Context and objectives
Nitrogen fertilization practices in Belgian agriculture are based on parcel nitrogen balances. In this N-balance, one term appears extremely difficult to estimate : the nitrogen provided by the mineralization of the soil organic matter (SOM). Indeed this term can be far from negligible as it can sometimes exceed the amount of nitrogen from the mineral fertilizer input on crops. One of the reasons explaining this difficulty stands in the high inter- and intra- variability of SOM. A better knowledge of the soil organic matter would allow not only to optimize the fertilization but also to reduce the impact of excessive fertilizations on the environment. This knowledge is necessary in precision farming.
Expected scientific results
In order to determine whether the spectral signatures given by laboratory, field measurements and remote sensing sensors are similar, these three signals were compared. Generally, the spectral signature from laboratory is lower than the two others from 530 to 2500 nm. This can probably be explained by higher water content in soils measured in the laboratory. In fact, soil samples for laboratory analysis were taken on a 20 cm depth while the two other approaches are limited to the soil surface that was dry the day of flight. The flying sensors and the ASD give similar signatures except for CASI-2 sensor in the range of 730 to 950 nm.
Concerning CASI and SWIR sensors, each of them revealed its ability to predict SOM from its reflectance information. However the SWIR bands seem more powerful and using both sensors in the all spectral range (450-2500 nm) showed the best results.
A set of stepwise multiple regressions models with a number of explanatory variables (bands) from 3 to 12 were proposed. It should be noted that the prediction equation developed in this study is appropriate only for the area of investigation. These preliminary results appear quite promising despite some disturbing factors (soil moisture and soil roughness variability, vegetation debris on the soil surface, …) not yet taken into account. A new APEX campaign in October 2003 will serve to study these factors more carefully.