Presowing nitrogen fertiliser management for aerial-sown rice on puddled soil

Abstract

Presowing nitrogen (N) fertiliser management for aerial-sown rice was investigated for 2 soil cultivation methods, conventional cultivation and puddling, in the Coleambally Irrigation Area of New South Wales. Two N sources, urea and anhydrous ammonia (NH3, ColdFlo), were used. Urea was applied at 3 depths (0,7, 17 cm) and NH3 at 2 depths (7, 17 cm). These 5 treatments were compared with an unfertilised control and with a novel method of applying NH3 in the same operation as puddling. Urea was applied at 60 kg N/ha; the application rate of NH3 appeared to be higher than the intended rate of 60 kg N/ha. The site was responsive to N: agronomic efficiency of the urea-fertilised treatments averaged 39 kg grain yield increase/kg applied N. Dry matter yield, N uptake, and grain yield were similar on conventionally cultivated and puddled soil. There were no significant interactions between cultivation and N treatments in their effects on crop growth or N uptake. Application depth of fertiliser also had no significant effect on crop performance. Applying NH3 in the same operation as puddling was as effective as other methods of applying N, with the advantage of allowing soil preparation and fertiliser application to be completed in 1 pass. Urea labelled with 15N was applied at depths of 0, 5, and 15 cm to microplots at a rate of 60 kg N/ha. Recoveries of 15N in plants and soil were similar for both methods of soil cultivation and for different N application depths, consistent with results from the large plots described above. There was no interaction between soil cultivation and urea application depth treatments. Recovery of applied 14N averaged 32% in the plant shoots and 24% in the top 30 cm of the soil. Recoveries of 15N from presowing urea application have not previously been reported for aerial-sown rice in New South Wales. The results suggest that puddling can be readily integrated into the rice management system without changing current fertiliser practices. However, soil N uptake was very high, accounting for around 90% of the plant N uptake in the urea-fertilised treatments. Therefore, extrapolation of the results of the treatment comparisons to other sites with lower available N should only be done with caution.