AgMemo Southern Agricultural Region

Topsoil slotting - an opportunity if deep ripping in 2017 

Topsoil slots formed using topsoil inclusion plates bolted behind deep ripping tines in yellow sand.

Topsoil slotting is a technique being developed to move topsoil and other soil amendments down the soil profile into the subsoil while deep ripping. 

Topsoil inclusion plates are bolted behind the deep ripping tine at a suitable depth to run at the base of the topsoil, approximately 100-150mm below the soil surface.

The plates hold the slot open to allow the topsoil down the slot.

Topsoil slots formed in a pale sand sown with wheat.

Benefits

While topsoil slotting aids incorporation of soil ameliorants such as lime and gypsum their original purpose was to help incorporate organic matter to help stabilise deep ripped soils.  

Ripping responses in yellow sands and heavier, sodic soils and morrell soils, that can naturally self-settle under wetting and drying, can last longer with organic matter at depth. 

Organic matter in the topsoil and new root growth and microbial activity can help minimise soil collapse and hardening. 

The hardening is due to ‘effective stress’ from water menisci between soil particles and aggregates and clay dispersion.

Department of Agriculture and Food, Western Australia (DAFWA) and Grains Research and Development Corporation (GRDC) funded trials in 2015 that showed a benefit of 1.0 to 1.7 tonne/hectare (t/ha) at four of six trial sites using inclusion plates in various soil types. 

The ripping for these trials was done with dry topsoil, moist subsoil and a shallow leading tine configuration on the ripper to rip to 500mm or deeper.

Five trials across the state in 2015 highlighted several other benefits that could help farmers manage dry seasons:

1. Creating ‘underground topsoil’ that remains moist and functioning for crop and pasture nutrition when the ‘upper topsoil’ has dried out in the growing season. This done without soil inversion or mixing and preserving sufficient anchored stubble to minimise the risk of wind erosion.
2. Encouraging very vigorous crop and pasture root growth below the surface to increase the amount of root that can supply water rapidly to flowering heads on very hot winter and spring afternoons and help minimise the loss of grains and grain size from heat shock.
3. Immobilising some toxic aluminium ions by mixing in humic and fulvic acid from the soil organic matter. The result of chelation where metal ions are locked up in ring structures of these complex organic molecules derived from the decomposition of plant materials. This may be sufficient on dune sands and deep white sands which contain little clay and are acid. Extra immobilisation may be needed from gypsum, in the short term and pH increase by carbonates for the long term.

Top soil slotting may also provide an opportunity to establishing ‘zonal tillage’ system with rips and slots spaced at about 600-700mm spacing (approx. 2-3 crop row spacings) that gives short term and longer term water supplies from the subsoil to crops and pastures trying to survive heat stress after sufficient previous rain.  

An added benefit of using wider row spacing is being able to rip more country in one season and achieve a similar benefit to doing smaller areas with narrower tine spacing. 

This theory was explored in a trial at Binnu in 2016 with 500mm and 1m row spacing. 

Results will be presented in a paper from Research Updates in Perth in February 2017.

Setting up plates on the ripper

The key design features of the topsoil inclusion plates for the research trials were:

• a rear taper (a tear drop appearance from above) to reduce cohesion of moist and sticky soil
• bolt to help hold plates apart
• lower plate cut-out to avoid fouling rear frame
• multiple holes for depth adjustability.

DAFWA’s trial shallow leading ripper developed by Paul Blackwell with topsoil inclusion plates attached bolted to the back of the tine rip below 500mm and slot topsoil with or with out ameliorants into the subsoil.

The plates must be set at a depth to run at the base of the topsoil usually about 100mm.

Topsoil slotting in action. The plates, set at a width of approximately 100mm, hold the slot open and allow topsoil to fall down the rip line.

Topsoil slotting plates do add draft to the deep ripper therefore optimal ripping width is about 4-6m (a half or a third the width of the seeder), particularly if ripping below 400mm. 

Alternatively to rip at 12.2m/40ft options include use less tines, reduce plate depth, run plates on only half the tines each season or pre-rip shallow then come back in and rip deeper with plates. 

A risk of using inclusion plates on less tines is that you may see a striping effect in the crop.

A hydraulic cage or heavy rubber tyre roller pulled after the ripper is essential to firm the surfaces and smooth any ridges or break up soil clods particularly in heavier soil types like grey clays or red loams. 

On-farm observations

In 2016 more than 50 rippers were fitted with topsoil slotting plates. The following are some observations and adaptions made by growers and manufacturers running plates:

1. Topsoil plates made of mild steel can wear quickly particularly in fine sands after about 200ha of ripping. To alleviate this problem growers have welded sacrificial steel on the outside leading edge of the plate, or made plates from hardox steel. The hardox plates showed little wear after 400ha of ripping in sand at Ogilvie. The wear is soil type dependent. Finer sands will often wear more quickly. Use experience with tine wear as a guide for suitable material/coatings.
2. If the soil is too wet, smearing can occur on the edges of the slot and wet soil can stick to the plates. Inverting the plates in moist soil conditions can also help reduce soil sticking and allow more topsoil to drop down the slot. Testing this theory at Brady Green’s at Nabawa also found that at slow speeds more topsoil falls down the slot and faster speeds more mixing occurs (Figure 1). Longer plates at normal speeds should have the same effect as reduced speed. 
3. The between furrow ridging left by plates is worse in moist soil conditions and a coil packer is not heavy enough to smooth the ridges.
4. Taper the plates slightly towards the back to help with soil flow and sticking on the plate face.
5. If the soil is moist reducing plate depth can mean more plates can be fitted to a wider ripper so it can be pulled and get the benefits of soil falling down and as the soil slot stays open longer before collapsing.
6. The soil after deeper ripping with or without plates is very soft and can cause problems with plant establishment due to poor depth control or bogging of the bar wheels or press wheels depending on the bar design. Some growers found reducing the tyre pressure in the bar wheels and locking the castor wheels helped reduce the bogging. 
7. The bolt holding the plates apart can increase stubble blockages. It is possible to run the plates without the bolts however they can help hold the plates apart if there are passing rocks.

Figure 1. The effect of inclusion plate orientation and tractor speed on the profile of the topsoil slot.

Observations suggest topsoil slotting works best when the topsoil is dry and the subsoil is moist, therefore it may be an opportunistic method of renovating soils given conditions at the time of ripping.

Further development of topsoil slotting plate design and understanding of the effects is required.

Acknowledgements

DAFWA’s GRDC funded project DAW00243 “Minimising the effect of compaction on crop yield”.

Co-author Dr Paul Blackwell, Research Officer, DAFWA (retired). We wish to thank Paul for his significant contribution to soil research in Australia for more than 25 years.

For more information contact Bindi Isbister, Development Officer, Geraldton on +61(0)8 9956 8532 or Wayne Parker, Research Officer, Geraldton on +61 (0)8 9956 8511.