Faculty of Natural and Agricultural Sciences
School of Agricultural and Food Sciences
Department of Plant Production and Soil Science
Selected Highlights from Research Findings
The findings of this PhD research project entitled: Heavy metal extractability and plant bioavailability from two sacrificial biosolids soils as influenced by intensive liming, indicate that extractable heavy metal levels increase after the intensive liming of high organic matter sacrificial soils.
This aspect raised concern about remediation and future management of these soils through the application of lime. Dedicated trials indicated that the increased metal extractability was linked to the choice of extractant. NH4-EDTA is often used in heavy metal pollution studies and in the trials it was shown that increased quantities of organic matter were extracted from the soils as the liming rate increased.
Trials with plants indicated an opposite trend; namely that a weaker extractant (NH4NO3) performed better in predicting the pollution risk of metals from sacrificial soils as well as metal uptake by the plants.
Dr JH van der Waals
Plant Production and Soil Science
+27 (0) 12 420 3214
johan.vanderwaals@up.ac.za
Part of a small group of international scientists was tasked with updating the highly successful FAO publication, Irrigation and Drainage No. 33 Yield response to water. This involved three trips to Italy, the last being in December 2005. The static and outdated approach to determining the response of crops to water supply is to be replaced with a dynamic crop model called AquaCrop.
There is much interest in some of the approaches followed in the SWB (Soil Water Balance) model that has been developed and refined at the University.
Nutrient balances (nitrogen and phosphorus) determined for various cropping systems at ERWAT have shown that sewage sludge, if properly managed, can be seen as a valuable resource, rather than as an unwanted waste product that is expensive to dispose of.
Extremely high crop yields were achieved, and large numbers of N and P can be safely alienated from water care works by marketing agricultural products. This project has also involved modelling of nutrient balances, and once model development is complete, a very valuable management tool will be available to guide the environmentally responsible utilisation of sewage sludge.
The nutrient balance work is also being used in a large, multi-institutional, multi-disciplinary project on determining the agricultural contribution to non-point source (NPS) pollution. Small catchment scale research sites have been set up in KwaZulu-Natal and the Western Cape.
Preliminary investigations suggest that dryland cropping on deep soils may not contribute significantly to NPS pollution, but our modelling efforts should prove invaluable to highlight the specificity of each situation, and to quantify the long-term impacts of certain agricultural practices.
The researchers have been able to show that a coarse ash waste dump that is used as a salt sink by the petrochemical industry, can be successfully vegetated without the need for an expensive soil cap. They were also able to simultaneously utilise a bio-solid waste product as a nutrient source for the vegetation, potentially saving millions of Rands in incineration costs.
Their modelling of the root zone water balance with the University’s SWB model and the Dutch SWAP model that is used by the CSIR, both show that with the coarse growing medium that has a low water holding capacity of only sixty millimetres water per meter depth, an evergreen tree canopy with a root depth of only three meters will be sufficient to practically eliminate percolation through the salt laden dump.
This will drastically limit pollution of ground and surface waters. Trees are able to grow roots well below these depths, so it seems very promising that this will be a cost effective, environmentally responsible approach to limiting the impact of such waste sites.
Follow-up research will now have to focus on measuring such water and salt balances to assure researchers that model predictions are reliable. The group has been investigating the feasibility of irrigating with saline mine water for several years. They have established a very valuable research site at Kleinkopje Colliery, where crops have been successfully irrigated with such waters for 16 seasons.
Research findings indicate that it is imperative to take into account what ions are being added to the field through the water. This has important implications for the fertilisation of the crops. The team’s modelling of the effect of large scale, long-term irrigation on groundwater quality, has shown the impact to be site specific, but manageable. They are confident that they have developed a valuable tool in the form of the SWB model to enable regulators to make informed decisions about the desirability of permitting irrigation with these waters on a large scale.
Prof JG Annandale
Plant Production and Soil Science
+27 (0) 12 420 3223
john.Annandale@up.ac.za
The aim of this research project is to develop a cost effective soil cap for disposable sites to minimise the infiltration of water into the dump. An engineered soil cap is normally required to achieve this. The cost however, is extremely high.
An alternative way of manipulating the water balance of the dump to reduce percolation and salt movement, is through the establishment of vegetation. Trees may be useful in order to obtain maximum exploitation of the available water and mineralised nutrients because of their deep root systems and low water holding capacity of the coarse ash medium. The selection of appropriate trees is important to ensure sustainability of the system.
Researchers at the University tested two models as mentioned above. Findings indicate that both models provide similar positive results on the use of deep-rooted vegetation to control drainage of excess rainwater, thereby preventing possible pollution of underground water.
Mr H Smith
Plant Production and Soil Science
+27 (0) 12 420 4585
hendrik.smith@up.ac.za
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