I believe all you have to do is set fertile level to 16 (well, 15+) to the mdirt material in materials.xml
<material id="Mdirt">
<property name="damage_category" value="earth"/>
<property name="surface_category" value="earth"/>
<property name="Hardness" type="float" value="1"/>
<property name="stepsound" value="dirt"/>
<property name="fertile_level" value="5"/>
<property name="stability_glue" value="20"/>
<property name="Mass" type="int" value="10"/>
<property name="MaxDamage" value="200"/>
<property name="Experience" value="2"/>
</material>