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ON THE ROAD to PRECISION AGRICULTURE
Remote Sensing – How Soils Interact with Electromagnetic Energy
Everything around us (including us) emits electromagnetic energy (assuming they are at
temperatures above absolute zero). However, much of this is at wavelengths outside the
visible band. Sunlight is important in remote sensing. The spectral reflectance of soil is
controlled primarily by four variables:
 | moisture content – water absorbs energy primarily in the NIR band; research
has also shown that well drained soils are more reflective in all wavelengths |
| organic matter content – soil reflectance will decrease as organic matter
increases; dark soil (low reflectance) are higher in organic matter than the lighter,
sandy (higher reflectance) soils |
| particle size distribution – large diameter particles have more surrounding
air space, which acts as a "light trap"; assuming the other soil factors are
equal, the finer particle sizes exhibit greater reflectance |
| iron oxide content – exhibits a broad absorption feature centered at 900 nm;
organic matter and ion oxide content are the two most important soil properties affecting
the spectral responses of eroded soils |
It is important to note that much of this reflectance is from a shallow layer on the
surface of the soil. Remote sensing of soils depending on the reflection of sunlight gives
little clue to the sub-surface characteristics of the field.
Roger Brook
Abstracted from The Physical Basis of Remote Sensing by Dr. David
Lusch, Center for Remote Sensing & GISci, Michigan State University
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