Soil amelioration

Author: | Date: 25 Feb 2014

Roger Groocock,

Groocock Soil Improvements

Take home messages

  • The development of delvers and the timing of delving, such as when the clay is near to the plastic stage, results in a much cheaper operation, the option to delve deeper and a dramatic increase in yield.
  • The consistent increase in biomass above and below the ground was also very important, potentially resulting in increased organic matter in the soil.
  • With good ongoing management of our water repellent soils, productivity gains are at least maintaining.


Farming north of Bordertown, SA on sandy soils has been a challenge and a great journey for Sue and myself since we started in our own right in 1979.

When the water repellency issue emerged as a having a serious impact on production, we embraced the clay spreading idea on the deep rising ground.

In the early 90s the Wirrega Agricultural Bureau members noticed that our shallow duplex sandy loam over clay was becoming water repellent. The clay from these areas, when spread and incorporated, gave us great results on the deep sands and so the thought was put forward – why not lift some of the clay with tynes and mix it in the top soil? This was the beginning of clay delving in our area.

The first Landcare funded local trial, using a trench digger 125 millimetres wide and 600 mm deep, spaced at 1.2 metres apart and incorporating gypsum, lifted yields from 1.8 to 4.5 t/ha. This gave us the target of trebling yields.

The trench digger was very expensive, so an economical way to undertake the process had to be found.

While attending a water repellency workshop in Western Australia, I saw Dr Paul Blackwell from DAFWA demonstrate a light weight modified rabbit ripper (delver configuration) which led us to approach Professor Riley from the University of SA to build a delver to handle the job. This was the second Landcare funded trial.

Being in a position to view the rainfall simulation work that David Malinda from SARDI demonstrated to us, we could see the big increase in infiltration rate on the clayed (spread and delved) sites compared to no treatment. The simulated application rate on only a 3 per cent slope showed the small pieces of organic matter floating away with the runoff water.

We started clay delving to avoid water repellence in the shallow soils. This led to the recognition of more important subsoil constraint issues, such as:

  • low fertility – some in defined layers down the profile;
  • compaction – both natural and also wheel compaction;
  • silicon sand particle crust on the dome structured clays;
  • sodic clay layer;
  • bleached sand in A2 layer; and
  • variable pH down profile.

Clay spread country is always going to be limited until organic carbon is lifted, which can be achieved with a crop phase followed by a lucerne based pasture phase before coming back into crop.

The further development of delvers and the timing of delving, such as when the clay is near to the plastic stage, results in a much cheaper operation and the option of going to a deeper depth.

Let nature do its bit, wetting and drying over summer. The clay cracks open and breaks down the lumps before smudging and mixing. Then there is an opportunity to spade green material 300 mm deep in spring and then sow summer producing crop, such as fodder rape or millet.

In 2010-11 our fodder rape was knee high twice and gave two grazings from January to the end of March, putting 100 kg of weight on 90 weaner heifers at $1.60/kg. Then in 2011, the same paddock yielded 2.8 tonne of wheat.

We were able to change crop rotation to enhance soil health, for example we grew beans on soil where previously it was not possible.

Trial results 12 years after treatments in 1994 - operation fine tune

Table 1. Old Landcare trial site, sown to barley in 2006

 Treatment  kg/ha % site mean
Slotting 30cm control 1175  115
Slotting 60cm 1t/ha gypsum 1156 113
Slotting 60cm 1/2t/ha gypsum 1153 113
Ripping 1994 45cm 1t/ha gypsum 1139 111
Ripping 1994 30cm 1t/ha gypsum 1138 111
Ripping 1994 30cm 1079 105
Slotting 30cm 1t/ha gypsum 1029 101
Slotting 60cm 2t/ha gypsum 993 97
Ripping 1994 45cm 981 96
No treatment control 729 71
 Surface 1t/ha gypsum 690 67
 Site mean 1024  
CV% 8.49  
lsd(0.05) 163.7  
While there has been a general lift in returns of 70 to 75 per cent from pre-1990, this year's water logging was not good on the shallow soils. There have been difficult finishes to the recent seasons, and early rain cut-off. The grain yield is one result, but the consistent increase in biomass above and below ground is also a very important result with potentially more organic matter going into the soil. 

Lessons learnt

  • Soil pits are critical to see what is happening.
  • Lifting clay content in A1 and A2 to at least five to 10 per cent which

a) retains water in the root zone longer, and

b) deeper more extensive root structure therefore more organic matter.

  • Need to know the pH and also the chemical make up of the clay.
  • Manage nutritional balance, such as trace elements and pH, using gypsum, lime, and potassium.
  • The placement of trace elements into the A2 zone should be broad band not pencil stream behind deep tynes – alabamas.
  • Need to find deep rooted plants to use modified soils.


With good ongoing management, productivity gains are at least maintaining. These productivity gains have always funded the next year's activities.


Many thanks to all the soil scientists and funding bodies who have helped the whole process evolve.

Contact details

Roger Groocock