Helen McMillan and John Small
Central West Farming Systems
GRDC Project: CWF00019 Soil acidity and pH management for central west farming districts.
Key Words: pH, acidification, lime, legumes
Take home messages
- Soil acidification is a natural process accelerated by high crop yields and fertilizer use. It is an unseen cost of doing business.
- To maintain a good soil pH profile, producers should aim for a pHCa 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 pHCa8.
- Correct pH is paramount when growing legumes, especially if the decision to grow them is to increase soil nitrogen levels.
- 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).
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 pHCa 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 pHCa 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.
Crop types and varieties vary greatly on their ability to tolerate soil acidity and high aluminium levels. Symptoms of acidity damage are difficult to see in the crop above ground and may also be misdiagnosed as environmental, such as dry springs. A key area to take note of is legume acidity tolerance and how this compares to its rhizobia acidity tolerance. The ability of a legume to fix nitrogen is determined by the number and functionality of its nodules. If the soil pH is not in the range of the rhizobia tolerance the plant will not be fixing any nitrogen and will actually be mining it from the soil.
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. The key message is to be conscious of a gradual decline in soil pH and to take a proactive approach towards limiting the decline.
Soil pH trends in the Central West
The GRDC funded “CWFS Soil acidity and pH management for central west farming districts” project involved identifying and retesting historic pH monitoring sites from previous publicly funded projects. New monitoring sites were also established (GPS located for future reference).
Six historic sites were confidently identified and the results of testing are shown in Table 1. The critical observation is that pH has generally declined in the 14 years since initial testing. Soil pH at three of the farms is more than likely resulting in a yield penalty. Changes in land use practise may help in explaining the observed change in pH. The Nymagee site had changed from cropping to native pasture. Cropping programs at Tottenham and Euabalong West remained relatively unchanged in mixed farming systems. The sites at Wirrinya, Ungarie and Condobolin West have become more intensive cropping enterprises with more fertilisers and legumes in the cropping cycle. Soil pH at these sites would be likely limiting grain production and sub-surface acidification is imminent.
Table 1: Observed changes in pH at 6 locations between 2000 and 2015
Soil acidity and nodulation of legumes
The use of a legume crop in a rotation has numerous benefits including providing a disease break, control of weeds, increasing soil nitrogen and many others. When growing legumes it becomes essential that the soil pH matches that of not only the legume, but also its compatible rhizobia. Table 2 provides information on some common legumes, their partnering rhizobia and their optimal soil pH. Serradella, narrow leaf lupin and their rhizobia are highly tolerant of low soil pH: however, they can have issues nodulating at high pH. In comparison, medics, lucerne and their rhizobia are highly sensitive to acidity and nodulation suffers below pH 5.
Work by De Meyer et al. (unpublished) in the central west found that ¾ of paddocks had pHca <5.5 and 95% of these paddocks had a pHCa <7 (n 60). When relating this information to Table 2, it means that almost all of these paddocks are unsuitable to grow lucerne and medics and 75% of these paddocks are unsuitable to grow peas, faba beans, vetch and lentil. Trying to grow legumes out of their preferred pH range prevents optimum nodulation which risks a decline in yield and increases the chance of the crop actually mining nitrogen from the soil, rather than storing it.
Table 2: Sensitivity of key rhizobia to pH, where red is sensitive and green is optimal (GRDC Inoculating Legumes)
|Cowpea, mungbean, lupin, serradella
|Rhizobium leguminosarum bv. trifolii
|Rhizobium leguminosarum bv. viciae
|Pea, faba bean, lentil, vetch
Liming and the importance of incorporation for optimum nodulation in legumes
Lime applied to the soil to increase the pH to suit the target legume crop must be thoroughly incorporated to at least 10cm to prevent pH stratification. Issues in growth and nodulation of faba beans in NSW and Victoria were found due to lime not being incorporated properly (Burns 2016). This caused a stratification of soil pH resulting in soil 5cm and below being too acidic for good root growth. Incorporation of lime in the top 10cm is important due to rhizobial activity being the highest in this soil fraction and this is where the oldest and most valuable nodules form. These nodules are referred to as crown nodules and they are active for the longest period of time allowing them to fix the most amount of nitrogen. The division of soil samples for pH testing could be challenged to shifting towards testing 0-5cm and 5-10cm, allowing for a better understanding of the stratification of soil pH.
The application of lime can change the availability of nutrients within the soil. Figure 1 shows the impact that soil pH can have on the availability of nutrients to the plant. As the soil becomes more acidic the availability of most nutrients decline, with the exception of iron. This chart helps explain why the optimum soil pH for agricultural production is between 5.5 and 6.5, as that is where the least amount of nutrient tie-up occurs. Nutrient tie-up is very important when considering molybdenum (Mo) and its requirements in successful nodule function and nitrogen fixation in legumes. When Mo is below legume requirements in acidic soils due to tie-up, adding fertilisers with trace Mo could be considered as a short-term solution. However, care must be taken when applying additional Mo as too much can cause toxicity when the soil pH is corrected. Correction of soil pH should be undertaken before additional fertilisers are applied
Is liming worthwhile in Central West districts?
A fundamental mind shift is required in low rainfall districts so that the application of lime is considered as an integral part of maintaining the system’s financial and environmental capital base, rather than being considered a stand-alone crop input.
If a yield response to liming is observed, the reality is that production has historically been lost to soil acidity. Where there is no response but liming was undertaken on the basis of pH and soil testing to determine rates, the liming was not wasted but is acting to maintain a good soil pH profile and will prevent yield decline in the future. The management of pH and the application of lime should be considered similar to changing the oil in the tractor motor: according to the manufacturers specifications to maintain reliability and asset value as opposed to purchasing a crop input, like nitrogen fertiliser, to improve yields or protein and receive a dollar return for the investment in a cropping cycle.
If one of the drivers behind choosing to grow a legume is to increase soil nitrogen then soil pH should be tested. If the soil pH does not support the legumes rhizobia, lime should be applied and incorporated to ensure the rhizobia are given the best chance to fix nitrogen. It should be kept in mind that if the legume is not fixing nitrogen then it is using it from the soil.
Heenan DP, McGhie WJ, Conyers MK (1998) Soil pH change over time in relation to rotation, N fertiliser, stubble management and tillage.
Scott BJ, Ridley AM, Conyers MK (2000) Management of soil acidity in long-term pastures of south-eastern Australia: a review. Australian Journal of Experimental Agriculture 40, 1173-1198.
Burns H (2016) Soil acidity holds back pulse potential. GRDC Ground Cover Issue 120: Jan-Feb 2016
The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC. The author would like to thank them for their continued support.
I would also like to acknowledge the support of Nick Hill, former CWFS project manager and the help provided by Belinda Hackney, NSW LLS.
This paper is a continuation of the paper John Small presented at the GRDC Update in February 2016 at Nyngan. For more information on lime decisions, support tools and management options, please see his paper here:
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