Use the group "soil" for nodes that saplings grow on

doc/lua_api.txt

@@ -510,7 +510,7 @@ Usage:
 - Groups are stored in a table, having the group names with keys and the
   group ratings as values. For example:
     groups = {crumbly=3, soil=1}
-    ^ Default dirt (soil group actually currently not defined; TODO)
+    ^ Default dirt
     groups = {crumbly=2, soil=1, level=2, outerspace=1}
     ^ A more special dirt-kind of thing
 - Groups always have a rating associated with them. If there is no
@@ -583,6 +583,7 @@ Special groups
 - attached_node: if the node under it is not a walkable block the node will be
                   dropped as an item. If the node is wallmounted the
                   wallmounted direction is checked.
+- soil: saplings will grow on nodes in this group
 
 Known damage and digging time defining groups
 ----------------------------------------------

src/content_abm.cpp

@@ -99,14 +99,10 @@ class MakeTreesFromSaplingsABM : public ActiveBlockModifier
 {
 private:
 	content_t c_junglesapling;
-	content_t c_dirt;
-	content_t c_dirt_with_grass;
 	
 public:
 	MakeTreesFromSaplingsABM(ServerEnvironment *env, INodeDefManager *nodemgr) {
 		c_junglesapling = nodemgr->getId("junglesapling");
-		c_dirt            = nodemgr->getId("mapgen_dirt");
-		c_dirt_with_grass = nodemgr->getId("mapgen_dirt_with_grass");
 	}
 
 	virtual std::set<std::string> getTriggerContents()
@@ -127,8 +123,7 @@ public:
 		ServerMap *map = &env->getServerMap();
 		
 		MapNode n_below = map->getNodeNoEx(p - v3s16(0, 1, 0));
-		if (n_below.getContent() != c_dirt &&
-			n_below.getContent() != c_dirt_with_grass)
+		if (!((ItemGroupList) ndef->get(n_below).groups)["soil"])
 			return;
 			
 		bool is_jungle_tree = n.getContent() == c_junglesapling;