Greener Pastures: soil acidity

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Lime quality

Lime quality is now evaluated by effective neutralising value (ENV).

ENV is a lime quality testing and reporting procedure that has replaced neutralising value (NV) and particle size previously used. It is a measure of the ability of a lime source to neutralise (ameliorate) soil acidity. In the past, NV was considered the major factor that determined the ability of a lime to reduce soil acidity. Recent research has shown that over 70% of the effectiveness of a lime product in ameliorating soil acidity can be explained by particle size, whereas less than 10% can be explained by NV.

ENV considers four factors: NV, particle size and two other factors, which will be explained below. The NV of a lime source is compared to the NV of pure calcium carbonate which is defined as 100%. If a lime source has a NV value of 80%, then it is 80% as effective as pure calcium carbonate at reducing soil acidity.

Most lime sources in WA have NV values of 60 to 90%, with the best deposits having NV values of 90% or more.

A lime source with fine particles has a greater surface area exposed to the acid (hydrogen ions) in the soil and will neutralise soil acidity faster than coarser lime particles. The carbonate component of lime exposed at the surface of lime particles reacts with hydrogen ions in soil solution or on the surfaces of soil constituents.
The greater the number of carbonate ions exposed at the surface of lime particles, achieved by using finer lime, the quicker soil pH is increased.

The third measure contributing to ENV is a measure of the change in soil pH due to the lime source. This is a laboratory test carried out by mixing a given amount of lime with a given amount of soil, adding water to the mixture and measuring soil pH after the moist mixture has been incubated for a set length of time.

In another sample of the same soil, the same procedure is repeated but no lime is applied. From the two samples, the change in soil pH due to adding lime to the soil can be calculated.

The fourth measure contributing to ENV is a measure of the solubility of the different size fractions of the lime source. To measure lime solubility, the lime sample is separated by sieving into five fractions: 0 to 0.125mm, 0.125 to 0.25mm, 0.25 to 0. 5mm, 0.5 to 1.0mm and greater than 1mm. The percentage of particles that pass through each sieve is recorded. The solubility of lime in the different sieved fractions is then determined.

The combination of all four factors determines the ENV which is expressed as a percentage. The higher the ENV value, the more effective is the lime source at increasing soil pH.

How much lime to apply

There is no simple answer to the question of how much lime is required to achieve a pHCa of 5.5 or greater in the top 10cm of soil. The amount depends on many factors, particularly on how low the pH is currently. To cope with these varying factors, general recommendations are given in Table 1 for the typical amount of lime to apply to raise soil pH.

Amelioration of soil acidity depends on a chemical reaction between the lime and the soil. Because of this, the properties of both soil and lime determine the amount of lime required.

Solubility of lime takes place in moist soil, so rainfall is important. Other climatic factors, such as temperature and humidity, also influence the reaction between soil and lime. Factors that determine the amount of lime required include:

  • The soil pH: for any given soil and lime source, more lime is required to raise soil pH
  • The lower the soil pH is to start with.
  • The pH buffering capacity of the soil, which is the capacity of the soil to retain hydrogen ions. This in turn is largely determined by the amount of clay, iron and aluminium oxides, and organic matter in the soil. The larger the pH buffering capacity, the more lime is required to increase soil pH.
  • The bulk density of the soil (mass of soil per unit volume). Soils with more pore space relative to mass of soil particles have lower bulk densities and so need less lime, while soils with larger bulk densities have larger mass of soil per unit volume and so require greater amounts of lime.
  • The amount of lateritic ironstone gravel stones, which are particles of greater than 2mm diameter, in the soil. Gravel particles have little impact on soil chemical properties but do influence soil mass and bulk density. Many sandy gravel soils in WA contain more than 60% gravel. These relatively chemically inert gravel stones decrease the amount of lime required per volume of soil to raise soil pH.
  • ENV determines the ability of the lime source to increase soil pH. ENV depends on the NV and particle size of the lime source used, how soluble different size fractions of the lime are in the soil and the magnitude of the increase in soil pH due to adding the lime source to the soil.
  • Soil moisture. The speed at which lime dissolves in the soil to consume hydrogen ions, and the speed at which it moves down the soil to ameliorate subsoil acidity, depends on soil moisture, including rainfall, soil temperature and humidity. The longer the lime is in moist, warm soil, the more rapidly the lime reduces soil acidity.
  • Tillage. The extent of mixing between soil and lime by means of tillage is a major factor which determines the speed with which lime alters soil pH.

Other important factors to consider

Pasture renovation

Clover and ryegrass are our most productive pasture species but soil acidification can lead to pastures becoming dominated by poorly productive species, reducing animal production. Therefore, once liming has started, the pasture often needs to be renovated by oversowing with clover and ryegrass.

Grazing management

Grazing management is the key to high pasture use and high pasture use is the basis of profitable grazing industries. Good grazing management also helps maintain good pasture composition. Once low soil pH has been corrected and the pasture has been renovated, it becomes essential to apply good grazing management to use as much paddock-grown feed as possible.

Fertiliser

Pastures growing in low pH soils are usually dominated by poorly producing pasture species so it is a waste of money to apply fertiliser to these pastures. It is also a waste of money to fertilise under-grazed pastures to grow more unused feed. It is only profitable to apply fertiliser to productive pastures that are managed to maintain high pasture use.

Acknowledgements

Numerous people have contributed to the Greener Pastures study between 2003 and 2011.

The project would not have been possible without the support, contributions and dedication from the entire team: John Baker, Don Bennett, Mike Bolland, Graham Blincow, Tess Casson, Len Chinnery, John Crosby ,Patrick Donnelly, Hamish Downs, Ian Fillery, Kevin Gardiner, Gordon Gibbon, Ian Guthridge, ‘Tex’ Hahn, Peter Jelinek, Kathy Lawson, Andrew Lindsay, John Lucey, Corrine Mack, Nola Mercer, John Milligan, Richard Morris, Peter Needs, Leonarda Paszkudzka-Baizert, Bill Russell, Dennis Russell, Greg Sawyer, Neroli Smith, Martin Staines, Frank Treasure, Judy Wills and David Windsor.

We are grateful for the guidance and support by Michael Blake, Laurie Cransberg, Grant Evans, Peter Evans, Dale Hanks, Brynley Jenkins, David Kemp, Ben Letchford, Ian McGregor, Miles Mottershead, Ian Noakes, Peter Oates, Paul Omodei, Ralph Papalia and Victor Rodwell.

We also thank our interstate colleagues for their support and guidance: Roger Barlow, David Chapman, Tom Cowan, Anne Crawford, Tom Davidson, Richard Eckard, Warren Mason and Mark Paine.

We acknowledge funding and support of this project by the Department of Agriculture and Food WA, Dairy Australia and Western Dairy. Additional funding and/or contributions in kind, were provided by the Chemistry Centre (WA), CSIRO Plant Industry, South West Catchments Council and Land and Water Australia.