Benefits of maintaining an appropriate pH profile

Production benefits

Since most agricultural soils in Western Australia are already below the recommended targets, treating soil acidity to remove it as a production constraint often produces a yield response. Wheat yields in WA increase by 12% on average after the application of 2t/ha or more of good quality lime to acidic soils, based on Department of Primary Industries and Regional Development (DPIRD) data from hundreds of trial-years (Table 1).

The time taken to observe this response depends on many factors, such as the starting pH profile; the amount, frequency and quality of lime applied; the sensitivity of the rotation; and the degree of reliance on subsoil moisture at the end of the season. Typically, amelioration and potential yield responses can take four or more years after liming.

In DPIRD trials, there is consistently little or no yield response for wheat in the year of lime application, but on average, there is a yield increase in the following years. Canola appears to have a large initial response to liming, although this has only been looked at in a limited number of field trials (Table 1). For all crops in Table 1 with sufficient lime and time to treat subsurface acidity, responses are significant. Where pasture is a part of the rotation, production is usually increased after liming.

Canola crop at Boothendarra. Liming as an integral part of farming practice will maintain crop choice, production and profitability

Profitability

The profitability of liming will differ according to individual circumstances and many factors need to be taken into account.

Gross margin of the paddock

Compare the potential profits from crops that can be grown when pH is not limiting, compared to low pH constraining both yield and choice of crops/pastures in the rotation. Also consider the ‘lime’ cost of acidity caused by the rotation from nitrogen fertiliser leaching and product removal. Increases in grain yield or pasture production as a result of liming are an indication that productivity has been lost.

Asset value

Liming improves land value. Liming has a long-term value and investment in lime to improve soil pH should be considered in much the same way as fertiliser history when buying or selling farming land.

Lime costs

The pH of the soil profile, lime quality, soil type and farming system will determine the amount of lime required to raise pH to, or maintain pH at, the desired level. For example, clays are slower to acidify but require more lime to lift pH while sands have less capacity to resist pH change but amelioration requires less lime.

Time required to change pH

The initial soil pH and soil type are important. Topsoil pH responds quickly to liming. Surface applied lime usually takes four to seven years to treat acidity in the subsurface layers, provided that sufficient lime is applied to raise and maintain the topsoil pH above 5.5. Limes with a high proportion of fine particles increase pH quicker.

Liming interval

Repeat applications of lime need to be determined by monitoring the change of the soil pH profile over time. The interval will usually be three to seven years depending primarily on initial soil pH and soil type.

Sustainable production and resource protection

Productivity and profitability are usually the immediate concerns to farmers but other issues such as long-term access to markets need to be considered. Some markets insist that farming practices are sustainable, such as Quality Assured production systems. Maintaining optimum soil pH by appropriate lime use is both environmentally friendly and economically profitable helping to ensure sustainability.

Long-term prosperity will be enhanced by maintaining the quality of the soil resource. If soil acidification is untreated, the subsurface soils can become highly acidic meaning amelioration is difficult, expensive and long-term, and all the while productivity is reduced. If pH becomes too low, clay in the soil can be dissolved and soil structural damage can be permanent.

Where acidic soil has reduced plant growth, the susceptibility of the land to wind and water erosion is increased. Such erosion can cause the loss of topsoil, nutrients and soil organic matter.

Offsite impacts

Soil acidity has impacts reaching far beyond the farm gate and liming to reduce or prevent on-farm soil acidity will have the offsite benefit of reducing these impacts.

Most offsite impacts of soil acidity stem from reduced plant growth leading to less stable soil, poor plant cover and reduced uptake of nutrients and water.

Streams and waterways may be impacted through sedimentation (from eroded soil) and eutrophication (increased nutrients leading to algal blooms). These impacts can result in reduced abundance and diversity of aquatic life, particularly if it is an ongoing problem. Degradation of the quality of waterways from these impacts can be long-term and have a wide ecological effect.

Groundwater is an important natural resource in WA and can be polluted by leaching of nutrients. More nutrients are leached when acidic soil restricts root growth and therefore nutrient uptake by the plant.

Salinity can result from insufficient water usage, which is a greater problem if root growth is restricted and the roots are unable to access moisture deeper in the soil profile. When deep-rooted species are unrestricted in growth, salts remain deeper in the soil and are not problematic.

Permanent stream running through farmland at Quindanning. When soil pH is maintained at appropriate levels, offsite impacts are reduced