def sti(b11, b12):
    """
    Soil Tillage Index (Van Deventer, 1997).

    .. math:: STI = b11/b12

    :param b11: SWIR 1.
    :type b11: numpy.ndarray or float
    :param b12: SWIR 2.
    :type b12: numpy.ndarray or float

    :returns STI: Index value

    .. Tip::

       Van Deventer, A. P., Ward, A. D., Gowda, P. H., Lyon, J. G. 1997. \
       Using thematic mapper data to identify contrasting soil plains and \
       tillage practices. Photogrammetric Engineering and Remote Sensing \
       63, 87-93.
    """

    STI = b11/b12
    return STI