Despite year-in, year-out problems with non-wetting soils and kangaroo grazing, there were some interesting results from a soil-fertility field trial, recently concluded by the West Midlands Group. With support from NACC and the Australian Government’s National Landcare Programme, the project aimed to characterise the residual value of different phosphate sources and changes in soil fertility after 30-plus years, using soil measurement and bioassay cropping.
West Midlands Group consultant Bill Bowden said the results “were a little hit-and-miss with the measurements of the residual, original source of phosphate treatments being poor. However, this project highlighted several important factors of soil fertility dynamics.”
The key findings of the study included:
- Long-term pasture phases allow build-up of soil organic matter reserves – with only a small response to cumulated pasture composition and production.
- Non-wetting of soils relates directly to the level of soil organic matter and there was a slight relationship to the cumulated level of blue lupin production.
- The build-up of soil organic matter increases the cation exchange capacity of the soil.
- Increases in plant available nitrogen was probably a result of increased organic matter, while increases in plant available potassium were most likely due to recycling of the nutrient from depth.
- Soil pH in the surface increases slightly with time, probably due to the ash alkalinity of the deposited residues.
- Phosphorus cycling and availability followed treatments. Water soluble superphosphate leached from the surface layers and its availability decreased through time. Less soluble phosphorus in C 500 initially increased in availability and then decreased through time while the poorly soluble C ore had an initial boost in availability due to the dissolution of the fines, but then stayed poorly available for 35 years.
- The losses of P from the root zone and into the water table, were parallel to the solubility of the sources.