Show soft keyboard when the device is landscape mode
You need to use show forced
InputMethodManager imm;
imm.toggleSoftInput(InputMethodManager.SHOW_FORCED,InputMethodManager.HIDE_IMPLICIT_ONLY);
imm.showSoftInput(this.editText,InputMethodManager.SHOW_FORCED);
The reason is that landscape mode most often put soft keyboard in a new full screen window. As Bakih said, force will work but the full screen window have more effects and so do SHOW_FORCED.
I prefer adding
<item name="android:imeOptions">flagNoFullscreen</item>
to my EditTextStyle so I can always catch onGlobalLayout() and so on. Now you can use SHOW_IMPLICIT. Just make sure your UI looks good in such a small area and remove autoCorrect if not needed.