diff --git a/backend/apps/fertilizer/migrations/0002_alter_fertilizationentry_fertilizer.py b/backend/apps/fertilizer/migrations/0002_alter_fertilizationentry_fertilizer.py
new file mode 100644
index 0000000..6e4e48c
--- /dev/null
+++ b/backend/apps/fertilizer/migrations/0002_alter_fertilizationentry_fertilizer.py
@@ -0,0 +1,19 @@
+# Generated by Django 5.0 on 2026-03-01 08:19
+
+import django.db.models.deletion
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('fertilizer', '0001_initial'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='fertilizationentry',
+            name='fertilizer',
+            field=models.ForeignKey(on_delete=django.db.models.deletion.PROTECT, to='fertilizer.fertilizer', verbose_name='肥料'),
+        ),
+    ]
diff --git a/backend/apps/fertilizer/models.py b/backend/apps/fertilizer/models.py
index 3a5ffee..22cc524 100644
--- a/backend/apps/fertilizer/models.py
+++ b/backend/apps/fertilizer/models.py
@@ -55,7 +55,7 @@ class FertilizationEntry(models.Model):
         'fields.Field', on_delete=models.CASCADE, verbose_name='圃場'
     )
     fertilizer = models.ForeignKey(
-        Fertilizer, on_delete=models.CASCADE, verbose_name='肥料'
+        Fertilizer, on_delete=models.PROTECT, verbose_name='肥料'
     )
     bags = models.DecimalField(max_digits=8, decimal_places=2, verbose_name='袋数')