|Promising future for Victorian brown coal in Agriculture|
|By Assoc. Prof. Tony Patti, Monash University, School of Chemistry|
|Brown coal or lignite is widely marketed for agricultural uses in different parts of the world. The high humic acid content, ion exchange properties, water holding capacity and relatively stable forms of carbon make it an ideal soil amendment, source of humic acids and a likely candidate for organo-mineral fertiliser formulations. Victorian brown coal possesses all these qualities and this resource holds great potential for developing new industries for agriculture and improving soil quality.|
|The research team led by Associate Professor Tony Patti at Monash University has been investigating various aspects of agricultural applications of Victorian brown coal for over 15 years and more recently, a 3-year project sponsored by BCIA has realised some promising outcomes.|
The various components of the investigation have led the group to a recent focus into improving the efficiency of nitrogen and phosphorus delivery to plants through improved fertiliser formulations involving Victorian brown. Improving the efficiency of N and P fertilisers is of paramount importance for food security, minimising detrimental environmental effects and delivering economic benefits to the farmer. In addition, the brown coal provided through ongoing fertiliser applications leads to cumulative increases in soil carbon. Two PhD researchers have independently been investigating blends of brown coal with urea and a commercial brown coal derived potassium-humate with triple superphosphate (Biplob Saha and Azita Kargosha respectively).
Biplob Saha has shown that NH4+ release is suppressed from urea-brown coal blends in water and ammonia gas release from soil columns was also suppressed (Figure 1).
|Figure 1: Release of NH4+ from urea brown coal blends in water.|
|Furthermore, brown coal-urea blends showed a remarkable decrease in the daily emissions of N2O from soil columns, compared to urea (Figure 2).|
|Figure 2: Cumulative N2O emissions from the surface of the soil columns.|
|The blends that included brown coal had lower N2O and NH3 emissions and maintained higher mineral N in soil. These results suggest that blending of urea with the coal can reduce gaseous nitrogen emissions and retain more mineralised N in the soil, thus providing greater amounts of available N to crops. |
Fertiliser blends involving brown coal are not yet available on the Australian market, though humic materials derived from low rank coals have been used commercially to coat urea pellets. Our research work at Monash University has greatly benefitted from the assistance provided by two local companies, Torreco and Feeco Australia, who prepared the high quality granulated fertiliser blends used in our experiments.
Another significant and completed part of the Monash study involved an extensive investigation into commercial humic acid based products derived from Victorian brown coal. These products are typically prepared by potassium hydroxide extraction and/or digestion of the coal and are sold in both liquid and solid forms. They are claimed to have significant beneficial effects on plant growth and soil microbial populations. Dr Karen Little (BCIA-sponsored PhD graduate) undertook an extensive study on the effects of a commercial potassium humate on plant growth and soil properties. This study showed that the abundance of soil bacterial families associated with nitrogen cycling was altered in response treatments with the soluble potassium humate granules (Figure 3), though the implications of this are not well understood.
|Figure 3: Effect of commercial potassium humate on N-cycle soil microbial population.|
|In addition, Dr Little was also able to show that ryegrass growth in a limited outdoor trial was significantly stimulated by treating the soil with a commercial potassium humate (Figure 4) in the early growth stage of the trial. The effect was not sustained in the longer term.|
|Figure 4: Ryegrass treated with a commercial potassium humate.|
|BCIA-funded postdoctoral researcher, Dr Michael Rose, played a central role in the Monash project and conducted a number of laboratory and field studies on commercial humates and as-mined Victorian brown coal. Despite a range of plant growth investigations covering pasture (ryegrass and mixed pasture) cereals (wheat) and vegetables (leeks), responses to these humates were spasmodic and no clear trend emerged. A number of these plant growth studies were conducted in the field on operating farms and the results were communicated to local farmers through public information sessions.|
Where brown coal itself was applied, soil properties were improved and carbon level increases were observed. Suppliers of commercial humates and humic acid containing products generally recommend application rates that are significantly below the levels of the organic matter (and hence humic materials) already present in the soil, yet beneficial effects, even at these low levels, have been reported. These applications rates were also used in the Monash work, as well as some higher levels.
The variability in the observed responses encouraged the Monash team to undertake an extensive survey of the literature where humic substance effects on plant growth were reported. A meta-analysis of 81 peer-reviewed research papers was undertaken (Rose, Patti et al., 2014) reporting data from 181 different experiments. The findings revealed that positive, negative or neutral responses have all been reported. It is likely that negative and neutral responses are underreported, as unsuccessful experiments tend not to be reported in the scientific literature.
The literature analysis revealed that humic materials from compost sources (25%–29% increases in plant growth) significantly outperformed brown coal (12% increase) and peat-derived (4% increase) humates. The plant growth responses are linked to complex interactions between humic substances and their source, application rates, soil types, environmental conditions and plant species, hence a ‘one-size fits all’ product or solution is unlikely. Given the low application rates used, changes in soil characteristics are only likely over longer time periods of time through cumulative effects, so may not show in short-term field trials.
An important development in modern agriculture is the growing use of microbial inoculants as plant growth promoters, often marketed as ‘biostimulants’. Master of Science graduate Tran Thi Kim Cuc (AusAid and BCIA supported) investigated the potential of Victorian brown coal as a microbial inoculant carrier. Cuc’s work showed that brown coal supported survival of nitrogen fixing bacteria during a 3-month storage period to almost the same level as a conventional peat carrier used in a commercial product.
Cuc’s work also provided important information by revealing that microbial factors are likely to play a significant role in the variable plant growth responses observed with humate-containing amendments and treatments added to soils. A glasshouse pot trial with rice showed significant differences between treatments, with Victorian brown coal having the most positive effect on plant growth, particularly when inoculated with nitrogen fixing bacteria. The coal preparation also outperformed a conventional peat carrier. These results combined with the previous work of Dr Rose and Dr Little also assist in understanding why very low levels of humic materials applied to soils and plants can have such dramatic effects (Figure 5).
It is still not well understood how the interactions between microbial populations, plants and humic-rich substances lead to improved nutrient acquisition, increased soil carbon through greater biomass production and generally improved soil health. However, the evidence suggests that there are potentially significant benefits to be realised. It is anticipated that brown coal will provide a very important source of humic materials for new commercial products in Australia.
|Figure 5: Rice in glasshouse trial after 30 days.|
|The initial Monash team of Associate Professor Antonio Patti, Professor Roy Jackson, Associate Professor Tim Cavagnaro (now at Adelaide University, Waite) and Dr Mick Rose (NSW DPI, Wollongbar), continue to work to together and are optimistic that the way forward is to combine Victorian brown coal with conventional chemical fertilisers and possibly microbial inoculants, hence improving fertiliser use efficiency and improving soil fertility through the additional soil carbon hat can be stored.|