Analysis of variation of spectral vegetation index measured in differently fertilized field barley
2002
Petersen, C.T. | Jensen, C.R. | Mogensen, V.O.
Information on crop conditions obtained from spectral reflectance measurements may be used in site specific farming systems. This study investigates the response of spring barley as measured with a relative vegetation index (RVI) to different fertilization treatments, aiming to analyze the response at different growth stages. Ground-based spectral reflectance readings were obtained in a split-plot experiment with nitrogen (N) in ammonium nitrate (49, 98, and 147 kg N ha(-1)), potassium (K) in potassium chloride (0 and 136 kg K ha(-1)), and calcium (Ca) in calcium carbonate (0 and 400 kg Ca ha(-1)). RVI, defined as the ratio between reflectance in a near-infrared (740-820 nm) and a photosynthetically active (400-700 nm) band, responded similarly and consistently during two consecutive growing seasons. Responses of RVI to fertilization were significant (P < 0.05) from Feekes growth stage 1 (one shoot, 1-3 leaves) to stage 11.1 (milky ripe) or 11.2 (mealy ripe). Between 58 and 90% of the daily RVI-variability observed at growth stages between initial stem elongation and heading could be attributed to N-effects. The slope of the forced linear increase of RVI with time obtained during the tillering period, s(t), correlated non-linearly with the amount of applied nitrogen (R2 greater than or equal to 82%). A model is formulated using s(t) to predict the crop nitrogen supply. Independently predicted nitrogen application levels differed with 11.3 kg N ha(-1) from known values, on the average. It is concluded that there may be a potential for prediction of the site specific need of sidedress N, based on derivations of site specific s(t)-values, including a s(t)-value for a reference area with known N-supply chosen within the field.
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