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
 ----------------------------------------------

games/minimal/mods/default/init.lua

@@ -741,7 +741,7 @@ minetest.register_node("default:dirt_with_grass", {
 	description = "Dirt with grass",
 	tiles ={"default_grass.png", "default_dirt.png", "default_dirt.png^default_grass_side.png"},
 	is_ground_content = true,
-	groups = {crumbly=3},
+	groups = {crumbly=3, soil=1},
 	drop = 'default:dirt',
 	sounds = default.node_sound_dirt_defaults({
 		footstep = {name="default_grass_footstep", gain=0.4},
@@ -752,7 +752,7 @@ minetest.register_node("default:dirt_with_grass_footsteps", {
 	description = "Dirt with grass and footsteps",
 	tiles ={"default_grass_footsteps.png", "default_dirt.png", "default_dirt.png^default_grass_side.png"},
 	is_ground_content = true,
-	groups = {crumbly=3},
+	groups = {crumbly=3, soil=1},
 	drop = 'default:dirt',
 	sounds = default.node_sound_dirt_defaults({
 		footstep = {name="default_grass_footstep", gain=0.4},
@@ -763,7 +763,7 @@ minetest.register_node("default:dirt", {
 	description = "Dirt",
 	tiles ={"default_dirt.png"},
 	is_ground_content = true,
-	groups = {crumbly=3},
+	groups = {crumbly=3, soil=1},
 	sounds = default.node_sound_dirt_defaults(),
 })
 

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");
+		c_junglesapling = nodemgr->getId("junglesapling");
 	}
 
 	virtual std::set<std::string> getTriggerContents()
@@ -127,8 +123,7 @@ class MakeTreesFromSaplingsABM : public ActiveBlockModifier
 		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;