John Small, Central West Farming Systems
- Soil acidification is a natural process accelerated by high crop yields, fertilizer use and potentially direct drilling and stubble retention. It is an unseen cost of doing business.
- To maintain a good soil pH profile, producers should aim for a pH(CaCl) above 5.0 in the 0-10cm of topsoil or 5.5 if subsoil acidity issues are present. The target in the 10-30cm zone is greater than pH (CaCl) 4.8.
- Retesting during 2015 of historic soil pH datasets confirms soil profiles continue to acidify.
- Left unmanaged sub soil acidification is likely to occur in most Central West NSW soils.
- Liming needs to be thought of as a farm input, like checking and changing the oil in the tractor, (maintaining capital) rather than buying urea (dollars returned per dollar invested).
- Cost in managing soil pH are easier to quantify than returns.
Soil acidification is the natural process accelerated by agriculture. Most produce (grain, meat, fibre) is alkaline and harvesting it causes an increase in acidity. Agriculturally generated sub surface soil acidity is a threat to the sustainability of intensive cropping in low rainfall districts. Preventing sub surface acidity is the preferable option, as the cost of attempting amelioration after sub surface acidification has occurred is time consuming, expensive and most likely cost prohibitive. The vast majority of research on ph management, liming response and economics has been conducted in the medium and high rainfall grain production zones in eastern Australia or in Western Australia. Unfortunately, the risk reward scenario for producers in low rain areas of NSW, where pH is likely to be a developing issue, means these research findings and economic models are not readily transferrable.
Soil acidification is not as obvious as other soil issues such as salinity, erosion or structural decline. Symptoms are less visible, production declines are gradual and these changes are often attributed to other factors such as weather. To maintain a good soil pH profile, producers should aim for a pH (CaCl) above 5.0 in the 0-10cm of topsoil or 5.5 if subsoil acidity issues are present. The target in the 10-30cm zone is greater than pH (CaCl) 4.8.
In soils where aluminium is present a small drop in pH can result in a large increase in soluble aluminium which retards root growth, restricting the crops ability to access water and nutrients. At harvest this results in a yield penalty and smaller grain size, usually most noticeable in seasons with a dry finish as plants have restricted access to stored subsoil water for grain filling.
The rate of acidification will depend on the pH buffering capacity of the soil, its initial pH, cumulative crop yields and the frequency of use of acidifying fertilisers and production of legume crops. Heenan et al reported that a higher rate of acidification was observed with direct drilling and stubble retention at Wagga Wagga. These findings are not a reason to stop stubble retention or using legume break crops, as there are other indicators that business profitability and soil health are significantly improved by these practices. The key message is to be conscious of a gradual decline in soil pH and to take a proactive approach towards limiting the decline.