Greener Pastures: soil acidity

What did we learn about soil acidity in Greener Pastures?

Soil sampling at Vasse Research Centre

During 1999-2009, soil testing for pH was used to determine lime application for 48 paddocks at the Vasse Research Centre at Busselton, in the south-west of Western Australia (WA).

Paddocks had been grazed intensively by dairy cows and their young stock over a period of 10 years, as part of the Vasse Milk Farmlets and Greener Pastures farming system projects.

Pasture consisted of annual ryegrasses with some subterranean clover. Soils in the 48 paddocks were 1–2m sand to sandy loam over massive clay, known locally as Abba sand. For many soils in the region, including Abba sands, the topography is flat and the soils are waterlogged from June to early September in the typical May to November growing season.

No major liming program had been undertaken in the 48 paddocks before April 1999 and soil testing in 1999 indicated soil pH_Ca for the top 10cm of soil was 4.0–5.0 in all paddocks.

Soil acidification was therefore identified as a major problem, and a liming program was undertaken to rectify the problem, starting in 1999.

The soil sampling program

Samples of the top 10cm of soil were collected from each paddock in April 1999 and January–February 2000–2009, during the dry period before fertiliser was applied. These are the standard sampling depth and sampling time for soil sampling of dryland pastures in WA.

Soil samples were collected while walking on the same diagonal path across each paddock each year between two permanent markers located on fences. Samples were collected using 2.5cm diameter metal tubes (10cm long; known locally as pogos) that were pushed into the soil by foot every 2–3m, with 50–100 samples collected per paddock, depending on the size of the paddock.

The samples from each paddock were bulked, air dried and sieved through a 2mm sieve to exclude coarse material. The samples were sent for analysis to CSBP Laboratories in Perth, WA. Soil pH was measured by mixing soil and 0.01 molar calcium chloride in the ratio of 1 weight of soil to 5 volumes of calcium chloride, which is the standard procedure for measuring soil pH in Australia and much of the world.

 Liming the 48 paddocks

Local ground limestone with a neutralising value of 80–90% was used to ameliorate soil acidification in the 48 paddocks. The lime was spread over the soil surface in March-May, before the start of the growing season, and was not incorporated into soil after application. Finance limited the amount of lime purchased and applied each year and no lime was applied in 2001, 2002 and 2005.

The same amount of lime was applied to all 48 paddocks in 1999 (3t/ha), 2000 (4t/ha) and 2003 (2t/ha). In 2004, 17 of the 48 paddocks with the lowest soil pH were treated with 1t/ha lime.

Lime was applied each year during 2006-2009 and the level of lime applied varied with soil pH. The pH levels that determined lime quantity are shown in Table 1.

What happened to soil pH?

Liming increased soil pH_Ca from 4.0–5.0 in 1999 to 5.2–6.0 in 2009 (Figure 1).
Soil pH_Ca of 5.5 was achieved in individual paddocks by 2007–2009, 9–11 years after the liming program started, and required a total of 12–21t/ha lime for individual paddocks.

For the last soil testing in 2009, 14 paddocks (about 29% of paddocks) were yet to achieve pH_Ca 5.5, but pH_Ca values for these paddocks were 5.2– 5.4, so close to the target value.

Measuring soil pH identified soil acidification as a problem in all 48 paddocks and soil testing for pH each year enabled the progress of liming to be monitored.

false

Progress in ameliorating soil acidification was disappointing during 1999-2003 (Figure 1). There were three possible reasons for this:

1. Lime was not incorporated after application to the soil surface so liming was less effective.
2. Continued soil acidification during 1999–2009 increased the requirement of lime to ameliorate the acidity.
3. Amounts of lime applied were limited by finance, so no lime was applied in some years and only low levels of lime were applied in other years.

As the liming program continued and liming eventually increased soil pH, applications of lime after 2006 were more effective at ameliorating the problem (Figure 1).

Though lime is more effective at ameliorating soil acidification when it is incorporated after application, farmers are often reluctant to cultivate soils because, in many situations, the topography is flat and the soils are waterlogged for most of the growing season. After cultivation, managing wet, unconsolidated soils can be difficult. In addition, cultivating soil increases mineralisation of soil organic matter, resulting in loss of soil fertility built up over many years when the soils were not cultivated.