The correct posture for shikantaza meditation in Soto Zen is known as the "full lotus" or "half lotus" position. This posture involves sitting cross-legged on a cushion or mat, with one foot resting on top of the opposite thigh and the other foot tucked underneath the opposite thigh. The hands are placed in the lap, with the palms facing upwards and the thumbs lightly touching. The spine should be straight, but not rigid, with the shoulders relaxed and the chin slightly tucked in. The eyes can be closed or kept slightly open with a soft gaze.

The full lotus position is considered the most stable and balanced posture for shikantaza meditation, as it allows for a strong and grounded foundation. However, for those who find it uncomfortable or physically challenging, the half lotus or even a simple cross-legged position can also be used.

It is important to maintain a comfortable and relaxed posture while practicing shikantaza meditation. This allows for the mind to settle and the body to remain at ease. It is also recommended to sit on a cushion or mat that is firm enough to support the body, but not too hard or soft.

In addition to the physical posture, there is also a focus on the mental posture in shikantaza meditation. This involves maintaining a sense of openness and non-judgment towards thoughts, emotions, and sensations that arise during the practice. Rather than trying to control or suppress these experiences, the meditator is encouraged to simply observe them without attachment or aversion.

In Soto Zen, the emphasis is on the present moment and the act of "just sitting" without any specific goal or expectation. The correct posture for shikantaza meditation is therefore one that allows for a relaxed and open awareness of the present moment, without any effort or strain.

In conclusion, the correct posture for shikantaza meditation in Soto Zen is the full or half lotus position, with a relaxed and open mental posture. However, the most important aspect of this practice is to find a posture that feels comfortable and supports a sense of present moment awareness.