Research is finding where subsoil acidity is the main constraint to crop production, there is little chance of a short-term return on investment in top-dressed lime. Incorporating top-dressed lime through strategic tillage helps get it deeper in the soil, treating the problem faster. Incorporation can also move more alkaline topsoil and residual free lime (from previous lime applications) to where it is of more use.
If you are not prepared to top-dress lime and wait a decade or more for it to reach the subsoil, how do you decide whether - and where - to incorporate lime?
Find the acidic layer(s)
First, you need to know if there is subsoil acidity. Like acidity in the topsoil, subsoil acidity first shows up on the deep, coarse-textured soils before the heavy loams and clay soils. It is important to know where the acidity is in the profile, so sampling to depth is critical. If early growth is good, but crops run into problems when moisture starts to dry up (premature senescence), then subsoil acidity might be stopping roots from growing deeper. It is advised to dig a hole in the worst patches and check:
- how deep the roots are;
- if there is an obvious physical barrier to further growth (for example, compacted subsoil); and
- if there is moisture below the roots.
If there is no obvious physical barrier, roots are stunted with few root hairs and there is unused water at depth, this indicates a constraint to root growth and a missed opportunity to exploit soil moisture. Target this area for soil testing to check the pH and other soil characteristics.
If there is a physical barrier to root growth, this may need addressing first, perhaps at the same time as subsoil acidity (see below on incorporating lime). If roots cannot reach a soil layer because of a compacted layer above, it does not matter what the pH is below that layer at this stage. Sampling soils in 10-centimetre depth increments is standard and good practice. But beware - it can mask narrow acid layers or 'chokes' (two to three centimetres thick) within a 10cm sample. If a 10cm increment suggests pH is OK, but field observations suggest otherwise, it may be worth sampling 5cm increments.
Another option is to use a pH test kit from a hardware store or nursery on a deep soil core or in a soil pit. Spray the indicator dye along the whole core or on the side of the pit, then tap the barium sulphate powder over the top. On lighter-coloured sandy soils, you might not need the powder to show colour change. If the colour change is hard to see, use the powder as per kit instructions. Detailed instructions on this method are available in the Soil Quality ebook on Soil Acidity, available as a download on iBooks.
"If early growth is good, but crops run into problems when moisture starts to dry up (premature senescence), then subsoil acidity might be stopping roots from growing deeper. It is advised to dig a hole in the worst patches and check: how deep the roots are; if there is an obvious physical barrier to further growth (for example, compacted subsoil); and if there is moisture below the roots."
These kits are not as accurate as laboratory testing, but can show where to look closer and collect samples. Test to a minimum of 30cm depth, ideally deeper; some growers are sampling to 70cm. Take care when collecting samples at multiple depths. If the topsoil falls into the sample hole and is collected in a deeper sample, it will compromise the results. Even a small amount of alkaline topsoil can contaminate subsoil samples, increasing the pH and lowering extractable aluminium results.
Where incorporation is not an option, top-dressing lime in large quantities may eventually deal with subsoil acidity but it might take 10 years or more. This is because lime is only partially soluble and - left to its own devices - moves very slowly through the soil. Soil type, pH and rainfall influence just how slow that movement is. The topsoil pH must be at least 5.5, ideally above 5.8 for lime to move. Otherwise, it will just get used up neutralising the acidity in the surface soil. The soil also needs moisture for lime to work and needs to be near saturation for even very slow lime movement, which is uncommon in medium to low-rainfall zones.
Because most broadacre crops grow well at about pH 5.0 to 5.5, lime applications to address subsoil acidity will not do much in terms of improving plant growth or returns until the lime moves into the subsoil. Increasing topsoil pH to reduce subsoil acidity is a 'long game'. Even with a high-quality lime source with a small particle size and high neutralising value, you need to apply about eight to 10 tonnes of lime per hectare over 10 years for top-dressed lime to start fixing subsoil acidity. Plus, the more acidic the topsoil is, the more lime you will need.
Where possible, prevention is the best strategy for tackling subsoil acidity. Top-dressing suitable rates of lime before the subsoil becomes acidic helps avoid acidification and future production losses. If lime has time to move through the soil profile, it can negate subsoil acidity as it occurs and maintain subsoil pH.
When to incorporate
If you have identified subsoil acidity, carefully consider whether you are willing and able to implement strategic tillage - and whether incorporating lime in the next couple of years is an option. On heavier soils with a high buffering capacity, you will need higher rates of lime to increase the pH and some form of incorporation is probably necessary. Preventing acidity is even more important on these soil types.
Many growers who are ripping, mixing or inverting soil to tackle compaction and/or non-wetting use the opportunity to get lime deeper into the soil. This is only effective for subsoil acidity if the operation is working down to the acidic layer. Soil renovations are expensive, so addressing multiple constraints at once can reduce costs in the long-term. You need to know the pH levels through the main root zone and determine how much lime is needed. In some cases, where lime has been top-dressed in the recent past, there might be enough neutral to alkaline topsoil with residual lime that mixing alone - without applying more lime - can raise the subsoil pH. Incorporating lime does not mean you can reduce the recommended rate. Incorporation merely increases the speed at which the rate of lime neutralises the acidity at depth and brings forward the benefits to the crop and the return on investment. If 3t/ha of lime is recommended, apply 3t/ha. Cutting rates just means you get less of a neutralising effect and have missed the opportunity to properly address the problem for several years.
How to incorporate
For incorporation to work best, you need to mix the lime into the acidic layer. If the tillage is too shallow this will not address the problem - and going too deep may place lime into soil layers that do not need it. If only the top 10cm is acidic, shallow ploughing works well. One-way ploughs are better than two-way ploughs, although they are usually used for deeper mixing/inversion.
Scarifiers with wide sweeps can do a good job and some growers are claiming reasonable lime incorporation and topsoil mixing using stubble crunchers. Deep ripping tends to move limed topsoil through the soil surface, but often not much deeper than 10 to 15cm because the slot behind the ripping tyne closes quickly.
Inclusion plates that keep the slots open can help bury topsoil. Using a shallow leading tyne and inclusion plates can get lime quite deep - up to 30 to 40cm if you are ripping at 40 to 50cm. Wings on the ripping point tend to lift the soil, allowing more topsoil to fall into the space opened up.
If you need to get lime deeper than 45cm, deep ripping with inclusion plates is currently the only option. While ripping can get lime to depth, the limed topsoil will only fall into the rip lines, creating vertical zones of increased pH soil among otherwise hostile subsoil. Narrower rip lines are better but increase fuel and maintenance costs.
Ploughing provides some mixing. Hybrid ripping/ploughing machines incorporate well to nearly the full working depth, but are less effective in soils with a hardpan or gravel at depth. Large offset discs, such as Plozza ploughs (more than 70cm diameter), incorporate lime from about two-thirds to three-quarters of the working depth. Rotary spaders are most effective at mixing limed topsoil throughout the profile to the working depth. Spaders can also bring acid subsoil to the surface, so it is important to test subsoil pH to know how much lime to apply before (and after) spading.
A mouldboard plough can incorporate lime to about 35cm, but complete soil inversion will bring an acid layer to the surface. This is why many growers apply lime before and after mouldboarding. One trick with mouldboard ploughing is to work a bit shallower and faster to do an 'incomplete' inversion. This can create an angled path of limed soil to depth for the roots to follow. The trade-off is that incomplete inversion does not give as good weed control (seed burial) or burying of water repellent topsoil.
Depending upon the type of lime incorporation and soil renovation, some seedbed preparation may be required. Herbicide applications and nutrition may also need modification. Incorporating lime usually creates a highly variable subsoil pH profile because lime is unevenly distributed through the profile. Monitoring pH change after incorporation can be difficult and requires more intensive sampling.
Banding lime into acidic subsoils has been evaluated experimentally over the past couple of decades. Overall, this technique provides good crop responses in a short time by increasing soil pH around the lime bands. But a large proportion of lime within the band remains undissolved and it is difficult to apply the large quantities of lime needed through banding equipment. Other lime banding approaches have been tested, including the application of 200 kilograms/ha as part of seeding operations over multiple years. These have also proven impractical.