Our ability to effectively manage the Phosphorus (P) nutrition of dryland grain crops is hampered by the strong influence of background soil fertility and seasonal conditions, especially rainfall.  Phosphorus fertilizer use efficiency by grain crops has rarely been quantified directly in the field and in the first year is often assumed to be 15-25% (McLaughlin et al. 1988). In larger scale field trials fertilizer efficiency can be measured using indirect methods where a control of no P is compared with plus P treatments, but this measurement is susceptible to interference from other factors (disease, soil variability, etc.) and lack of response does not mean that the fertilizer did not contribute P to the crop.  

We used ³³P isotope to directly measure the first year fertilizer efficiency of wheat at seven sites across the Mallee and Eyre Peninsula cropping regions of Southern Australia. We compared the effect of simulated decile 2-3 with decile 7-8 rainfall on single-year fertilizer efficiency. Seasonal conditions also influence the relationship between fertilizer and topsoil and subsoil P uptake by crops. Our hypothesis was that under dry conditions the crop might push more roots into the subsoil and access nutrients from deeper in the profile, due to the inaccessibility of nutrients in the dry topsoil. To test this we measured topsoil and subsoil contribution to plant P uptake in response to low and high rainfall at three of the seven sites using a double isotope labelling method (³²P and ³³P isotopes) under field conditions. Hence we were able to measure the contribution of fertiliser P, topsoil residual P, and subsoil P, to crop P nutrition.

The percentage of added P fertilizer that was absorbed by the crop plant ranged from 3-33% (accounting for 7-44 % of total plant P uptake) and was greater for the decile 7-8 rainfall treatment, but there was no clear relationship between phosphorus use efficiency and the plant available P status of the soil as measured with current commercial soil tests. Residual soil P supplied in the order of 56-93% of plant P uptake and played a major role in crop P nutrition. This highlights the importance of the maintenance and monitoring of soil P status using accurate soil P tests.

Plants accessed more subsoil P under high rainfall conditions at all sites; furthermore the use of subsoil P was increased by P fertilizer application at at two of the three sites tested, suggesting that addition of P fertilizer stimulated root growth into the subsoil and uptake of subsoil P reserves.