Tackling subsoil constraints - what are some options?

Author: | Date: 18 Mar 2015

Take home messages

  • The impact of subsoil constraints on agricultural productivity both at the individual farm and nation level is huge.
  • Identifying, understanding and managing these constraints require a variety of approaches; it is often difficult, but if successful has the potential to increase productivity dramatically.
  • Subsoil manuring (SSM) increases crop yields in South Western Victoria.
  • More needs to be known about Gippsland sub soils.
  • In situ subsoil manuring may address some problems with the present reliance on chicken litter.
  • Modelling may aid in identifying those soils/regions that are most likely to respond to SSM.

Background

Subsoil constraints are any physical or chemical characteristic existing below the topsoil that limit crop (or pasture) root access to moisture and nutrients. These constraints are widespread across all agricultural regions of Australia and both locally and nationally have a huge impact on crop and pasture production. Table 1 (below) shows examples of subsoil constraints in different cropping regions of Australia and details of the extent and severity of impacts on crop production.

Chemical constraints include; nutrient deficiencies, acidity, alkalinity, salinity and sodicity.

Physical constraints can be gravel layers and layers of high bulk density soil which impede root growth and are often caused by compaction or sodicity.

Table 1. Examples of subsoil constraints and the regions they exist in.

 Region Nature of constraint Affected area Estimated potential impact on grain yield
South western Victoria Sodicity, acidity, magnesium, etc 2.17 million ha, 50-75% of pasture/cropping land area (Nicholson et al 2015) Up to 3.5t/ha (Sale and Malcolm, 2014)
Victoria Mallee and Wimmera Sodic subsoils on neutral to alkaline sodosols > 90% of cropping land affected Wheat ~ 1t/ha (BCG)
South Australia Various boron toxicity, sodicity - -
SW western Australia Duplex sands  Most cropped areas All crops, 0.2-1.5t/ha (Farre et al, 2010)

Identifying, understanding and managing these constraints both at a paddock and regional level are the biggest challenges to crop and agricultural production in affected areas.

Work undertaken over the past 15 years in Western Victoria has investigated various ways in which the typical subsoil constraints can be addressed. Initially it was thought that deep ripping conducted during dry soil conditions would break up the dense compacted subsoils and allow root penetration and increased crop and pasture production. Experience has shown that whereas short term improvements may have occasionally been seen the subsoils soon reverted to their original condition and any yield advantage was short lived. The addition of gypsum immediately prior to ripping (often referred to as “rip & gyp”) was thought to possibly solve this by allowing gypsum to aggregate sodic clay subsoils and maintain open root pathways for longer. Again experience showed that whereas a slight, short term response may have been noted the quantities of gypsum required to be meaningful were huge and the speed with which it entered the subsoils was very slow. “Biological ripping” using crop and pasture species with aggressive root systems such as lucerne, sula, maize, sorghum and sunflowers have similarly shown mixed results. Their impact on soil moisture has often been significant and in many cases has caused reduction in subsequent crop yield, thereby negating any biological ripping benefits in that year.

This led progressive farmers and researchers at Southern Farming Systems and La Trobe University to consider the addition of large quantities of organic amendments such as chook manure during the ripping process. The theory being that high quantities of organic nutrients would actively encourage root development into the ripped cracks, biological activity and root exudates would then aggregate the sodic soil particles and hold the cracks open and encourage further root development. Thus effectively allowing crop and pasture roots free access to subsoil layers that in the past were unavailable to them. It was hoped that once conducted, it would be a self-maintaining system and hence require only a “once off” treatment.

This access by crop roots to greater volumes of soil effectively “increases the bucket” size in soils; increasing plant available water capacity (PAWC) by as much as 37% (Peries, 2013). Following on from this significant impacts on grain yields in crops have been measured with increases of between 27% and 96% in grain yield across different soil types and rainfall regimes. Water available for end of the season production through the critical grain fill period has been shown to be approximately three times as valuable to crop yield as that which falls over the entire growing season (pers.comm Kirkegaard, 2015).

Peries (2013) conducted many crop trials across the Western districts of Victoria between 2005 and 2011 and in wheat recorded grain yield increases between 42% and 96% with the addition of 20t/ha of poultry litter during the ripping process to the top of the constraining subsoil layer. Two of these sites at Derrinallum and Penshurst were analysed in detail by Peter Sale and Bill Malcolm over four years of crop production and found the initial high investment of around $1,300/ha was repaid by the first or second year and thereafter an increased net benefit of approximately $780/ha pa was seen. By any assessment this is a dramatic increase seen by addressing a subsoil constraint and has generated much interest and enthusiasm amongst grain growers (http://www.grdc.com.au/Research-and-Development/GRDC-Update-Papers/2014/02/The-economics-of-subsoil-manuring-the-numbers-are-out).

Constraints to the constraint

Even though the potential for the practice of SSM to dramatically improve crop yield and profitability has been demonstrated and is now widely accepted, large scale adoption of the practice by growers is dependent on overcoming the following barriers to adoption:

  • Lack of commercial available machinery for broadacre application of SSM.
  • High cost of poultry litter as the SSM amendment (commonly $320/ha for product purchase), representing approximately 25% of total SSM costs.
  • High cost of transport and handling of amendments (commonly $530/ha)' representing 40% of total SSM costs.
  • Expected impacts on availability and cost of amendments with increased demand as adoption of SSM increases: It is estimated (Nicholson et al. 2015) that annual poultry litter production in Victoria is approximately 250,000t which would allow 12,500 ha to be treated at an application rate of 20t/ha. This may be further exacerbated by the phasing out of the caged egg industry. Given that there are some 2.17 million hectares of land suitable for treatment any adoption of the technology will impact on both supply and demand and the subsequent effect on price and availability is expected.

During a recent survey of growers who had hosted subsoil manuring trials, and therefore, had firsthand experience of the process, most identified the production of on-farm amendments as a possible means of solving the high cost of both the amendment and its transport.

With this in mind SFS commenced trials in 2013 examining some aspects of the production and incorporation of amendments produced on farm or from local waste streams. The aim was to reduce costs to a more acceptable level and also to examine ways in which the amendment production process could contribute to other aspects of the farming system.

In situ subsoil manuring

The concept; to grow an amendment on site (either within the paddock or on farm) of an appropriate volume and quality such that when incorporated the anticipated impact on the subsoil constraint and improvement in “bucket size” (PAWC) and crop yield would be similar to that experienced with conventional subsoil manuring using poultry litter.

Concept validation

2013

In the spring of 2013 as part of the Pasture Crop Sequence program a replicated trial was installed at Inverleigh using:

  1. Mixture of peas and wheat at 8t/ha Dry Matter (DM)
  2. Mixture of peas and wheat 16t/ha DM
  3. Lucerne pellets at 10t/ha DM
  4. Control

The lucerne pellets were used as a convenient “subsoil manuring control” since these were used in early subsoil manuring trials and achieved similar results to chicken litter. The peas and wheat mixture was harvested at flowering stage, processed through a forage harvester and incorporated into a trench of approx. 200mm X 300mm which was ripped below the amendment to a depth of approx. 400mm. This was backfilled and allowed to sit over the summer prior to the establishment of a wheat crop in 2014.

Inverleigh experienced a very dry spring in 2014 and little impact of any treatment on yield was seen or expected. However visually during early grainfill all subsoil treatments were much greener and thicker than the control with grain protein at harvest approx. 3% greater. This trial will be ongoing to compare rotation performance of grain yield and quality.

If successful in producing similar results to imported poultry litter then the in situ SSM concept will:

  • Use legumes to provide the majority of nitrogen required in the amendment.
  • Target a balance of species so that the amendment will have a carbon nitrogen ratio of approximately 25:1 which is required for effective breakdown (composting) within the subsoil.
  • Capture the large amounts of carbon and nitrogen lost to the atmosphere during surface composting and manuring for use by crop and soil biota. As much as 50% and 80% respectively is lost during surface composting.
  • Be useful in an integrated weed management (IWM) program where weed seed set of weeds such as ryegrass will be controlled.

2014

In 2014/15 this concept was expanded to include waste streams that could be available locally to farmers and two trials were established at Inverleigh and Wingeel with the following treatments (SSM = subsoil manure):

  1. Wheat control
  2. Legume clover – not SSM
  3. High volume (oat/pea mix) SSM
  4. High quality (clover) SSM
  5. Cereal stubble + nitrogen SSM
  6. Poultry litter SSM
  7. Bio prill (Black Rock) SSM
  8. Compost (municipal green waste)SSM
  9. Compost (animal manure)SSM

These trials have now been established and will be sown to wheat this year.

Where to now?

  • Preliminary modelling using the APSIM, Grassgrow and @RISK programs of crop and pasture response to SSM under different topsoil, subsoil and seasonal (rainfall) conditions has been encouraging in reasonably predicting observed trial results. It is expected that further development in this field will enable growers to determine, with some confidence what response to expect on their farm under a range of seasonal conditions and so make informed investment decisions in SSM technology.
  • Suitable soil characterisations needed to run these models do not exist in Gippsland as they do in most other cropping areas. The establishment of a number of sites for this purpose would be useful.
  • Machinery needed to harvest, process and incorporate the in situ amendment will need to be developed and to this end a 12 month program with the School of Mechanical Engineering, Melbourne University, has been initiated whereby a functional prototype machine will be developed. The commercial development of a machine capable for this purpose should proceed more quickly than is currently being experienced in the production of commercial machinery capable of using poultry litter. The latter has been under private development for more than five years and may still be many years away from commercial reality capable of large scale application.
  • Proceed with the in situ and locally sourced amendment trials as a viable alternative to using chicken litter as a sub soil amendment.

References and useful resources

Farre, I et al, Removal of a subsoil constraint – when does it pay? Dept of Agriculture and Food Western Australia (DAFWA) Australian Agronomy Conference, 2010.

Sale, P & Malcom, W. (2014)The economics of subsoil manuring – the numbers are out. Dept Ag Science, La Trobe University. Dept Land and Food Systems, University of Melbourne.

Peries, R. 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.

BCG.  Armstrong, R and Nutall, J et al Subsoil constraints. Limiting crop profitability on alkaline soils in South Eastern Australia (see Factsheet on SARDI website)

Nicholson, C et al, 2015. Increased farm productivity through injection of organic wastes into subsoils. Part 1, 2 & 3. Southern Farming Systems.

Contact details

David Watson, Agvise Pty Ltd, david@agviseservices.com

GRDC Project Code: ULA00008,