Rice N fertilization guided by plant nutritional status using proximal sensing

With warming climate, the conditions north of the alps become more favorable for growing paddy rice as a niche product to diversify the crop production, while simultaneously utilizing wetlands with their benefits for high biodiversity and the prevention of greenhouse gas emissions. However, growing paddy rice in these climatic conditions remains challenging, and therefore, the Nitrogen availability might not be a dominant limiting factor to reach the relatively low yield objectives of Swiss growers (3-4 t / ha). Here, we assess the occurrence of N deficiencies in paddy rice fields across Switzerland and the relative importance of the two main fertilizations (basal at transplantation and at panicle initiation) to reach the yield objectives. To achieve these goals, we used proximal sensing (SPAD (soil plant analysis development) and a near-infrared leaf spectrometer) to estimate the nitrogen nutrition index (NNI) as a fast and affordable method as needed for precision agriculture and targeted fertilization. We calibrated the methodology to determine N and chlorophyl critical values at panicle initiation for the short duration rice variety (Loto) grown in Switzerland. 

In nine paddy rice fields throughout Switzerland, proximal sensing measurements were done between transplantation and panicle initiation (determined as the best moment for the second application of fertilizer). In addition, in one paddy rice field we implemented an experiment consisting of four treatments: a standard practice, where the field was fertilized once at transplantation together with the plant (40 kg N/ha) and once before panicle initiation with a spreader (40 kg N/ha), zero fertilizer and two treatments of only one fertilizer application, namely one in which the fertilizer was applied with the transplantation, and one where the fertilizer was applied before panicle initiation.

Plant leaves were measured with two proximal sensing devices, (Hansatech SPAD meter and SpectraVue leaf spectrometer) before the second fertilization, and in the case of the experiment also one week after fertilizer application. In parallel, plant samples were collected to be analyzed for biomass, leaf N content and phenology.

Preliminary results of the SPAD values showed, that they tended to reach a maximum at ca. 18 ± 4 before panicle initiation, especially in the high yielding fields. In other fields, the SPAD values were much lower (ca. 9 ± 5), indicating the need for adapted fertilization even at low yield objectives.

In terms of yield, the experiment resulted in significantly different (p<0.05) grain yield differences between the treatment without fertilizer and with the two doses of fertilizer applied. The SPAD values showed significant differences after the second fertilizer application between the treatments that received the second fertilization and those who didn’t. No effect could be seen from the first fertilization in that case as the recent fertilization overrode the other differences.

This method could be used in the future to guide precision fertilization based on crop needs and to account for the high interannual variability.