Vegetation Indices to Support Nitrogen Sidedressing in Potatoes.

Cultivation of potato (Solanum tuberosum L.) in The Netherlands contributes significantly to the pollution of groundwater with nitrates. Previous work has shown that in-season determination of sidedress N rate based on proximal sensing of crop N uptake allows savings of 25-30 kg N ha^-1 without a negative effect on yield. A crucial element of the in-season sidedress system is the vegetation index (VI) that is used to measure N uptake. The objective of our study was to determine whether a chlorophyll-specific index such as TCARI/OSAVI allows for a better measurement of potato N uptake than the WDVI that has been used so far. We used a previously published dataset on potatoes (cv. Bintje) growing in Wageningen, The Netherlands, in the years 1997-2000. We found that TCARI/OSAVI seems to be a slightly worse predictor of aboveground biomass and of aboveground organic N than WDVI. These results are surprising TCARI/OSAVI is sensitive to the amount of chlorophyll in the canopy and can therefore be expected to be a better predictor than WDVI of organic N content, and, given the strong and linear correlation between total N and organic N in the aboveground parts of the crop, a better predictor of total N in the aboveground parts. We consider our results not to indicate a failure of TCARI/OSAVI which seems to perform well in many cases described in the literature, but rather as the outcome of potato plants adjusting canopy growth to the availability of N and thus keeping N concentration in the canopy within narrow bounds, thus effectively making biomass a good predictor of N uptake. We conclude that determination of N uptake by potatoes using WDVI is at least as precise as with TCARI/OSAVI.