Fluids more effective on alkaline soils
GroundCover™ Issue: 49
Field trials are showing that fluid fertilisers are likely to be far more effective than granular fertilisers in highly alkaline soils, such as those characteristic of the Eyre Peninsula and, potentially, those of the Mallee of South Australia and Victoria. It is emerging that these soils have a high capacity to ‘fix’ phosphorus and trace elements such as zinc. Consequently, a high proportion of phosphorus applied to these soils can remain inaccessible to crops.
Researchers with the South Australian Research and Development Institute (SARDI), CSIRO Land and Water and the Victorian Department of Primary Industries have demonstrated in SA fields trials that fluid fertilisers (such as phosphoric acid, urea and micronutrient blends, liquid MAP [dissolved technical grade MAP with micronutrients] and ammonium polyphosphate) are more effective on these soils than granular fertilisers (such as DAP and MAP) – especially in dry years. Preliminary evidence suggests that the greater efficiency of fluid fertiliser results from more diffusion (spread) and less ‘fixation’ compared to granular forms.
The research is helping to explain anomalies that have been identified in recent years, such as significant yield responses to added fluid phosphorus – even when soil tests indicate there is already plenty of phosphorus present. Another anomaly is the failure of crops to respond to granular forms of phosphorus fertiliser such as Triple Super.
These findings now explain, for example, the common observation on many of the soils of the Eyre Peninsula that even when high rates of phosphorus fertiliser are applied, crops are still stunted and phosphorus deficient. Similarly, other elements such as zinc are also likely to be fixed, and thus less available for plant uptake. Fluid fertilisers are widely used in the US in broadacre cropping, although their use in Australia has been mainly confined to horticulture.
However, fluid fertilisers can offer growers handling advantages compared to solid formulations, as well as the potential for farmers to simultaneously apply macro and micronutrients and herbicides in a single pass. Research in the US has generally concluded that nutrients derived from fluid fertilisers and granular formulations have similar uptake levels. But in Australia the research is showing a superior performance by some fluid formulations on alkaline calcareous soils in dryland grain production.
Field trials on the Eyre Peninsula found that at similar rates of phosphorus application, fluid fertilisers increased wheat yields by an average of 19 percent over five years, in 26 direct comparisons on grey, highly calcareous soils. There were three trials on these soils in which fluid fertiliser did not produce increases in wheat yields.
On red calcareous soils, there were 17 direct comparisons between fluid and granular fertilisers with a 14 percent mean yield increase from fluids. There were nine further comparisons in which fluids did not out-yield granular products.
Field trials conducted at three sites in 2003 in Victoria on alkaline soils (central and southern Mallee and Wimmera Plains), demonstrated that fluid forms of P fertiliser promoted the seedling growth of wheat more than granular forms at all sites – but this translated to a yield response at only one site (Birchip).
Whether or not fluids performed better than or similar to granular, and the degree of difference between the two, depended on the form of fluid used. For instance, phosphoric acid with urea often produced the same result as granular (coated or incorporated micronutrients with MAP and urea) on red calcareous soils, if micronutrients were applied at a separate time, such as pre-sowing or a later foliar application.
But if the micronutrients were applied in the solution at sowing, the results tended to favour the fluid mixture. No differences were recorded at sites that were not responsive to phosphorus. There was also a trend for the fluids to be much more efficient than granular forms in dry years.
While the trials have centred on highly calcareous soils, there is some evidence that fluids may offer yield benefits to crops on other soil types, but these are still under investigation.
Glasshouse trials have indicated that dry matter yields were greater using fluid fertilisers (such as ammonium polyphosphate and phosphoric acid) than granular forms (such as Triple P) in Sodosols and Calcarosols collected throughout the major cropping areas of SA and Victoria.
Although not as generally responsive to phosphorus as other alkaline soils types, fluid fertilisers were also superior to granular forms in Vertosols (cracking clays). Surprisingly, fluids also produced significantly improved growth responses by wheat in some acid soils compared to Triple P, although only a limited number of these soils were tested.
Current research is now trying to understand why fluids are more effective than granular formulations. It has been established that the growth benefit resulting from using fluids is not due to greater water supply (the 200 l/ha applied equates to 0.02 mm of rain) or to a pH effect.
At this stage the research suggests a chemical process is involved. CSIRO studies indicate there is a greater rate of phosphorus diffusion away from bands of fluid fertilisers compared to the granular formulations.
Diffusion is the major form of movement for immobile elements such as phosphorus and zinc. This contrasts with elements such as nitrogen, which move primarily by mass flow in soil. Consequently, phosphorus derived from the fluid form occupies a larger volume of soil surrounding the fertiliser, which increases the likelihood that roots will come into contact with the nutrient.
Experiments have shown that not only is the phosphorus from the fluid fertiliser more mobile, but is potentially more ‘available’ for plant uptake – due to a tendency for granular phosphorus to become insoluble more rapidly.
This research is funded by the GRDC, South Australian Grain Industry Trust Fund, and Fluid Fertiliser Foundation (USA).
The research team is M. McLaughlin and E. Lombi from CSIRO Land and Water, B. Holloway, B. Frischke and D. Brace from SARDI and R. Armstrong, J. Nuttall and R Argall from the DPI, Horsham, Victoria.
Shoot response of wheat to fluid fertilisers (APP and H3PO4) compared to granular fertiliser (Triple P) and a control growing on a Vertosol collected from Minyip in the Wimmera under glasshouse conditions.
Roger Armstrong, 03 5362 2336, firstname.lastname@example.org
See also: Legumes lend a hand