Renick Peries, Research Scientist with the Department of Primary Industries in Victoria, reports on work that is tackling hostile subsoils in Southern Australia
Work by Southern Farming Systems (SFS) in the past five years has advanced our understanding of what happens to soil under raised beds.
Raised beds alleviate waterlogging and reduce compaction through controlled traffic. When crops or pastures are grown on beds, the soil in the beds is rapidly re-aerated after heavy rainfall events. These repeated events of wetting and drying, together with biological activity in the root zone, appear to help build aggregates in the soil resulting in reduced soil bulk density, improved porosity and consequently the storage of increased amounts of plant-available water (PAW).
Our work on vertosols (cracking clay soils) has also shown that following the installation of raised beds, the porosity of the soil can improve even below the depth of the initial tillage. Consequently there appears to be an improvement in hydraulic (water) conductivity at depths that would favour movement and storage of PAW.
However, we are yet to successfully address the issue of low harvest indices of crops, which is most likely a result of inadequate PAW during grain-fill to meet the higher demand of bigger crops produced on raised beds.
It has also been observed that although the soil structure at depth under beds is improving, there sometimes appears to be a problem zone under the bed surface that occurs around the furrow depth.
Initially, following the tillage undertaken when the beds were formed, aeration and conductivity increased at this depth. However, if water sits at this depth in the furrow for extended periods, some of the initial benefits appear to reverse.
The saturation of this band of soil can lead to anaerobic conditions, which slow biological activity. The problem is further exacerbated in dispersive soils and it can lead to a decline in macro-porosity. It can also lead to fewer of the large pores that move water rapidly and provide oxygen for root respiration.
In our current work, we are concentrating on achieving the connectivity between the topsoil and subsoil that is crucial to enhancing the soil structure. The incorporation of organic matter, or any other material that could improve and maintain the porosity at that depth, could contribute to this enhancement.
In 2003, SFS conducted trials at Gnarwarre on slotting the soil with poultry manure and wool scour waste. The operation also makes it possible to use other ameliorants at depth, such as gypsum and straight fertiliser phosphorus, where sodicity or lack of root proliferation were issues.
The results were encouraging, with most treatments giving better yields than the control. In 2004, we also followed a mechanical approach where we attempted to use a slurry of poultry manure in a slot that was created behind the tine of a deep ripper. These treatments clearly showed the qualitative differences brought about in the proliferation of roots and increased biological activity in the subsoil.
As yet there have been no significant yield responses, and this is thought to be the result of sub-optimal rainfall conditions in the last three years.
In 2005 we plan to use two soils to further test our hypothesis and also to test the modified deep ripper on two different subsoils. Both soils are marginally sodic at depth.
However, the soils are different in the amount of clay in the profile. The heavy clay vertosol (Mt Pollock) is just coming out of a lucerne phase and its subsoil structure is very different to the "Moorabool Viaduct" soil (Inverleigh), which is very low in clay in the top 50 centimetres.
We are concentrating mainly on the connectivity between topsoil and the subsoil (10 to 30cm below the surface of beds) and believe that our approach will throw some light on soil specific management options for these problem subsoils.
GRDC Research Code SFS00007
For more information: Dr Renick Peries, 03 5226 4827, firstname.lastname@example.org
[Photo by Brad Collis: Renick Peries: advancing the understanding of what happens to soil under raised beds.]
[Photos: A slurry of poultry manure is slotted into the soil behind the tine of a deep ripper.]
[Photo: Improved root proliferation in the subsoil where poultry manure has been slotted.]