Applying lime

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.

Figure 1 Surface application of lime south of Northam. Sufficient surface applied lime can prevent the subsurface becoming acidic

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.

Figure 2 Extensive modification of machinery is required for good distribution of deep-placed lime

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.

Figure 3 Soil profiles of deep-placement DAFWA lime trials at Maya and south Bodallin stained with universal pH indicator. The purple colour shows poor distribution of the lime (left) and good subsurface distribution (right; note that additional surface-applied lime can also be seen in the topsoil)

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 (pH_Ca=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.

Figure 4 Poor distribution of lime after mouldboard ploughing at Wongan Hills can be clearly seen where universal pH indicator has stained limed soil purple and green. Lime has subsequently been applied to the surface to treat the acidic subsurface soil that now covers the limed 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).

Figure 5 Rotary spading achieved good distribution of lime in the soil profile (stained purple and green by universal indicator) and amelioration of soil acidity to 35cm at Three Springs

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 pH_Ca 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 pH_Ca 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).

Figure 6 Shallow incorporation of lime using offset discs achieved good distribution of the lime (stained purple and green by universal indicator) and was effective in quickly ameliorating topsoil acidity at Badgingarra

Efficacy and cost of incorporating lime

There are a range of options available to incorporate lime to depth. It's important to carefully consider the applicability of different options in different circumstances

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.

Table 1 Details of tillage implements and a summary of their efficacy when assessed for lime incorporation

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)   A standard deep ripper used to break out compacted subsoil	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   Root proliferation in a rip line. Although limed topsoil is often mixed in the surface layer, there is variable movement of lime down the profile
Shallow leading tine ripping ($40-50/ha)   This shallow leading tine ripper has four sets of tines and is typically used for research purposes. Commerical shallow leading tine rippers have two sets of tines which rip at 20-30cm and then again at 30-50/60cm in the same pass	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   A yellow sand deep ripped to 500 mm with a shallow leading tyne ripper
Shallow leading tine ripper with topsoil inclusion plates ($40-55/ha)   Topsoil inclusion plate fitted to the rear tine of a shallow leading tine ripper	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   Seems of topsoil with good pH placed to depth through the use of topsoil inclusion plates
Ripper with wings ($45-55/ha)   A ripping tine with wings allows for greater soil disturbance as it operates below the soil surface and tends to lift subsurface soil	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)   Ripping tine with a 'Horwood attachment' to hold open the soil slot longer and allow limed topsoil to drop into the subsoi	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)   A 'railway fishplate' opener is essentially a plate bolted onto the side of the ripping tine to increase the level of soil disturbance	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?)   The wide curved tines of the Deep Digger® are capable of ripping deeper than standard deep rippers	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   Incorporation of lime to a depth of approximately 25cm in West Binnu using the Deep Digger. The loose soil has been subsequently compacted by traffic
Offsets ($40/ha)   A standard set of offset discs that cultivate the topsoil	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   Incorporation of lime to 10-15cm in a yellow sandplain soil in Carnamah using offset discs
Large offsets ($50-60/ha)   Large offset discs can cultivate to a greater working depth than standard offsets	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)   A small one-way plough used for research trials	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)   A rotary spader used to bury some topsoil and lift up some subsoil	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.   A soil profile of a yellow sand in Carnamah after spading. The spader thoroughly mixed the top soil and subsurface soil throughout the profile, bringing acid soil to the surface and burying the top soil and lime    Soil profile at a trial site showing buried topsoil in spaded treatment. The rotary spader has buried some of the topsoil to depth of 25cm but also lifted some pockets or seams of subsoil into the top 10cm of soil
Mouldboard plough ($100-150/ha)   Growers will often topdress lime after mouldboard ploughing and bringing acidic subsoil to the surface	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.   A mouldboarded soil profile showing acidic soil brought to the surface and limed topsoil buried at depth after complete inversion. Additional lime has then been topdressed (stained purple from universal pH indicator) but there is no preferntial pathway of good pH to allow for root growth between the two layers      In this example in Carnamah, an incomplete soil iversion after mouldboard ploughing resulted in seams of limed topsoil alongside acidic subsurface soil
TopDown® plough (>$100/ha?)   A Topdown® plough has a combination of leading offset discs then curved ripping tines, levelling discs and packers	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   The TopDown plough achieved lime incorporation to 28cm in Carnamah, incorporating lime in large V shaped patterns as subsurface soil is dragged to the surface either side of the tyne

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.

Figure 7 Spring liming of pasture at south Bodallin