The chlorophyll content and level of nitrogen supply of the crop stand can also be measured indirectly with non-destructive optical instruments that make it possible to map a crop stand and to replenish nitrogen in a differentiated way. GreenSeeker Model 505 measures the relative chlorophyll content in the form of vegetation index (NDVI) which is calculated on the basis of the intensity of red and infrared rays reflected from the crop stand. During the indirect determination of nitrogen supply, it is important to know how the results of the optical measurement change in space and time and what correlation they have with environmental factors.

During the testing of GreenSeeker Model 505 in an alfalfa stand, it was established that the measurement results and their variability change in accordance with the given part of the day and the applied measurement height. At the 20 cm measurement height above the crop stand, the measured values and their variability are higher than those at the height values between 40 and 100 cm. There was no difference between the results of the measurements taken at 08:00 a.m., 11:00 a.m., 02:00 p.m. and 05:00 p.m. at the 20 measurement height, but the difference between various measurement times became increasingly significant with the increase of measurement heights. Measurement results increased at later measurement times, while their variability decreased.

In our opinions, the variability of measurement results is related to the change in temperature, humidity and light; therefore, we consider it important to examine how changes in weather parameters influence the correlation between normalized difference vegetation index and nitrogen supply.

Keywords: alfalfa, NDVI, GreenSeeker, measurement height, measurement time