diff -u b/src/Entity/MediaBundle.php b/src/Entity/MediaBundle.php
--- b/src/Entity/MediaBundle.php
+++ b/src/Entity/MediaBundle.php
@@ -250,18 +250,19 @@
   public function preSave(EntityStorageInterface $storage) {
     parent::preSave($storage);
 
-    // If the type plugin uses a source field, we'll need to store its name in
-    // our type configuration before saving. We'd need to double-save if we did
-    // this in postSave().
-    $type_plugin = $this->getType();
-    if ($type_plugin instanceof SourceFieldInterface) {
-      $storage = $type_plugin->getSourceField($this)->getFieldStorageDefinition();
+    // If the handler uses a source field, we'll need to store its name before
+    // saving. We'd need to double-save if we did this in postSave().
+    $handler = $this->getType();
+    if ($handler instanceof SourceFieldInterface) {
+      $storage = $handler->getSourceField($this)->getFieldStorageDefinition();
       // If the field storage is a new (unsaved) config entity, save it.
       if ($storage instanceof FieldStorageConfigInterface && $storage->isNew()) {
         $storage->save();
       }
-      // Store the field name.
-      $configuration = $type_plugin->getConfiguration();
+      // Store the field name. We always want to update this value because the
+      // field name may have changed, or a new field may have been created,
+      // depending on the user's actions or the handler's behavior.
+      $configuration = $handler->getConfiguration();
       $configuration['source_field'] = $storage->getName();
       $this->setTypeConfiguration($configuration);
     }
@@ -273,12 +274,12 @@
   public function postSave(EntityStorageInterface $storage, $update = TRUE) {
     parent::postSave($storage, $update);
 
-    // If the type plugin is using a source field, we may need to save it if
-    // it's new. The field storage is guaranteed to exist already because
-    // preSave() took care of that.
-    $type_plugin = $this->getType();
-    if ($type_plugin instanceof SourceFieldInterface) {
-      $field = $type_plugin->getSourceField($this);
+    // If the handler is using a source field, we may need to save it if it's
+    // new. The field storage is guaranteed to exist already because preSave()
+    // took care of that.
+    $handler = $this->getType();
+    if ($handler instanceof SourceFieldInterface) {
+      $field = $handler->getSourceField($this);
 
       // If the field is new, save it and add it to this bundle's view and form
       // displays.
diff -u b/src/MediaTypeBase.php b/src/MediaTypeBase.php
--- b/src/MediaTypeBase.php
+++ b/src/MediaTypeBase.php
@@ -158,6 +158,7 @@
         '#empty_option' => $this->t('- Create -'),
         '#empty_value' => NULL,
         '#options' => $options,
+        '#description' => $this->t('The field on media items of this type that will store the source information.'),
       ];
     }
     return $form;
@@ -184,9 +185,13 @@
    * {@inheritdoc}
    */
   public function getSourceField(MediaBundleInterface $bundle) {
+    // If we don't know the name of the source field, we definitely need to
+    // create it.
     if (empty($this->configuration['source_field'])) {
       return $this->createSourceField($bundle);
     }
+    // Even if we do know the name of the source field, there is no guarantee
+    // that it already exists. So check for the field and create it if needed.
     $field = $this->configuration['source_field'];
     $fields = $this->entityFieldManager->getFieldDefinitions('media', $bundle->id());
     return isset($fields[$field]) ? $fields[$field] : $this->createSourceField($bundle);
@@ -199,9 +204,13 @@
    *   The field storage definition. Will be unsaved if new.
    */
   protected function getSourceFieldStorage() {
+    // If we don't know the name of the source field, we definitely need to
+    // create its storage.
     if (empty($this->configuration['source_field'])) {
       return $this->createSourceFieldStorage();
     }
+    // Even if we do know the name of the source field, we cannot guarantee that
+    // its storage exists. So check for the storage and create it if needed.
     $field = $this->configuration['source_field'];
     $fields = $this->entityFieldManager->getFieldStorageDefinitions('media');
     return isset($fields[$field]) ? $fields[$field] : $this->createSourceFieldStorage();
@@ -228,12 +237,17 @@
   abstract protected function createSourceField(MediaBundleInterface $bundle);
 
   /**
-   * Determine a free field name to use as the default field.
+   * Determine the name of the source field.
    *
    * @return string
-   *   An appropriate field name that was determined to be available.
+   *   The source field name. If one is already stored in configuration, it is
+   *   returned. Otherwise, a new, unused one is generated.
    */
   protected function getSourceFieldName() {
+    if ($this->configuration['source_field']) {
+      return $this->configuration['source_field'];
+    }
+
     $base_id = 'field_media_' . $this->getPluginId();
     $tries = 0;
     $storage = $this->entityTypeManager->getStorage('field_storage_config');
diff -u b/tests/modules/media_entity_test_type/src/Plugin/MediaEntity/Type/TestType.php b/tests/modules/media_entity_test_type/src/Plugin/MediaEntity/Type/TestType.php
--- b/tests/modules/media_entity_test_type/src/Plugin/MediaEntity/Type/TestType.php
+++ b/tests/modules/media_entity_test_type/src/Plugin/MediaEntity/Type/TestType.php
@@ -48,8 +48,6 @@
       '#default_value' => empty($this->configuration['test_config_value']) ? NULL : $this->configuration['test_config_value'],
     ];
 
-    $form['source_field']['#description'] = $this->t('Field on media entity that stores the source information.');
-
     return $form;
   }
 
diff -u b/tests/src/Kernel/BasicCreationTest.php b/tests/src/Kernel/BasicCreationTest.php
--- b/tests/src/Kernel/BasicCreationTest.php
+++ b/tests/src/Kernel/BasicCreationTest.php
@@ -2,6 +2,7 @@
 
 namespace Drupal\Tests\media_entity\Kernel;
 
+use Drupal\field\Entity\FieldConfig;
 use Drupal\KernelTests\KernelTestBase;
 use Drupal\media_entity\Entity\Media;
 use Drupal\media_entity\Entity\MediaBundle;
@@ -114,2 +115,49 @@
 
+  /**
+   * Tests creating and updating bundles programmatically.
+   */
+  public function testProgrammaticBundleManipulation() {
+    // Creating a bundle programmatically without specifying a source field
+    // should create one automagically.
+    /** @var FieldConfig $field */
+    $field = $this->testBundle->getType()->getSourceField($this->testBundle);
+    $this->assertInstanceOf(FieldConfig::class, $field);
+    $this->assertEquals('field_media_generic', $field->getName());
+    $this->assertFalse($field->isNew());
+
+    // Saving with a non-existent source field should create it.
+    $this->testBundle->setTypeConfiguration([
+      'source_field' => 'field_magick',
+    ]);
+    $this->testBundle->save();
+    $field = $this->testBundle->getType()->getSourceField($this->testBundle);
+    $this->assertInstanceOf(FieldConfig::class, $field);
+    $this->assertEquals('field_magick', $field->getName());
+    $this->assertFalse($field->isNew());
+
+    // Trying to save without a source field should create a new, de-duped one.
+    $this->testBundle->setTypeConfiguration([]);
+    $this->testBundle->save();
+    $field = $this->testBundle->getType()->getSourceField($this->testBundle);
+    $this->assertInstanceOf(FieldConfig::class, $field);
+    $this->assertEquals('field_media_generic_1', $field->getName());
+    $this->assertFalse($field->isNew());
+
+    // Trying to reuse an existing field should, well, reuse the existing field.
+    $this->testBundle->setTypeConfiguration([
+      'source_field' => 'field_magick',
+    ]);
+    $this->testBundle->save();
+    $field = $this->testBundle->getType()->getSourceField($this->testBundle);
+    $this->assertInstanceOf(FieldConfig::class, $field);
+    $this->assertEquals('field_magick', $field->getName());
+    $this->assertFalse($field->isNew());
+    // No new de-duped fields should have been created.
+    $duplicates = FieldConfig::loadMultiple([
+      'media.' . $this->testBundle->id() . '.field_magick_1',
+      'media.' . $this->testBundle->id() . '.field_media_generic_2',
+    ]);
+    $this->assertEmpty($duplicates);
+  }
+
 }