Combine this feature with Fix Issue #935 in another pull request.
diff --git a/system/language/english/form_validation_lang.php b/system/language/english/form_validation_lang.php
index 703f2e7..3d77e5e 100644
--- a/system/language/english/form_validation_lang.php
+++ b/system/language/english/form_validation_lang.php
@@ -43,8 +43,8 @@
$lang['regex_match'] = 'The {field} field is not in the correct format.';
$lang['matches'] = 'The {field} field does not match the {param} field.';
$lang['is_unique'] = 'The {field} field must contain a unique value.';
-$lang['is_natural'] = 'The {field} field must contain only positive numbers.';
-$lang['is_natural_no_zero'] = 'The {field} field must contain a number greater than zero.';
+$lang['is_natural'] = 'The {field} field must contain an ordinary number greater or equal to zero.';
+$lang['is_natural_no_zero'] = 'The {field} field must contain an ordinary number greater than zero.';
$lang['decimal'] = 'The {field} field must contain a decimal number.';
$lang['less_than'] = 'The {field} field must contain a number less than {param}.';
$lang['greater_than'] = 'The {field} field must contain a number greater than {param}.';