Soma yagam is a highly spiritual and effective ritual performed as prescribed in the Vedas. This lasts for six days. This Yagam is called Soma Yagam because Soma rasam (juice) is used as the main oblation. The Soma plant, the king of medicinal herbs, is said to be ‘relished by the celestials’. In Soma Yagam there is the full complement of priests, with each of them being is assisted by three others. In all sixteen priests participate in the Soma Yagam. Agni stoma which is the first of the seven soma Yagams is the ‘Prakriti’ (archetype) and the other six are its ‘Vikriti’.
Soma Yagam has an unparalleled importance in the yagna culture. This special Yagam provides an extra powerful boost of healing energy for those who participate and also to the whole world. The atmosphere becomes medicinal, nutritious and disease-free. Soma Yagam yield a qualitative and quantitative improvement in the psyche. The benefits of Soma Yagam for the public are similar to those of the other Shrauta yagnas, viz., cleansing of the environment off toxins and harmful pollutants, primarily. The performance of the same needs some pre-requisites apart from the knowledge of Yajurveda in full together with the auxiliary sutras viz., - Srauta grihya-dharma-kalpa pertaining to the particular recension of the Veda.
--somayagam.org