 1 and 2).
Table 1: The yield increases, and extra costs and benefits from 20t/ha of subsoil manuring at the Penshurst and Derrinallum sites in 2009.
Costs and benefits
|2009 Wheat||2010 Canola||2011 Wheat||2012 Canola||
||2009 Wheat||2010 Canola||2011 Wheat||2012 Canola|
|Yield increase (t/ha)||2.8||1.2||4.5||2.0||
|Extra costs ($/ha)||1398||27||67||39||
|Extra benefits ($/ha)||830||789||1184||1100||
|NET BENEFIT ($/ha)||-568||764||1201||1061||
Table 2: The yield increases, and extra costs and benefits from subsoil manuring with 10 t manure/ha at the Penshurst and Derrinallum sites in 2009
costs and benefits
|Yield increase (t/ha)||2.0||0.6
|Extra costs ($/ha)||717||7||43||22||674||0||38||45|
|Extra benefits ($/ha)||678||398
|NET BENEFIT ($/ha)||-39||377||757||308||228||66||381||617|
But what might drive a farmer to consider subsoil manuring?
Dr Peries says clay subsoils with tightly packed together clay particles which have very little pore space for roots to move or for water to infiltrate are a prime target for subsoil manuring.
“In order for the subsoil manuring technology to work, the soil should have a minimum clay content of around 20 per cent, as that is the material that helps in the formation of aggregates,” he said.
“This technology helps improve the bucket size of the soil, or the soil water-holding capacity. Some areas only have 5pc or less of their natural macro-porosity left in the subsoil, resulting in poor aeration and impeded root growth, which is where we find crops haying off and greater loads of stubble on the ground which causes issues in itself such as a greater habitat for slugs and snails.
“What we end up getting after subsoil manuring is a larger bucket size and soil capable of holding three times its original capacity of plant-available water, which can be stored deep in the profile to be used in the spring grain-fill period, therefore achieving much higher water use efficiency.”
Organic matter, manures and rates
Besides chicken manure, Dr Peries has also looked at subsoiling duck manure and any other locally available composts, with the thinking being that chicken manure will not be available everywhere that subsoiling is conducted.
While results are still inconclusive as to which manure or organic matter might be best to use, Dr Peries says they do need to find a material that is more abundantly available should the popularity of subsoil manuring increase.
“We are hoping one of the materials available is wheat and barley stubble,” Dr Peries said. “It contains a lot of carbon, but it needs to be enriched with nutrients. There has been some work done by a PhD student which has shed some light on where to proceed with this, but we haven’t got the resources at the moment to follow-up and bridge the knowledge gaps.
“We are looking at impregnating the stubble with chemicals and nutrients and investigate ways to increase the density of the material to get it flowing through the machinery better. But for farms close to cities, there are options such as bio solids and municipal green waste which some councils are saying can be delivered to farms if they want to do the composting.”
Research into what rates to apply and where is also continuing.
“We’re not really at a stage where we can say which soils will respond to 10t/ha and which will respond to 20t/ha,” Dr Peries said. “That work is being done at the moment by looking at what has been done over the years to provide guidelines to farmers.
“From a series of years of experiments between 2009 and 2012, we’ve got some sites responding to 10t/ha and giving a significant and profitable yield, but on those same sites rates of 20t/ha have given stronger results. However, some sites haven’t responded to 10t/ha but have responded to 20t/ha.”
While Dr Peries conceded they had not yet been provided the resources to get into the details of soil biology as a result of subsoil manuring, he did have some educated estimates on the process that was occurring below the topsoil.
“The manure will help the water storage – it will improve the conductivity of the soil so in cases of heavy rainfall it will infiltrate deeper into the profile and stimulate more root growth and then the microbes will begin to react on the root exudes,” he said.
“Then, as a result of a number of chemical compounds including polysaccharides, large clay structures begin to break down and start to form aggregates, which increases the macro pores by up to 20-30 times.
“When there is greater macroporosity in the soil it not only helps to hold water but it drains water better and prevents waterlogging as the water is taken away rapidly and stored in deeper profiles of the soil.
Dr Peries said as a result of this, farmers were seeing an increase in the size of the soil water ‘bucket’.
Repeat … or not?
As for a second treatment of subsoil manuring, he said growers were still seeing the benefits of their first treatment almost 7 years ago.
“They are reporting incremental benefits from 2005 right up until today,” he said. “Who knows? Ten years down the track we might need to go to a deeper layer, but as it is, we feel they won’t need to repeat the practice for 7-10 years.
“As long as the crops are growing, the roots are reacting and the clay is transforming, which will be happening over a number of years, it won’t need to be repeated. One of the earliest sites set up in 2009 bordering Vic and SA – from the road you can still see the difference between the control and where it was subsoiled.”
Depths and machinery
Dr Peries said the depth of subsoil manuring depends on where the topsoil meets the subsoil, which he said could be anywhere from 5-10 centimetres.
However, he reminded growers considering subsoil manuring to be cautious as not all soils had a clay layer at a uniform depth. In such cases, he advised growers to take soil cores.
The greatest depth that the subsoil manuring trials had to go to was 40cm.
Dr Peries believes that as subsoiling machinery becomes more available, the practice will become more widespread. He says the same thing happened when raised bed cropping was at its peak and in southern Victoria, machinery dealers were bombarded with requests from growers for machinery to make raised beds.
One of the machines currently operating is the Peries-Wightman subsoiler, named after Dr Peries and agronomist Bruce Wightman. However, it is only a small, twin-boot machine capable of doing 0.5ha an hour.
The first commercial subsoiler was produced in 2012. It features a six-metre bar and an air suction system which takes manure from a trailer and delivers it into the soil, similar to an airseeder.
Dr Peries says land earmarked for subsoil manuring needs to be mostly dry so a “shattering” process occurs in the clay, allowing the manure to get in, but some subsoil moisture was needed in the profile.
He says in most cases, farmers subsoil manure in January or February. However, the practice is also gaining traction with livestock farmers wanting to improve irrigated pastures who would chose a time when the soil is not waterlogged.
Research is continuing into the soil carbon benefits of subsoil manuring and the potential of the practice to sequester carbon, in line with the Federal Government’s response to climate change.
Dr Peries said the major research will be into soil biology as a result of subsoil manuring.
“We will look at what it is in the manure that is driving the change,” he said. “From that, there is the potential to develop a liquid concentrate which could be dropped into the subsoil, which would also mean we could use less powerful machinery.”
Taking subsoil manuring to the Mallee and beyond
While subsoil manuring was born and bred in the high rainfall zone of southern Australia, a new project is demonstrating the practice in the lower and medium rainfall zones to determine whether there are benefits on improving the productivity of soils.
In an effort to address the declining soil condition and subsoil constraints in areas of Victoria such as the Wimmera, Mallee, North Central and South West, subsoil manuring is being trialled alongside controlled traffic farming to see if it will benefit the long-term productivity, sustainability and profitability of soils in those regions.
The two-year project, funded by the Federal Government’s National Landcare Program, is being led by the Victorian Department of Environment and Primary Industries (DEPI) with partners including the Victorian No-Till Farmers Association, La Trobe University and the North Central Catchment Management Authority.
The demonstrations are located across Victoria at Ouyen, Hopetoun, Propodollah, Toolondo, Rhymney, Charlton, Kamarooka and Werneth.
DEPI team leader, land management and livestock extension Melissa Cann said although it was only early in the project, visual differences in crops on soils that had been subsoil manured were observed.
“We’ve seen positive effects on sandy and sodic soils in the Hopetoun area, which we hope translates into yield, but it's too early to tell yet,” Ms Cann said. “Where the ground has been ripped and the manure placed, there has been a lot more plant root development.”
While visual inspections had been promising, Ms Cann said it was too early to be too confident about any positive impact from subsoil manuring.
“Most important is the practice’s cost effectiveness,” Ms Cann said. “On one hand it could have a positive long-term effect on improving subsoil constraints, however accessing manure or compost will weigh heavily on the economics. Freight is the biggest cost."
“We’ve still got another growing season to go, so by end of the project we will have two years’ worth of data. By then we might be able to get a better picture of how subsoil manuring has improved the subsoil, whether we’ve been able to increase the rooting depth and if crops are able to hang onto more moisture in deeper sands by having compost below the surface.”
Of particular interest at the Mallee sites will be whether subsoil manuring improves the soil water-holding capacity and nutrient use efficiency in the deep sands. The Kamarooka sight has a focus on the benefits of subsoil manuring on the remediation of salinity.
Data collection will focus on the physical and chemical properties of the soil and plant growth and yields over the two growing seasons in comparison with the rest of the paddock.
Fast-tracking adoption in Victorian HRZ
Southern Farming Systems (SFS) is continuing its research into subsoil manuring with projects focusing on the commercialisation of the concept.
SFS research and extension officer Corinne Celestina says the concept of subsoil manuring has been proven through research by the Victorian Department of Environment and Primary Industries (DEPI) and La Trobe University. However, they are working to grow the practice on a broad scale through a grant from Regional Development Victoria (RDV) and federal funding in the form of an innovation grant from the Department of Agriculture, Fisheries and Forestry (DAFF).
There are lots of barriers for growers to the adoption of subsoil manuring, mainly because of the cost and access to chicken manure, with only one commercial grower adopting the practice. For that reason, we are developing and testing other amendments that are more cost-effective and accessible than chicken manure, such as stubble, and looking at brought-in materials such as green waste, bio solids and manures.”
These materials are being trialled on a large scale at Westmere, while there are two smaller field trials at Inverleigh and Wingeel, all in Victoria.
“The other key aim of the project is to design commercial subsoil manuring machinery, in which we have enlisted the help of some people from the University of Melbourne and to develop suitable machinery and use it to test different amendments for use in manuring," Ms Celestina said.
Ms Celestina says, ideally, materials used in subsoil manuring will come from on-farm.
“We’re looking at whether farmers can grow a legume crop into cereal stubbles, windrow it when its green and then collect it and put it underground, rather than carting in chicken manure and putting that underground,” she said.
“The idea with commercialisation is to tackle barriers to adoption. Obviously not everybody can access chicken manure and almost nobody has the machinery to subsoil, so we’re working on overcoming all those barriers.“At the AgriFocus Field Days we had so much interest in subsoil manuring, so if we can get some machinery developed and find the right amendment, hopefully it will take off.”
Dr Renick Peries, firstname.lastname@example.org
Melissa Cann, DEPI, email@example.com
Corinne Celestina, firstname.lastname@example.org
GRDC Project Code ULA00008