UAL Crop Nitrogen and Irrigation Lab

Proximal optical sensors are hand-held sensors that measure optical properties of crop canopies. Generally, these sensors are held some distance (e.g., 0.5 m) from the canopy and are then passed over the canopy thereby providing information on a representative area of a crop. There is considerable international interest in using these sensors to assist with N management of diverse agricultural crops. We are evaluating these optical sensors for the management of N of vegetable crops within greenhouses.

Canopy reflectance sensors have been shown to be very sensitive to crop N status and are the subject of considerable international research. Canopy reflectance sensors are being used for controlling N fertilizer application rates in commercial cereal crops in North America and North-West Europe. We are adapting this technology for vegetable crops grown in greenhouses. These sensors are manually held parallel to the crop at a distance of approx. 50 cm and passed over varios metres of the crop. The surface area of the canopy measured is the 30 cm high vertical section measured by the length of the pass, multiplied by the length of the pass which can be many metters. Consequently, large representative surface areas are measured.

The reflectance of various wavelengths are measured, and mathematical indices (e.g., NDVI) are used to integrate the different wavelengths. The indices are then related to crop N status. Relationships are derived to (a) determine crop N status from values of the indices and (b) subsequently assess whether or not fertilizer N is required, and (c) to calculate how much N fertilizer is required. We have demostrated that canopy reflectance is a very sensitive indicator of crop N status of vegetable crops.