diff --git a/README.rst b/README.rst
index 7e3c912..373c8c7 100644
--- a/README.rst
+++ b/README.rst
@@ -87,3 +87,20 @@ buildozer.spec
 
 See `buildozer/default.spec <https://raw.github.com/kivy/buildozer/master/buildozer/default.spec>`_ for an up-to-date spec file.
 
+
+Default config
+--------------
+
+You can override the value of *any* buildozer.spec config token by
+setting an appropriate environment variable. These are all of the
+form ``$SECTION_TOKEN``, where SECTION is the config file section and
+TOKEN is the config token to override. Dots are replaced by
+underscores.
+
+For example, here are some config tokens from the [app] section of the
+config, along with the environment variables that would override them.
+
+- ``title`` -> ``$APP_TITLE``
+- ``package.name`` -> ``$APP_PACKAGE_NAME``
+- ``android.p4a_dir`` -> ``$APP_ANDROID_P4A_DIR``
+
diff --git a/buildozer/targets/android.py b/buildozer/targets/android.py
index 75dae05..0aaf462 100644
--- a/buildozer/targets/android.py
+++ b/buildozer/targets/android.py
@@ -301,7 +301,7 @@ class TargetAndroid(Target):
         cmd = self.buildozer.cmd
         self.pa_dir = pa_dir = join(self.buildozer.platform_dir, 'python-for-android')
         if not self.buildozer.file_exists(pa_dir):
-            system_p4a_dir = self.buildozer.config.getdefault('app', 'p4a_dir')
+            system_p4a_dir = self.buildozer.config.getdefault('app', 'android.p4a_dir')
             if system_p4a_dir:
                 cmd('ln -s {} ./python-for-android'.format(system_p4a_dir),
                     cwd = self.buildozer.platform_dir)