Finally I found out what generates the fouling around fields.
As the manual states (on page 49):
"This rule would surround farmland fields with small trees."
The rule looks as follows:
<rule type="forest">
<filter type="key‐value">
landuse=farmland
</filter>
<density>200</density>
<perimeter‐only>true</perimeter‐only>
<pass-through>true</pass-through>
<forests>
<forest>shrubs.for</forest>
<forest>mixed.for</forest>
</objects>
</rule>
Looking into the configuration file I use (originated from the HD config file) the following rule (including the marked change) solves my problem:
<rule type="forest">
<filter type="key-value">
landuse=farm
<!-- landuse=farmland -- THIS FINALY SOLVED MY PROBLEM! -->
natural=meadow
landuse=meadow
landuse=recreation_ground
landuse=village_green
natural=grassland
landuse=grass
<!-- landuse=field -- I ALSO CHANGED THIS - BELIEVE IT IS ALSO A GOOD IDEA :-) -->
natural=field
</filter>
<simplify>0.5</simplify>
<perimeter-only>true</perimeter-only>
<pass-through>true</pass-through>
<random-density-min>170</random-density-min>
<random-density-max>235</random-density-max>
<skip-airport>true</skip-airport>
<regions>
<region>!MED</region>
</regions>
<forests>
<forest>forests/Europe/shrubs.for</forest>
</forests>
<random-execute seed="farming" min="0" max="100"/>
</rule>