while i dont use soap myself, but as long as its fragrance and bleach/antibacterial free i think its okay. i believe the soap acts as a emulsifier and a surfactant, so yes you need something. the emulsifier keeps the oils from seperating and the surfactant makes it stick to the leaves. my recipe now is 1 gallon of 70 degree RO water, 1.5tsp of neem oil, 1.5 tsp of karanja oil, 2 tsp of liquid silica(for emulsifier) and 1tbsp of liquid yucca extract for the surfactant. my plants absolutely love the yucca. i mix this concoction in a baby food jar with a little bit of water. then add my yucca to the RO water, and slowly add the neem/karanja mix to it. im sure i could go heavier on the oils but this was the closest to happy i came while experimenting with mixes, looking for a even milky mix. there are soaps made espicaly to mix with neem oil, soap nut maybe? hope some of that helps.
