Modeling of sugarbeet quality variables from canopy spectral indices in a fertility trial

Sugarbeet quality varies inversely with N fertility, while gross yield varies positively with N. Existing models of yield from remotely sensed data do not consider crop quality. A fertility trial was used as the basis for a canopy reflectance study to correlate quality variables to canopy characteristics. Sugarbeet canopy reflectance and radiance were measured with a portable spectroradiometer, and from airborne images of the trial plots. Canopy data were collected five times from early August to early October of 1999. Linear regression models were used to correlate canopy indices to sugarbeet quality variables on each date. Both a green NDVI and the conventional NDVI indices showed strong correlation to recoverable sucrose concentration in sugarbeet roots. Correlations to harvest quality in October were highest in spectral measurements taken in early to mid-August. Other quality variables including root concentrations of sodium, potassium, and amino-N were poorly correlated to spectral properties of the canopy. The results suggest that a single image of a field in August could provide spatial indication of variability in root quality.