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Development of technologies to alleviate soil acidification in legume-based production systems in the tropics of Asia and Australia

Overview of Project

This project sought to develop production systems that minimise soil acidification and its negative effects, through research undertaken in laboratory and field-based trials in China, Thailand and Australia.

Legumes are widely used in farming systems to improve soil fertility and organic matter accretion through their nitrogen fixation, and to provide protein for human and animal consumption. Some legumes are crops (for example, soybean and peanut) while others are pasture plants (such as Stylosanthes) or trees (such as Leucaena). Stylosanthes, or stylo, has proved to be an ideal, cheap method of improving the quality of native pastures in Asia, Africa, South America and northern Australia. It adapts well to low-fertility soils and tolerates a range of climate types. The importance of introduced legumes such as these for increasing the productivity of tropical agricultural systems has long been recognised. However, it is now clear that legumes make the soil more acid.

Recent research has shown that acidification has occurred in pastures in Australia’s semi-arid tropics following the introduction of Stylosanthes. This is also occurring in northeast Thailand, southern China and southern India, at similar rates to those measured in northern Australia. Acidification of soils makes nutrients less available to plant roots, by altering the balance of ions.

The soils that have shown the greatest degree of acidification are light-textured, and are characterised by their poor fertility, and inherently small ability to buffer pH changes. Such soils are becoming increasingly important agriculturally (and hence economically). If soil organic matter increased this would provide greater buffering capacity. Acidity can also be remedied by applying a base, but given the large areas and depth to which the acidity occurs, this would be impractical."

Key Outcomes of Project

Soil acidification can reduce productivity by restricting root growth through toxicities of aluminium and manganese or calcium deficiency. Soil fertility decline is a consequence of soil organic carbon decline, and nutrient removal in harvested product not being matched by nutrient inputs. Net acid addition rates were measured for several key production systems in Australia and SE Asia, using a paired site (degraded versus undegraded) approach. Lime equivalents of removed product ranged from 50 kg/ha/year for paddy rice in Thailand to 2850 kg/ha/year for leucaena in China. This indicates the wide range in potential soil acidification arising from different agricultural production systems.

Several strategies to minimise or ameliorate soil acidification were studied in this project. It was shown that the use of nitrate-based rather than ammonium-based nitrogen fertilisers could ameliorate soil acidification to a depth of a least 120 cm in sandy soils in northern Australia and north-east Thailand. This finding is particularly significant because neutralisation to depth is rarely possible with conventional approaches using lime.

In legume plus grass pasture production systems in China, the use of fire in combination with over-sowing of grass and input of nitrogen was shown to successfully reduce stylo dominance compared to other management systems. Reduction of legume dominance should reduce the rate of soil acidification. A spin-off from studying the legume production system has been the incorporation of legume intercropping into plantation systems, giving the benefits of nitrogen input and erosion control. This system is being regularly monitored to ensure that soil acidification does not become an issue.

The soils of the project focus areas, in Hainan China, NE Thailand and northern Australia, were predominantly sandy. Sustainable production on such soils is dependent on increasing their cation exchange capacity. The researchers found that applications of materials such as bentonite clay can help restore fertility to degraded, sandy soils. Field experiments in NE Thailand and northern Australia demonstrated yield responses of forage sorghum, gamba grass and sugarcane to applications of calcium-rich natural bentonites. This was due to the improved nutrient-holding capacity and/or water-holding capacity of the amended soils. An adaptation of the technology is to use clay minerals in the composting process to reduce loss of nitrogen by ammonium volatilisation. Clay technology is seen as a way to ‘kick-start’ the remediation process for a degraded system, enabling intensive agriculture on sandy soils.

Assessing the risk of soil acidification is a prerequisite for identifying soil/land use prone to this degradation process. The development of pedo-transfer functions for estimating soil pH buffer capacity from simple routine soil analyses enabled ranking of soil types according to their susceptibility to rapid acidification. This information has been linked with spatial soil survey data to produce soil acidification risk maps for NE Thailand. These maps have provided input into regional land use and environmental planning at provincial level through collaboration with the Land Development Department and provincial staff.

A similar approach is to be applied in Hainan, allowing the identification of priority ‘hot spots’ for concentrating research and extension efforts. Because the complexity of the local farming systems makes predication of soil acidification based on crop parameters (carbon/nitrogen cycles) very difficult, risk assessment will be enhanced through the use of a simple field tool kit for measuring soil pH and pH buffer capacity. This kit will also be used to demonstrate concepts of soil acidity to farmers."

Project Dates

01 Jul 2000 - 30 Jun 2005

Partners

Queensland Department of Natural Resources and Mines - Australia
Chinese Academy of Tropical Agricultural Science - China
Khon Kaen University - Thailand

Leaders

Ms Suzanne Berthelsen

Email

suzanne.berthelsen@jcu.edu.au

Phone

0407 252 815

Website
Launch Website