Liming techniques
Whether liming to maintain or recover soil pH, surface application (at appropriate rates) is currently the recommended method for most situations (Figure 1). In time, sufficient surface application of lime will treat subsurface acidity.
Recently, incorporation of lime into the soil by mouldboard ploughing or rotary spading has shown promising results and can, if done appropriately, recover acidic subsurface soil to target pH rapidly. Rotary spading generally achieves a better distribution of lime through the soil profile than mouldboard ploughing, which buries the lime with the topsoil, leaving an acidic layer on the surface.
If you are ploughing or spading to remove another constraint, then it is worth considering the opportunity to incorporate lime to treat subsurface acidity. The risks, costs and returns need to be carefully evaluated before undertaking an expensive process such as ploughing or spading just to incorporate lime for quick recovery of acidic subsurface soil.
Good results have been achieved with deep placement of lime by direct injection using modified farm-scale machinery (Figure 2), but it is difficult and time-consuming to achieve the desired distribution, and ineffective and expensive if it is not achieved. Deep placement is only recommended for soils in which subsurface acidity is constraining production and, before it is attempted, detailed consideration should be given to whether it is likely to be profitable.
Surface application
The main aim when applying lime to the surface should be an even coverage of the ground. The spreading width should be approximately 6-8 metres (m) (depending on wind conditions) to get good coverage of fine particles (less than 0.5 millimetres (mm)). Spreading too wide results in uneven treatment of soil acidity. Larger particles will spread up to 15m but the effective distribution is poor and will result in variable change in pH.
Surface applied then deep-rip
It is good practice when applying lime to spread the lime prior to any soil disturbance such as deep-ripping, ploughing or spading. This enables better distribution of the lime particles and greater contact with the acidic soil. Liming before deep-ripping for compaction adds value to both treatments.
Direct injection
This technique deep-places lime during deep-ripping, using modified machinery (Figure 2). Research has shown that successful direct injection is possible (Figure 3 right) and subsurface acidity can be quickly removed as a production constraint. When the distribution of lime is correct, yield responses of 20-30% in wheat are common.
However, increased costs due to machinery modification and slow operation need to be considered. It is difficult to achieve adequate distribution of the lime. Poor distribution can result in the lime being placed below an untreated acidic layer (Figure 3, left), which continues to act as a barrier to root growth. Only where compaction is also a constraint, may direct injection be worth considering.
Mouldboard ploughing
Mouldboard ploughing is usually done to fully invert the soil profile, which can result in very effective weed seed burial, typically achieving weed control in excess of 90%. For lime incorporation, however, some mixing and distribution of the lime through the profile is needed to provide a continuous ameliorated pH pathway from the surface soil into the subsoil for root growth.
Soil inversion using mouldboard plough typically results in the lime being buried in a layer at depth together with the topsoil, above which is a layer of acidic soil brought up from the subsurface. Even if the soil is then limed, it can still take some time for this lime to ameliorate the acidic subsurface soil that is now at the surface creating a barrier to root growth.
Figure 4 shows an example of where mouldboard ploughing achieved excellent inversion but a thick layer of very acidic subsurface soil (pHCa=3.8-4.2) was brought to the surface and despite lime applications before and after ploughing, crop root growth is still being severely restricted by the acidic layer that is sitting above the buried topsoil.
Keeping the mouldboard plough working a little shallower, using a higher speed and setting it up to get incomplete inversion may result in the formation of an angled limed topsoil layer that goes from the surface to subsurface and provides a pathway for root growth into the subsoil. This will compromise weed control and the value of this approach to incorporate lime needs to be weighed up against the costs and compared to other incorporation methods such as rotary spading.
Mouldboard ploughing has proven to be successful for incorporating lime in sandplain soils with mild to moderate subsurface acidity but should be used with caution in more strongly acidic soils.
Rotary spading
Rotary spading is a very effective method for deep incorporation of lime to depths of 30-35 centimetres (cm). The spades bury some topsoil while also lifting seams of subsoil to the surface. While the mixing is not thorough or even throughout the profile there is good distribution of lime right through the cultivation depth with many areas of higher pH where the lime has been mixed into the subsoil (Figure 5).
This mixing action will also improve the contact of the lime with the acidic soil resulting in more rapid and effective neutralisation of the acidity in the soil. Because lime is mixed through a greater volume of acidic soil, higher rates of lime may need to be applied for effective neutralisation of the acidity and the opportunity for lime to move below the cultivation depth.
Usually it is recommended to take the opportunity to incorporate lime when you are spading to ameliorate non-wetting soil rather than to undertake spading for lime incorporation alone. In soils with an extremely acidic profile, spading may be the only realistic option for recovering the pH to reasonable levels. Careful consideration of the costs and benefits need to be undertaken to assess whether or not this would be a cost effective option.
Shallow incorporation
Some farmers use shallow incorporation of 200-300 kilograms per hectare (kg p/ha) of lime at seeding. This may be a convenient way to fit maintenance liming into the farming schedule. Soil testing should be done every few years to ensure sufficient lime is being applied to maintain topsoil pHCa at or above 5.5. Department of Primary Industries and Regional Development research shows that low rates of lime that do not maintain the surface pHCa at 5.5 or above are insufficient to treat ongoing subsurface acidification. This would not be an effective method of recovering acidic soil as insufficient lime is applied.
Shallow incorporation of appropriate rates of lime using offset discs can be beneficial. The cost is 70-80% cheaper than for rotary spading, although the incorporation depth is only 10-12cm. The advantage over surface application is the mixing and faster neutralisation, leading to more opportunity for the lime to then move deeper into the profile (Figure 6).
Efficacy and cost of incorporating lime
The efficacy with which various implements can incorporate surface applied lime depends on numerous factors including:
- Soil type, in particular clay content, which can affect the cohesion, fracturing and flow of the soil.
- Soil moisture conditions - in sandplain soils moisture can help the sand maintain its form (greater cohesion) allowing slots to remain open for longer but may reduce fracturing and soil flow. Wetter soils are softer so this can improve the penetration of soil by implements and reduce draft. Dry surface sand flows easier when worked (less cohesion) which can be an advantage for moving limed topsoil behind soil openers. Optimal movement probably occurs when there is some subsoil moisture but the limed topsoil is dry and can readily flow into the fractured subsoil.
- Implement type - variations between machinery brands such as width of tines, curved or laid-back tines which may promote a lifting (delving) action; curvature (dish depth) of discs are just some examples.
- Implement set up and use - for disc ploughs and mouldboard ploughs setup greatly influences the incorporation result. Having ploughs more open will increase the work rate and the space between ploughshares available for soil to move but may limit the working depth.
- Speed of operation - higher speeds can result in more soil throw and mixing but may require a shallower working depth.
The table below gives a brief description of various tillage implements, the mixing action of each implement, typical working depth and some supplementary information on the efficacy of lime incorporation. Note that every situation is different and this information is designed to be used simply as a guide when considering options for incorporating lime.
Incorporation implement (approx. cost range $/ha) | Overview of tillage action by implement | Typical working depth (cm) | Depth of lime incorporation achieved (cm) | Lateral spread of lime and incorporation efficacy *Images with purple soil are from application of universal pH indicator to identify placement of lime within the soil profile |
---|---|---|---|---|
Deep ripping ($45-55/ha)
| Narrow strong deep working tines used to break out subsoil compaction | 30-40 | 10-15, variable | Limed topsoil tends to be mixed in the surface layer where the tine passes through but generally the slot behind the tine closes rapidly so there is little opportunity for limed topsoil to fall deeper into the subsoil
|
Shallow leading tine ripping ($40-50/ha)
| Ripping with shallow leading tines allowing deeper break out by deeper working, trailing tines | 40-50 | 10-15 | Limed topsoil can be incorporated better due to multiple tines disturbing the soil in the one pass, although incorporation is still limited as tines are narrow and slots close rapidly behind the tines
|
Shallow leading tine ripper with topsoil inclusion plates ($40-55/ha)
| The addition of topsoil inclusion plates to the rear tines allows for the movement of topsoil rich in organic matter to depth | 40-50 | 30-40 | The topsoil inclusion plates keep the slot behind the rear ripping tines open for limed topsoil to fall into the subsoil. The result is vertical seems of organic matter rich/limed topsoil behind placed to depth behind the rear ripping tines
|
Ripper with wings ($45-55/ha)
| Wings mounted on ripper tines that operate below the soil surface when ripping which creates greater soil disturbance as they tend to lift subsurface soil | 30-40 | 20-25 | Limed topsoil can flow into the space opened up via the lifting (delving) action of the wings. Lateral incorporation is improved with 'tongues' of topsoil up to 8cm wide on either side of the ripping tine where the wings had passed |
Ripper with 'Horwood' opener ($45-55/ha)
| Plates extend behind the ripping tine to hold open the soil slot longer operating just below the topsoil | 30-40 | 20-26 | Holding the slot open for longer below the soil surface allows limed topsoil to drop into the subsoil. A continuous stream of limed topsoil was achieved but the slot narrowed with depth being only 1-2cm wide at depth |
Ripper with 'Railway Fishplate' opener ($50-60/ha)
| Plates bolted onto the side of the ripping tines effectively increasing the tine width and the degree of soil disturbance | 30-40 | 19-23 | More disturbance resulted in more mixing. Width of mixing was increased up to 14 cm in some instances but this was variable |
Deep digger® ($60-70/ha?)
| Large wide curved tines in a V-shaped arrangement capable of ripping deeper than standard deep rippers | 40-60 | 23-25 | Wider tines and some delving action allows some topsoil flow around and behind the tines but overall incorporation is fairly minimal for the cost. Tines would need to be modified to achieve better incorporation
|
Offsets ($40/ha)
| Standard offset (two-way) discs that cultivate the topsoil | 10-15 | 10-15 | Very little limed topsoil is incorporated into the subsoil layers due to inadeqaute working depth. Mixing will still improve the reaction of the lime in the topsoil that may then allow for faster lime movement into the subsoil
|
Large offsets ($50-60/ha)
| Large offsets (two-way) discs, typically greater than 70cm in diameter, that can cultivate deeper than standard offsets | 24-25 | 24-25 | Limed topsoil is effectively incorporated to the working depth. Some layering occurs on an angle from the surface but generally the mixing is good. Visually it appears about two-thirds to three-quarters of the profile is treated to the working depth. The incorporation depth can be less if hardpans or gravel layers prevent disc penetration |
One-way plough ($30-40/ha)
| Discs throw the soil one-way, can achieve partial turning of the soil but mixing occurs as soil tumbles off the disc | 15-25 | 15-25 | Limed topsoil is partially mixed and layered on an angle from the surface because of the cultivation process. Despite partial inversion and layering continuous pathways of limed topsoil are still available for root growth. About half to two-thirds of the topsoil is buried. Can bring acidic subsoil to the surface so more surface lime may be required post-ploughing. |
Rotary spader ($120-150/ha)
| Rotating spades bury some topsoil while lifting up some subsoil. About two thirds of the topsoil is buried below 10 cm. Soil tends to take on a marbled apperance | 28-35 | 28-35 | Very effective at mixing limed topsoil into the subsoil. Does lift some acidic subsoil to the surface so additional lime may be required in subsequent years. Because spades are offset and overlapping lime is incorporated through the entire profile to the working depth, although pockets of acidic subsoil may remain.
|
Mouldboard plough ($100-150/ha)
| Curved mouldboard shares lift, roll and invert the soil aided by skimmers that scalp the topsoil into the base of the furrow. Square ploughs achieve a similar result | 28-35 | 28-35 | Inversion buries limed topsoil in a layer and can bring a thick layer of acidic subsoil to the surface that needs treating with more surface-applied lime. Continuous ameliorated pathways are not always present if inversion has been effective.
|
TopDown® plough (>$100/ha?)
| A cominbination of leading offset discs then curved ripping tines, levelling discs and packers | 20-35 | 20-25 | Off set mixed well through to their working depth. Curved ripping tines then open a slot allowing surface soil to fall into 20-25 cm. This incorporation is a broad 'V' shape beginning at the width of the tine at the surface and finishing to a point at 20-25cm. Thr curved tines also lift acidic sub surface soils to the surface in seams. Not as effective in gravelly soils or soils with hard pans or layers that are difficult to penetrate
|
Timing of liming
It is sometimes difficult to fit liming into an already busy schedule. Most liming is carried out in autumn after summer soil testing results have been analysed. While there is some benefit of incorporation during sowing or working-in by stock of lime applied earlier, if time is a constraint, liming at other times should be considered.
Spring liming is a good option for pasture and allows the workload to be spread (Figure 7). Soil microbes responsible for the breakdown of soil organic matter and mineralisation of nitrogen are inhibited by acidic soil (Soil microbial activity). Liming in spring allows benefit from the warm, moist conditions, which favour microbial activity. Spring liming of pastures allows time for the lime to start neutralising soil acidity before a cropping rotation, particularly if there is summer rainfall.
With tramline farming systems, lime can be applied at any time without any detriment to the crop.