The lowdown on lime

When planning any fertiliser or top-dressing program, the first step should be to conduct some current soil tests across soil types on the farm.

This will allow you to prioritise paddocks based on their requirements and allocate where best to spend your dollars. Use of lime should be part of the overall strategy to improve farm profitability and animal production.

Several claims, which are reviewed in this article, have been made about what lime will do and likely responses, including:

• improved animal production
• more pasture (quality and quantity)
• release of phosphate
• reduction in some trace elements (for example, zinc and boron)
• reduction in toxic levels of some elements (for example, aluminium, manganese)
• release of other trace elements (for example, molybdenum).

The evidence

Improved animal production

Pasture trials at Lardner in Gippsland (in the 1960s) and in New Zealand looked at animal responses to lime and found no direct animal responses. In New Zealand, animal production improved only when there was more or better pasture. If there was no change in pasture, there was no animal response. The Lardner trial, where pH was reasonably high to start with, failed to find a response to lime in either pasture or animals.

Release of, or reduction in, nutrients

Most of the claims about the effect of lime on nutrient availability are based on a graph (derived from soil solution trials) showing that, at a pH of 7, all nutrients were at maximum availability. The data do not show the relative impacts of lower pH on nutrient availability or the cost-effectiveness of changing the pH.

Many responses that have been seen soon after application of lime have been due to the release of molybdenum. However, if molybdenum is limiting, it would usually be cheaper to spray or top-dress the pasture with molybdenum rather than to apply lime.

The belief that lime will make phosphate more available to plants is attractive, but there is very little or no evidence that this regularly occurs. A large pot trial experiment indicated that much (more than 70 per cent) of the time no release of phosphorus occurred after application of lime. In the remaining trials, half demonstrated a negative effect from applying lime and half a positive effect. When high rates of lime are applied, paddock trials have shown some phosphorus release. Lime therefore is generally an expensive and unreliable way of trying to get more phosphorus.

On some light soils where boron is limiting, top-dressing with lime can make the situation worse. Boron is needed in the growing points of plants. If boron is limiting in pastures, clover seed set is greatly reduced or eliminated, ultimately reducing subclover content in the pasture. However, boron deficiency has to be extremely severe to lead to a reduction in pasture production. The best test for a deficiency of boron is to conduct tissue tests.

The most significant impact of lime on pasture growth is a reduction in aluminium availability. Aluminium levels in soil affect different species of pasture plants to different extents, depending on the sensitivity of the species:

• Lucerne is affected at aluminium levels above 5 per cent.
• Phalaris is affected at levels above 10 per cent.
• Sub clover is affected at levels above 15 per cent.
• Perennial ryegrass is affected at levels of 20 to 25 per cent.

High aluminium levels stunt root growth. Where aluminium levels are high and root growth is compromised, plants have reduced ability to access and utilise phosphorus, which does not move far through the soil (so that the roots need to be close to the phosphorus).

Deciding to lime

Table 1 provides guidance for decisions on whether liming is needed.

Once soil pH starts to drop below 4.3, we may start to see long-term and more permanent changes to soil structure. At this level, liming becomes more critical and is independent of aluminium.

Table 1. Liming priority at different levels of aluminium

Lime application rates

Although a standard rate of lime is commonly 2.5 t/ha, the rates required will differ depending on:

• soil texture — for example, applying 1.8 t/ha of lime to a sandy soil will increase pH by the same amount as applying 3.7 t/ha to a loam soil
• starting pH — lifting a soil pH from 4 to 4.3 will require more lime than lifting the same soil type from 4.3 to 4.6 or from 5.2 to 5.5
• lime quality — limes that are lower in lime content, less coarse (larger surface area per volume of lime, so that it will dissolve more quickly) and softer can be applied at lower rates than coarser and harder limes (Lilydale and dolomitic limes are examples of harder limes). The fineness and effective neutralising value can be used to estimate the relative rates (and hence cost-effectiveness) of different limes.

Acidification

Soils will acidify faster:

• when rainfall is higher
• under an annual pasture system
• under high stocking rates (increasing the rate of transfer of nutrients to stock camps)
• under pastures that are high in legumes
• where hay is cut (as a result of product removal).

Some fertilisers have acidifying effects. Table 2 shows the lime requirement to neutralise 1 tonne of particular fertiliser products.

Table 2. Lime requirements for neutralising acidifying effects of fertilisers

Superphosphate has no direct acidifying effect. Indirectly, it may increase acidification through higher stocking rates and higher legume components.

Key points

• Don't just look at lime but assess all limiting factors to get the best response for your dollars and for the soil.
• Prioritise paddocks for the most cost-effective returns. This may mean that some of the most acidic paddocks are left to last—that is, spend money on paddocks that will give you the biggest financial returns so that you can then afford to address problem paddocks.
• Don't rely on lime to release nutrients: it may not, and there are generally cheaper options.
• Use lime to reduce the effects of aluminium.

Jim Shovelton, Meridian Agriculture (formally MS&A)

Editor's note: This article has been adapted from a phone conference run by BESTWOOL/BESTLAMB. If you would like to listen to the original audio or get details on past and future BWBL phone seminars, contact Jane Court (03) 5430 4579.