Improved technique for accurate prediction of soil phosphorus (P) fertiliser requirements in highly alkaline soils (PT10005)

This is a final research report from Hort Innovation’s historical archives. Please note that as these reports may date back as far as the 1990s, the content and recommendations within them may be superseded by more recent research.

What was it all about?

French fry and fresh potato production were significant horticultural industries in South Australia. Commercial potato growing required large amounts of fertiliser so the efficient application and most effective selection of fertiliser combinations were critical decisions for both financial and environmental sustainability of the industry. There had been little information available at the time to help growers determine the optimal phosphorus (P) and potassium (K) fertiliser requirements of potatoes grown for processing in the South-East region of South Australia (SA). It had been suggested that the current P rates may have been too high and K rates may have been too low.

This project evaluated whether two different technologies developed by University of Adelaide or New Zealand Plant and Food Research could be used assist growers better determine the P and K requirements to maximise yield.

Two replicated field experiments were established on commercial properties in Glenroy in the South East and Parilla in the Murryland districts of South Australia. Soil was collected pre planting for testing, with half sent to Adelaide University and half to a commercial laboratory for the Colwell soil test, used by the industry for many years to determine application rates of fertilisers. Various rates of phosphorous and potassium were applied after planting and yield measured at harvest. However the Parilla site was abandoned prior to harvest due to flooding in December and January that washed out the trial area.

The University of Adelaide used soil collected from the two trial sites to evaluate whether a new “Diffusive Gradients in thin Films” (DGT) soil test could provide more accurate data for growers than the Colwell test. Widely used in the mining industry to extract metals from soil, DGT had been successfully used to measure P in horticultural crops such as tomatoes, and cadmium in potatoes and potato soil.

The pre plant Colwell soil tests results and yields were also provided to New Zealand Plant and Food to compare with the phosphorus and potash best management practice determination using their proven technology “PARJIB” modelling. From results of previous trials in the South East of South Australia, PARJIB predicted clear yield benefits from K fertiliser but no yield response to P fertiliser unless soil P levels were lower than ~10 mg P/kg. A series of recommendations in the form of look-up tables were developed from the model to help growers determine optimal P and K fertiliser application rates that reconciled initial soil nutrient supply and the target yield potential in a given field.

At the South East site, both the Colwell P and DGT soil tests predicted no response from added P, which was confirmed by the yield results. The Colwell K also predicted no response from added K, again confirmed by the yield results. The PARJIB model look up tables also did not recommend any application of phosphorous or potassium.

At the Parilla site, the potassium level was low enough to predict a yield response with added K and the PARJIB model tables recommended application of K. However these predictions were not validated due to the loss of yield data from the flooding.

While the data did not produce conclusive results, it suggested that the DGT test could be an effective alternative to the Colwell method and that evaluation should be continued utilising a greater number of field trials with contrasting soil types. This better enabled the determination of the benefits of DGT technology in the potato industry.